Counter-intuitive yet efficient regimes for measurement based quantum computation on symmetry protected spin chains

Presenting Author: Arnab Adhikary, University of British Columbia
Contributing Author(s): Wang Yang, Robert Raussendorf

Quantum states picked from non-trivial symmetry protected topological (SPT) phases have computational power in measurement based quantum computation. This power is uniform across SPT phases, and is unlocked by measurements that break the symmetry. Except at special points in the phase, all computational schemes known to date place these symmetry-breaking measurements far apart, to avoid the correlations introduced by spurious, non-universal entanglement. In this work, we investigate the opposite regime of computation where the symmetry-breaking measurements are packed densely. We show that not only does the computation still function, but in fact, under reasonable physical assumptions, this is the most resource efficient mode.

Read this article online: https://arxiv.org/abs/2307.08903

Predicting Expressibility of Parameterized Quantum Circuits using Graph Neural Network

Presenting Author: Shamminuj Aktar, New Mexico State University
Contributing Author(s): Andreas Bärtschi, Abdel-Hameed A. Badawy, Diane Oyen, Stephan Eidenbenz

Parameterized Quantum Circuits (PQCs) are essential to quantum machine learning and optimization algorithms. The expressibility of PQCs, which measures their ability to represent a wide range of quantum states, is a critical factor influencing their efficacy in solving quantum problems. However, the existing technique for computing expressibility relies on statistically estimating it through classical simulations, which requires many samples. In this work, we propose a novel method based on Graph Neural Networks (GNNs) for predicting the expressibility of PQCs. By leveraging the graph-based representation of PQCs, our GNN-based model captures intricate relationships between circuit parameters and their resulting expressibility. We train the GNN model on a comprehensive dataset of PQCs annotated with their expressibility values. Experimental evaluation on a four thousand random PQC dataset and IBM Qiskit's hardware efficient ansatz sets demonstrates the superior performance of our approach, achieving a root mean square error (RMSE) of 0.03 and 0.06, respectively.

Quantum error supression by weak measurement reversal with current trapped-ion devices

Presenting Author: Andrea Rodriguez-Bla Alejandro Bermudez, University of California Berkeley
Contributing Author(s): K. Birgitta Whaley

Amplitude damping is an important mechanism of decoherence that is common to many atom-based platforms that can affect entangled states. For example, the T1-time sets the ultimate decoherence limit for trapped-ions optical qubits when all other sources of technical noise are suppressed. It can become relevant for trapped-ion hyperfine or Zeeman qubits when using two-photon Raman transitions via auxiliary excited states, or in Rydberg-atom quantum processors where several spontaneous emission decay channels limit the two-qubit gate fidelities. We introduce in [1] a low-overhead protocol to reverse this degradation by partially filtering out amplitude damping noise. In [1], we present two trapped-ion schemes for the implementation of a non-unitary probabilistic filter against amplitude damping noise, which can protect any maximally-entangled pair from spontaneous photon scattering during or after the two-qubit trapped-ion entangling gates. This filter can be understood as a protocol for single-copy quasi-distillation, as it uses only local operations to realise a reversal operation that can be understood in terms of weak measurements. [1] Suppressing Amplitude Damping in Trapped Ions: Discrete Weak Measurements for a Non-unitary Probabilistic Noise Filter, A. Rodriguez-Blanco, K.B. Whaley, and A. Bermudez PRA 107, 052409 (2023)

Read this article online: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.107.052409

Towards a resolution of the spin alignment problem

Presenting Author: Mohammad Alhejji, University of New Mexico CQuIC
Contributing Author(s): Emanuel Knill

We study a class of optimization problems that are inspired by the recently introduced spin alignment conjecture. In the original version of the underlying problem, each state in the mixture is constrained to be a freely chosen state on a subset of n qubits tensored with a fixed state Q on each of the qubits in the complement. According to the conjecture, the entropy of the mixture is minimized by choosing the freely chosen state in each term to be a tensor product of projectors onto a fixed maximal eigenvector of Q, which maximally "aligns" the terms in the mixture. We generalize this problem in several ways. First, instead of minimizing entropy, we consider maximizing arbitrary unitarily invariant convex functions such as Fan norms and Schatten norms. To formalize and generalize the conjectured required alignment, we define alignment as a preorder on tuples of self-adjoint operators that is induced by majorization. We prove the generalized conjecture for Schatten norms of integer order, for the case where the freely chosen states are constrained to be classical, and for the case where only two states contribute to the mixture and Q is proportional to a projector. The last case fits into a more general situation where we give explicit conditions for maximal alignment. The spin alignment problem has a natural "dual" formulation, versions of which have further generalizations that we introduce.

Read this article online: https://arxiv.org/abs/2307.06894

Randomised benchmarking of universal single- and multi-qudit gates.

Presenting Author: David Amaro-Alcala, University of Calgary
Contributing Author(s): Barry C. Sanders, Hubert de Guise.

Although universal benchmarking schemes exist for qubit gates, universal qudit-gate randomised benchmarking (RB) schemes, despite being in great need, have not been established, due both to mathematical challenges and the question of how RB costs will scale with Hilbert-space dimension. Here we devise a scheme for benchmarking a universal set of gates for single- and multi-qudit gates in terms of fidelity, with our scheme remarkably depending only on two parameters that can be estimated using the same quantum circuit. We introduce the `real hyperdihedral group' as a natural mathematical structure for the group generated by cyclic gates plus the magic gate, with the benefit of this group being the imposition of a minimal classical overhead in computing compositions of gates and their inverses, which are needed in our RB circuit design. An intriguing outcome of our extension to qubit dihedral RB is a novel mathematical identity for Bell numbers, which count set partitions. We further show that our RB circuit-design method is readily extended to recent RB approaches, namely, randomised compiling and mirror RB, thus making our new group-based method attractive as a testbed for relaxing the unitary 2-design assumption common in qudit RB schemes.

Quantum and classical steganography using optical systems

Presenting Author: Bruno Avritzer, University of Southern California
Contributing Author(s): Todd Brun

We detail methods of transmitting classical and quantum information using states that have statistics which mimic a thermal channel's, with provable secrecy. These methods consider transmission of coherent, state, and TMSV-type states with error correction techniques applied to compensate for issues arising from the constraints. We then study the performance of these methods in an information-theoretic context.

Read this article online: https://arxiv.org/pdf/2303.02307.pdf

Dynamics, geometry and measurement of entanglement in many-body systems

Presenting Author: Peyman Azodi, Princeton University
Contributing Author(s): Herschel A Rabitz

A new framework will be introduced to study and measure entanglement dynamics in many-body quantum systems along with an associated geometric description of entanglement. In this formulation, called the Quantum Correlation Transfer Function (QCTF), the system’s wave function or density matrix is transformed into a new space of complex functions with isolated singularities. Accordingly, entanglement dynamics is encoded in specific residues of the QCTF, and importantly, the explicit evaluation of the system’s time dependence is avoided. Notably, the QCTF formulation allows for various algebraic simplifications and approximations to address the normally encountered complications due to the exponential growth of the many-body Hilbert space with the number of bodies. Consequently, an exterior (Grassmannian) algebraic expression of many-body entanglement will be presented. This associated geometric description is shown to provide new means of measuring entanglement entropies in experimental settings, including spin and bosonic lattices as well as qubit systems. This QCTF-based geometric description offers the prospect of theoretically revealing aspects of many-body entanglement, by drawing on the vast scope of methods from geometry.

Optical network design for entanglement distribution

Presenting Author: Rohan Bali, University of Arizona
Contributing Author(s): Ashley Tittelbaugh, Shelbi Jenkins, Anuj Agrawal, Jerry Horgan, Marco Ruffini, Dan Kilper, Boulat Bash

We study the entanglement distribution in an optical network using a quazi-deterministic source-in-the-middle approach, such as zero-added-loss entangled-photon multiplexing (ZALM, see https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.19.054029). We focus on the quantum network system design as follows: 1) We characterize the network node architectures, including the design of entangled-photon-pair source, and consumer/relay nodes. We describe trade-offs between various approaches to implement these systems. 2) We devise routing and spectrum allocation (RSA) methods for entanglement distribution in optical networks using a single broadband entanglement source. We maximize the average entangled photon pair distribution rate while enforcing fairness across the consumer nodes. Since finding the exact solution is an NP-hard problem even in the single-source scenario, we consider approximations and heurists. We evaluate those on synthetic random networks, as well as models derived from existing optical networks. 3) We will discuss the path forward to employing multiple entanglement sources. This will improve the entangled photon pair distribution rates at the cost of additional complexity for routing and spectrum allocation, potentially requiring changes to node architecture to mitigate these challenges.

“investigation of the structural dynamics and topology of gp28 incorporated in lipid bilayers using EPR spectroscopy”

Presenting Author: BINAYA BARAL, Miami University

The lysis process induced by phages in gram-negative bacterial hosts involves two main pathways: the holin-endolysin pathway and the pinholin-SAR endolysin pathway. This process comprises holin creating a cytoplasmic membrane hole, endolysin breaking down peptidoglycan, and spanin proteins disrupting the outer membrane (OM). Some phages without spanins can still lyse the OM effectively using disruptin proteins. The disruptin protein gp28, an alpha-helical peptide on the surface, is crucial. Understanding gp28's interaction with the lipid bilayer and OM disruption mechanism requires knowledge of its helical tilt in the bilayer, determined by electron paramagnetic resonance (EPR) spectroscopy using TOAC spin labels. Previous work by the Lorigan lab in helical peptide topology using EPR spectroscopy aids this study. Combining CW-EPR and nitroxide spin labels allows insights into lipid membranes, proteins, and interactions. Mechanically aligned bilayers with CW-EPR power saturation and DEER spectroscopy will reveal gp28's topology relative to the lipid bilayer, advancing our understanding of phage-induced lysis, disruptin proteins, and phage-host interactions for potential antimicrobial strategies.

Two new principle-based characterizations of quantum state space

Presenting Author: Howard Barnum, Currently unaffiliated
Contributing Author(s): Joachim Hilgert, Cozmin Ududec, John van de Wetering

I give two characterizations of finite-dimensional quantum theory's framework of density matrices (states) and POVM elements (measurement outcomes) for describing systems, from simple postulates whose physical and informational meaning and appeal is clear. Each first characterizes a class of Euclidean Jordan-algebraic (EJA) systems. The simple EJAs were classified by Jordan, von Neumann, and Wigner: real, complex, and quaternionic quantum theory, systems whose state spaces are balls, and 3-dimensional octonionic quantum theory. Complex quantum theory then follows from "local tomography", or "energy observability": the generators of continuous symmetries of the state space (potential reversible dynamics) are observables. The first characterization uses: (1) Spectrality: every state is a convex combination of perfectly distinguishable pure states, (2) Strong Symmetry: every set of perfectly distinguishable pure states may be taken to any other such set (of the same size) by a symmetry of the state space. The second uses: (1) Homogeneity: any strictly positive element of the cone of unnormalized states may be taken to any other by a symmetry of this cone, (2) Pure Transitivity: any pure state may be taken to any other pure state by a symmetry of the normalized state space The physical, informational, and operational significance of the postulates will be explained.

Read this article online: https://arxiv.org/abs/1904.03753, https://winephysicssong.com/wp-content/uploads/2021/09/BarnumHilgertSpectralPropertiesOfConvexBodiesPreprintVersion2.pdf, https://arxiv.org/abs/2306.00362

Time-resolved shadow tomography of open quantum systems

Presenting Author: Joseph Barreto, University of Southern California
Contributing Author(s): Arman Babakhani, Onkar Apte, Daniel Lidar

Shadow tomography is a powerful and resource-frugal characterization technique which permits the estimation of a set of expectation values of an arbitrary quantum state using a number of measurements that scales only logarithmically in the size of the observable set. Allowing the state in question to undergo general non-unitary evolution, we leverage shadow tomography to resolve the time-dependence of this observable set and thereby efficiently estimate parameters of the open system dynamics, and discuss various extensions of the method in the dynamical setting.

Two-Photon Splitting in Chiral Waveguide Quantum Electrodynamics

Presenting Author: Tiberius Berndsen, Miami University
Contributing Author(s): Imran M. Mirza

We study the probability of two photons propagating through two chiral waveguides coupled via quantum emitters splitting into different ports. This routing probability is analyzed in two regimes: a plane wave regime and a regime that includes the effects of bound states. Within the plane wave regime, we observe an alternating pattern in optimal splitting probability as a function of the frequencies of the photons for both the ideal (negligible environmental loss) case and the critical coupling case. In addition to this alternating pattern, a general method of calculating an arbitrary number of quantum emitters is presented. A fully analytic treatment of the splitting probability, including bound states, is realized for up to two quantum emitters. The final result of this fully analytic study shows that under the critical coupling regime, the photons experience a higher optimal splitting probability when compared to the plane wave regime. The possible application of this work is in quantum networking and communication across long distances.

Cold atomic ensemble with high optical density for spin squeezing experiments

Presenting Author: Sudhan Bhadade, University of New Mexico CQuIC
Contributing Author(s): Francisco Elohim Becerra

Atomic ensembles can be used for high precision metrology and sensing. The interaction of light with these atomic ensembles allows for the generation of correlated states of many atoms. These quantum correlated systems can be used for sensing beyond the standard quantum limit (SQL), which is the classical limit for uncorrelated systems. Spin squeezed states are a class of quantum correlated atomic states that provide a reduction of noise below the SQL along one spin component, at the expense of an increased noise along the conjugate component. We are currently working on the preparation of an atomic ensemble with large optical depth (OD) to enhance the atom-light interaction for the preparation of spin squeezing. We trap Cs atoms in a Magneto Optical Trap (MOT) and prepare the atoms in the pseudospin formed by the clock states in the Cs ground-state manifold, |6S_(1/2),F=3,m_f=0⟩ and |6S_(1/2),F=4,m_f=0⟩. We plan to use a polarization-spin entangling interaction based on birefringent response of atoms encoded in clock states [1]. The birefringence acquired by an optical probe from the interaction with the atoms, followed by polarimetry measurements, will allow for the preparation of spin squeezing based on quantum measurement backaction. References: 1. Chaudhury, S., Smith, G.A., Schulz, K., Jessen, P.S., 2006. Continuous nondemolition measurement of the cs clock transition pseudospin. Phys. Rev. Lett. 96, 043001

Gibbs state observable estimation from thermal pure states

Presenting Author: Eric Bobrow, Sandia National Laboratories
Contributing Author(s): Lucas Kocia Kovalsky, Andrew Baczewski

Gibbs states are vital to the study of material properties beyond the ground state. A variety of algorithms have been proposed for preparing the Gibbs state on a quantum computer, from variational to Monte Carlo approaches. Instead of preparing the Gibbs state directly, the thermal pure state approach involves generating pure state samples by imaginary time evolution of a random initial state to reproduce observable expectation values in the Gibbs state. Thermal pure states have been recently shown to admit efficient classical shadow estimation for many observables in part due to favorable sample requirements under assumption of extensivity at high temperatures. We extend the analysis of thermal pure states to find beneficial sample requirements over a range of both high and low temperatures. In both regimes, we show a suppression of sample requirements by an exponential in system size. We demonstrate these limits as well as the higher-sample-cost intermediate regime by numerically examining the variance of thermal pure states for different systems and observables. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Encoding oscillators into oscillators with Gottesman-Kitaev-Preskill codes

Presenting Author: Anthony Brady, University of Arizona
Contributing Author(s): Jing Wu, Quntao Zhuang

Bosonic encoding of quantum information into harmonic oscillators is a hardware efficient approach to battle noise. In this regard, oscillator-to-oscillator codes not only provide an additional opportunity in bosonic encoding, but also extend the applicability of error correction to continuous-variable states ubiquitous in quantum sensing and communication. In this work, we derive the optimal oscillator-to-oscillator codes among the general family of Gottesman-Kitaev-Preskill (GKP)-stablizer codes for homogeneous noise. We prove that an arbitrary GKP-stabilizer code can be reduced to a generalized GKP two-mode-squeezing (TMS) code. The optimal encoding to minimize the geometric mean error can be constructed from GKP-TMS codes with an optimized GKP lattice and TMS gains. For single-mode data and ancilla, this optimal code design problem can be efficiently solved, and we further provide numerical evidence that a hexagonal GKP lattice is optimal and strictly better than the previously adopted square lattice. For the multimode case, general GKP lattice optimization is challenging. In the two-mode data and ancilla case, we identify the D4 lattice---a 4-dimensional dense-packing lattice---to be superior to a product of lower dimensional lattices. As a by-product, the code reduction allows us to prove a universal no-threshold-theorem for arbitrary oscillators-to-oscillators codes based on Gaussian encoding, even when the ancilla are not GKP states.

Read this article online: https://arxiv.org/abs/2212.11970

Iterative Quantum Algorithms for Maximum Independent Set

Presenting Author: Lucas Brady, NASA - Ames Research Center
Contributing Author(s): Stuart Hadfield

Quantum algorithms have been widely studied in the context of combinatorial optimization problems. While this endeavor can often achieve quadratic speedups, often the analytic study of quantum optimization algorithms is outstripped by the rush to apply them to more problems. We study a specific class of quantum optimization algorithms, termed Iterative Quantum Algorithms, focusing on a modified form of the Recursive Quantum Approximate Optimization Algorithm (QAOA), applied in a novel way to Maximum Independent Set (MIS). We show that for p=1 circuit depth this algorithm performs the exact same operations and selections as the classical greedy algorithm for MIS. We then go on to modify the quantum algorithm in ways that can no longer be easily mimicked by classical algorithms but which still show the outsized power of the classical portions of hybrid quantum algorithms.

Entangling gates in neutral atoms via the spin-flip blockade

Presenting Author: Vikas Buchemmavari, University of New Mexico
Contributing Author(s): Sivaprasad Omanakuttan, Yuan-Yu Jau, Ivan Deutsch

The Rydberg dipole-blockade has emerged as the standard mechanism to induce entanglement between neutral atom qubits. In these protocols, laser fields that couple qubit states to Rydberg states are modulated to implement entangling gates. Here we present an alternative protocol to implement entangling gates via Rydberg dressing and a microwave-field-driven spin-flip blockade [Jau et al, Nature Physics 12, 71-74 (2016)]. We consider the specific example of qubits encoded in the clock states states of cesium. An auxiliary hyperfine state is optically dressed so that it acquires partial Rydberg character. It thus acts as a proxy Rydberg state, with a nonlinear light-shift that plays the role of blockade strength. A microwave-frequency field coupling a qubit state to this dressed auxiliary state can be modulated to implement entangling gates. Logic gate protocols designed for the optical regime can be imported to this microwave regime, for which experimental control methods are more robust. We show that unlike the strong dipole-blockade regime usually employed in Rydberg experiments, going to a moderate-spin-flip-blockade regime results in faster gates and smaller Rydberg decay. We study various regimes of operations that can yield high-fidelity two-qubit entangling gates and characterize their analytical behavior. In addition to the inherent robustness of microwave control, we can design these gates to be more robust to thermal fluctuations in atomic motion as well to laser amp

Read this article online: https://arxiv.org/abs/2307.16434

Square root law for quantum-secure covert communication over classical-quantum channels

Presenting Author: Michael Bullock, University of Arizona
Contributing Author(s): Azadeh Sheikholeslami, Mehrdad Tahmasbi, Robert C. Macdonald, Saikat Guha, Boulat Bash

We investigate covert communication over general memoryless classical-quantum channels with fixed finite-size input alphabets. In contrast to standard secure communication methods which protect the information content being transmitted, covert communication guarantees that the transmission itself is undetectable. We show that the square root law (SRL) governs covert communication on this channel when product of $n$ input states is used: $L_{\rm SRL}\sqrt{n}+o(\sqrt{n})$ covert bits (but no more) can be reliably transmitted in $n$ classical-quantum channel uses, where $L_{\rm SRL}>0$ is a channel-dependent constant that we call covert capacity. We also show that ensuring covertness requires $J_{\rm SRL}\sqrt{n}+o(\sqrt{n})$ bits secret shared by the communicating parties prior to transmission, where $J_{\rm SRL}\geq0$ is a channel-dependent constant. We assume a quantum-powerful adversary that can perform an arbitrary joint (entangling) measurement on all $n$ channel uses. We determine the single-letter expressions for $L_{\rm SRL}$ and $J_{\rm SRL}$, and establish conditions when $J_{\rm SRL}=0$ (i.e., no pre-shared secret is needed). Our choice of the general memoryless classical-quantum channel model allows one to determine the ultimate limits of many practical channels. We evaluate scenarios where covert communication is not governed by the SRL. Finally, we relate our work to our prior work on covert communication over the continuous-variable bosonic channels.

Read this article online: https://arxiv.org/abs/1601.06826

Experimental demonstration of mirror circuit fidelity estimation

Presenting Author: Fernando Calderon-Vargas, Sandia National Laboratories
Contributing Author(s): Stefan Seritan, Timothy Proctor, Mohan Sarovar

Ensuring the accurate execution of a quantum circuit is crucial for dependable quantum computing. Mirror Circuit Fidelity Estimation (MCFE) is an efficient and scalable technique for verifying the low-error execution of a quantum circuit. In this work, we demonstrate the efficacy of MCFE in estimating the fidelity with which real quantum hardware implements certain quantum circuits. We also compare MCFE's performance against a similar technique, Direct Fidelity Estimation, via experiments and simulations.

Local Measurement Strategies for Multipartite Entanglement Quantification

Presenting Author: Luke Coffman, University of Colorado JILA
Contributing Author(s): Jacob Beckey, Graeme Smith

Despite being a fundamentally non-local phenomenon, quantum entanglement can be estimated using only local measurements. Methods of estimating multipartite entanglement using local random measurements (LRMs) have received a lot of attention in recent years due to their apparently minimal experimental requirements. The primary drawbacks of these techniques are the large number of measurement settings experimentalists must implement to estimate them and the inability to apply to standard statistical methods to prove convergence of the techniques. In this work, we address these limitations by showing that many well-known multipartite entanglement measures can be estimated in a de-randomized fashion by utilizing local POVMs that form projective 2-designs. As a particular example, we focus on SIC-POVMs, which allow for the estimation of many of these measures and 1) use only one experimental setting 2) converge faster than LRMs, and 3) allow for analytical performance guarantees via standard statistical tools. In addition to our main results, which have implications for both theory and experiment, we discuss several open questions, and progress towards their resolution, regarding upper and lower bounds on the sample complexity of multipartite entanglement quantification under various assumptions on the allowed measurement operators.

Verifying quantum phase estimation (QPE) using Prove-It

Presenting Author: Warren Craft, University of New Mexico
Contributing Author(s): Wayne M. Witzel, Warren D. Craft, Deepak Kapur

We use Prove-It [arxiv:2012.10987, pyproveit.org], an interactive proof assistant for organizing and verifying mathematical knowledge, to formally prove the success probability guarantee of the quantum phase estimation (QPE) algorithm, closely following Nielsen & Chuang’s (2000/2010) textbook presentation using similar notation [arXiv:2304.02183, accepted to PRA]. The interactive proof construction flows naturally from an informal proof by automatically deducing simple steps (given prior development of dependent theory packages as well as some training in using Prove-It). Our formal proof compares favorably with other approaches (using Coq and QBricks). We do rely upon well-established mathematical theorems that are not themselves required to be proven in the system, which demonstrates a useful feature of Prove-It that enables proofs based upon conjecture. Prove-It also provides hyper-links to explore the dependency structure of the generated human-readable proofs while maintaining a list of axioms and conjectures used (directly and indirectly) in each proof. This work was supported by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research under the Quantum Computing Applications Team (QCAT) and Quantum Systems Accelerator (QSA) programs, and the Laboratory Directed Research and Development program at Sandia National Laboratories. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Read this article online: https://arxiv.org/pdf/2012.10987.pdf, https://arxiv.org/pdf/2304.02183.pdf

Validating models of quantum computers in practice

Presenting Author: Megan Dahlhauser, Sandia National Laboratories
Contributing Author(s): Robin Blume-Kohout, Timothy Proctor, Kevin Young

Modeling low-level components of quantum computers is critical to understand quantum systems, identify errors, and pursue opportunities for engineering improvements. Tantamount to these tasks is the expectation that an effective and useful model should provide accurate predictions of circuit outcomes. Successfully predicting circuit outcomes validates our model and understanding of our quantum system, whereas failing to predict circuit outcomes indicates either a poor, inappropriate, or obsolete characterization. While evaluating the performance of a model is vital, it is not a binary metric and determining when a model is performing well or at least satisfactorily can be difficult. We present a generalized process of model validation. We show how to determine and report performance on circuit prediction tasks in practice using statistical tests and simulation. We demonstrate this process in experiment using gate set tomography and randomized benchmarking and evaluate performance on several distinct circuit prediction tasks. We find that our validation process is an effective tool in identifying model shortcomings and upgrading methodologies to create more accurate characterizations of quantum devices.

Quantum Dynamics is When Nothing Happens

Presenting Author: John DeBrota, University of New Mexico CQuIC
Contributing Author(s): Christopher Fuchs, Ruediger Schack

QBism interprets the quantum formalism as a decision theory that helps an agent navigate the consequences of their actions. Thus, the aspects of the formalism directly concerning measurement are fundamental. Less obvious is how a QBist should think about dynamics, as it concerns state change in the absence of measurement. In a very real sense, for a QBist, nothing happens to a system between their measurement actions. However, nothing prohibits a QBist from changing their quantum state in the absence of measurement; one option is to adopt a standard quantum channel, but this begs the question of why they would or should choose to do so. We find that the constraint of completely positive dynamics, both unitary and otherwise, follows from a simple judgment concerning actions an agent could take. In this way, QBism identifies dynamics as a consequence of action and belief, answering the begged question and advancing the position that it is measurement, not dynamics, that should be considered physically fundamental. Although a foundational result, this perspective shift has the potential to percolate into applied circles, especially as it suggests a fundamentally different view on decoherence. In particular, the standard metaphor of an environment measuring or monitoring a system to provide a mechanism for the phenomenon of decoherence has no force in QBism because it relies on ubiquitous unitary evolution and an improper treatment of measurement.

Quantum computer-enabled receivers for optical communication

Presenting Author: Spencer Dimitroff, University of New Mexico CQuIC
Contributing Author(s): John Crossman, Lukasz Cincio, Mohan Sarovar

In this work we demonstrate improved receivers for coherent communication that are enabled by quantum transduction of optical states to qubits and subsequent quantum computation. In particular, we aim to discriminate binary phase shift keying (BPSK) codewords, made up of a sequence of optical pulses where each pulse encodes one of two coherent states |+-alpha>. It is known that such states are optimally discriminated via joint measurements, but how to perform such measurements in an optical setting has long been a challenge in the field. Here, we consider transducing the optical BPSK codewords into qubit states to perform joint measurements that can discriminate the codewords better than any local, adaptive measurement in terms of the error probability in identifying the state (Helstrom bound). We analyze theoretically and numerically how to optimally transduce the distinguishing information contained in the BPSK states into qubits, i.e., the phase. We then implement the computational component of the joint measurement on NISQ devices (the qubits are initialized to the states that would be obtained if the transduction step was performed) and confirm that our method beats the Helstrom bound in the low mean photon number limit. This demonstration shows the utility of transduction for performing joint quantum measurements, with the potential to extend this to other discrimination and sensing tasks.

Read this article online: https://arxiv.org/abs/2309.15914

Using quantum Monte Carlo to study quantum phase transitions

Presenting Author: Nicholas Ezzell, University of Southern California
Contributing Author(s): Itay Hen, Lev Barash

We derive a scheme to estimate fidelity susceptibility in permutation matrix representation quantum Monte Carlo (PMR-QMC) and test it numerically. Fidelity susceptibility is a universal indicator of quantum phase transitions, and hence, our work allows for the study of quantum phases even without knowledge of an underlying order parameter. We remark that previous works have derived similar schemes for imaginary time and stochastic series expansion QMC (Wang et. al. PhysRevX.5.031007). However, PMR-QMC is neither an imaginary time nor stochastic series expansion method, and hence, our scheme does not follow directly from previous work. Instead, PMR-QMC is a sampling scheme over permutations with weights given by divided differences of the Boltzmann exponential. The PMR-QMC has many benefits over imaginary time and stochastic series expansion methods (Lalit et. al. J. Stat. Mech. (2020) 073105]), so our scheme inherits all these benefits over previous approaches.

Qubit resource states for measurement-based quantum gate teleportation beyond symmetry protected topological order

Presenting Author: David Feder, University of Calgary
Contributing Author(s): Zhuohao Liu, Emma C. Johnson

All known resource states for measurement-based quantum computation possess symmetry protected topological order, but it is not currently known if this is a necessary condition. This work investigates the ability of one-dimensional qubit states to perform universal measurement-based quantum gate teleportation (MBQT) in correlation space, within the framework of matrix product states with bond dimension four. A family of resource states is identified that possesses no symmetry but nevertheless permits deterministic MBQT of both single-qubit and two-qubit finite gates. We have also found a family of states that possesses a non-unitary Z2 x Z2 symmetry but surprisingly a generically non-degenerate entanglement spectrum.

Hardness results for decoding the surface code with Pauli noise

Presenting Author: Alex Fischer, University of New Mexico
Contributing Author(s): Akimasa Miyake

Biased noise decoders for quantum codes are decoding algorithms that find an error correction operation using not only the syndromes, but also a prior distribution of errors. Biased noise decoders are a useful tool for quantum computing because real quantum computers will be subject to complicated, qubit-dependent noise, rather than simple noise such as independent and identically distributed depolarizing or dephasing noise. While there are biased noise decoders for the surface code that are correct in the average case (i.e., for most sets of syndromes and for most error distributions), there are no known decoders that are correct in the worst case (i.e., for all sets of syndromes and for all error distributions). There are only a few special cases of noise models where any decoders are guaranteed to find the Maximum Probability Error (MPE), or find a Maximum Likelihood (ML) correction operation. We explain why this is the case by proving worst case computational hardness results for surface code decoding where the input to the problem is both the syndromes and a probability distribution of Pauli errors for each qubit. In this setting, we show that MPE decoding is NP-hard, and ML decoding is #P-hard. We reduce directly from SAT for MPE decoding, and from #SAT for ML decoding, by showing how to construct a distribution of Pauli errors that encodes the satisfiability properties of a boolean formula. We also give hardness of approximation results for MPE and ML decoding.

Describing Local Decoherence of Spin Ensembles using a Fokker-Planck Equation in a Bosonic Mode

Presenting Author: Andrew Forbes, University of New Mexico CQuIC
Contributing Author(s): Philip Daniel Blocher Ivan H. Deutsch

Many protocols seek to prepare nonclassical states of spin ensembles through an entangling atom-light interface. When the atoms are uniformly coupled to a common mode of the field, such systems are simply described by the collective spin in the symmetric subspace. However, local decoherence due to diffuse photon scattering breaks this symmetry as a scattered photon carries information into the environment about which atom spontaneous emitted. Such local decoherence is typically inefficient to describe for large ensembles of atoms. In this work we develop a formalism that represents local decoherence for a large number of spins using a Wigner function representation of a bosonic mode by making use of the Holstein-Primakoff approximation. The dynamics of the bosonic mode can, in many cases, be described using a Fokker-Planck equation for a damped harmonic oscillator, and we analyze the types of local decoherence which can be efficiently described in this way. We use this formalism to study the combined effect of Hamiltonian evolution, local and collective decoherence, and measurement backaction for preparing nonclassical spin states for applications in quantum metrology and continuous variable quantum computing.

Quantum Sensing for the Detection of Ionizing Radiation

Presenting Author: Matthew Freeman, Sandia National Laboratories
Contributing Author(s): Sueli Skinner Ramos, Rupert Lewis, Stephen Carr

Quantum sensing describes the use of a quantum system, quantum properties, or quantum phenomena to perform a measurement of a physical quantity. Quantum sensors capitalize on the central weakness of quantum systems, their strong sensitivity to external disturbances. Here we describe an approach to utilize a superconducting qubit, not for quantum information processing, but as a quantum sensor for the detection of ionizing radiation. Whereas ionizing radiation presents a potentially serious problem for quantum error correction due to spatially and temporally correlated errors, it represents an opportunity for quantum sensing. The proposed principle of detection is based on using quantum coherence as a sensitive probe for ionizing radiation. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Markovian and non-Markovian master equations versus an exactly solvable model of a qubit in a cavity

Presenting Author: Juan Garcia Nila, University of Southern California
Contributing Author(s): Zihan Xia, Juan Garcia-Nila, Keming He, Dawei Zhong

We investigate the dynamics of a qubit in a leaky cavity interacting with a bosonic bath charac- terized by three different spectral densities: an impulse spectral density, an Ohmic spectral density, and a proportional spectral density with a sharp cutoff. Specifically, we focus on the behavior of the first excitation state and explore its non-Markovian features, such as oscillatory amplitudes. We derived solutions from various approximation methods to investigate their ability to approximate the exact solution and discuss their optimal performance with respect to relevant parameters. We consider the time-convolutionless (TCL) master equation up to the second order (TCL2) and the fourth order (TCL4), the coarse-graining Lindblad equation (CG-LE), and the rotating-wave ap- proximation Lindblad equation (RWA-LE). Notably, we compare two variants of CG-LE: one based on a completely positive (CP) map that derives the semigroup master equation from first principles and another employing the Born approximation along with bath correlation functions. We obtain the optimal coarse-graining time by optimizing a metric that quantifies the deviation between the approximated and exact solutions. We demonstrate that CG-LE outperforms the Markov limit derived from RWA-LE for the cases of low coupling or high cavity frequency where the Markovian approximation is valid. In the presence of non-Markovian effects characterized by highly oscilla- tory and non-decaying behavior, the TCL approximati

Robust Estimators of Multiparticle Indistinguishability

Presenting Author: Shawn Geller, National Institute of Standards and Technology, Boulder
Contributing Author(s): Aaron Young, Scott Glancy, Emanuel Knill

The dynamics of noninteracting bosons has attracted interest due to the BosonSampling problem and its computational difficulty. A challenge in experimental systems implementing these dynamics is verifying that the output distributions are close to the desired ones. Motivated by a cold atom optical lattice experiment, we formalize the notion of multiparticle indistinguishability, making use of tools from representation theory. We then construct novel estimators of it, aiming for small variance and robustness to small miscalibrations of the unitary that governs the dynamics.

How to believe Quantum Bayesianism and Many Worlds

Presenting Author: Scott Glancy, National Institute of Standards and Technology, Boulder

The Bayesianism interpretation of quantum mechanics understands quantum states subjectively, as representations of an agent's knowledge. The Many Worlds interpretation understands quantum states objectively, as descriptions of the actual configuration of physical systems (or the entire universe). Researchers often debate which of these two (or many other) interpretations is correct. However, I argue that these debates are often misguided because Bayesianism and Many Worlds can be compatible with one another. The beliefs that quantum states describe one's knowledge and that quantum states can correspond to the actual configurations of systems do not contradict one another.

On the asymptotics of two-stage quantum estimation

Presenting Author: Zihao Gong, University of Arizona
Contributing Author(s): Boulat Bash

Suppose we wish to estimate an unknown parameter embedded in a quantum state. The ultimate quantum limit of the mean squared error, given $n$ identical copies of this state, is the quantum Cramer-Rao bound (QCRB). It can be achieved asymptotically, as $n$ increases, using a positive operator-valued measurement (POVM) constructed from the eigenvectors of the symmetric logarithm derivative operator associated with the desired parameter, and applying a classical estimator to the POVN outcomes. However, this optimal POVM can depend on the true value of the parameter of interest. The paradox of needing to know the parameter to estimate it is addressed by a two-stage approach proposed by Hayashi and Matsumoto (see https://arxiv.org/abs/quant-ph/0308150). The first stage yields a rough pre-estimate by measuring an asymptotically small fraction of state copies (say, $n^\delta$ states out of $n$, where $0<\delta<1/2$) via a sub-optimal measurement that is independent of the parameter. This pre-estimate is used to construct the optimal POVM to refine the estimate. Unfortunately, the existing analysis severely limits applications of this method by imposing conditions that severely restrict the class of classical estimators applied to the results of both measurements. We show that relaxing these conditions only slightly weakens the asymptotic properties of the two-stage method. We apply results to transmittance sensing and show that we can achieve QCRB asymptotically.

Automated Model Selection with First Order Gauge Invariant Quantities

Presenting Author: Juan Gonzalez De Mendoza, Sandia National Laboratories
Contributing Author(s): Corey Ostrove, Stefan Seritan, Robin Blume-Kohout

Tomography is one of the most powerful tools available for the characterization of quantum computers. However, this power comes with great cost, in part because the space of all possible errors that a quantum processor may experience is incredibly vast. In practice we observe that in experiments only a small fraction of possible errors are relevant for any given device, making the rest unnecessary for the description of that quantum computer’s operations. The inclusion of such unneeded errors in our models makes the task of performing and interpreting tomography results harder. Automated Model Selection is an algorithm that reduces this problem by finding a model with the least number of parameters that still can effectively describe the data collected from a device. An obstacle for this algorithm, is that due to a property called gauge freedom, many different models give the same physical predictions and thus are equally effective in representing empirical data. As a consequence, traversing the space of models with different a set of parameters is not trivial. In this project, we implement the automated model selection algorithm using first-order gauge invariant (FOGI) parameters, which eliminates the gauge freedom problem and simplifies the landscape. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Bifurcation diagrams for tunneling and entanglement dynamics in the kicked top

Presenting Author: Alex Gran, Carleton College
Contributing Author(s): Ryan Quinn, Noah Pinkney, Alex Kiral, Sudheesh Srivastava, Arjendu Pattanayak

We study the Quantum Kicked Top (QKT) as a function of nonlinearity K. Using adiabatic evolution of the parameter, we uncover the phenomenon even when the original spectrum is recovered the states have been shuffled(also termed exotic quantum holonomy). Further, we use measures of spectral bunching and averaged inverse participation ratios across phase-space to identify K values that yield unusual many-body quantum dynamics. In particular, for the 4 qubit QKT we find unusual dynamics for both linear entropy and tunneling at non-obvious K values 4π/3, 2π, ~2.76π, 4π corresponding to sharply-defined local minima in K for spectral bunching. We also see differing K-periodicities for the 4 qubit QKT with the period of 4π for the linear entropy, 8π for measures of tunneling and spectral bunching, and 16π for the adiabatically unraveled spectrum itself. Finally, we show that with increasing number of qubits n, the density of these local minima increases along with the K period, nonlinearly accelerating the number of K values with these degeneracies. We discuss the N → infinity limit.

Characterize pulses with qubits

Presenting Author: Jonathan Gross, Google
Contributing Author(s): Elie Genois, Zhang Jiang

Precise characterization of control pulses is essential to building reliable quantum computers. We use qubits as probes to measure distortions in microwave pulses in the Fourier domain. This allows us to identify and characterize several mechanisms for pulse reflection without using a prescribed model. We also show that the distorted pulses can be deconvolved with one percent precision. Our method can also be used to characterize timing differences between different pulses with 10ps precision. Finally, we show that the pulse shapes for crosstalk are significantly distorted and delayed.

Partial characterization of quantum gates using character phase estimation

Presenting Author: Andrew Guo, Sandia National Laboratories
Contributing Author(s): Jordan Hines, Winton Brown, Timothy Proctor, Kevin Young

In experimental quantum systems, an accurate understanding of underlying noise processes is pivotal for implementing targeted calibration strategies. One common approach to characterizing errors is phase estimation. However, existing schemes for phase estimation are not robust to state prep and measurement errors and limited to characterizing specific types of noise, e.g. overrotation error. To overcome these challenges, we present character phase estimation (CPE), a technique that harnesses tools from representation theory to robustly and efficiently estimate parameters of general Markovian noise processes. CPE uses linear combinations of data to isolate individual eigenvalues of a noisy gate, which can then be learned to high precision with a simple exponential fitting routine. We validate our method’s efficacy through simulations with error models encompassing both coherent and stochastic noise. We also discuss how to use CPE to perform gate set tomography without computationally intensive maximum-likelihood estimation, which would offer a pathway towards robust and scalable partial tomography of quantum gate sets.

Hong-Ou-Mandel Effect in Waveguide QED

Presenting Author: Dingyu Guo, Miami University

We study the Hong-Ou-Mandel effect (a two-photon interference phenomenon that can be used to characterize indistinguishability between photons) in waveguide QED. In particular, we focus on the question that how strong light-matter interactions can impact the spatial and temporal properties of two photons generated by two initially excited atoms coupled with a bi-directional waveguide. This work may find applications in the area of long-distance quantum communications.

Quantum-Enhanced Parameter Estimation With a Single Qudit

Presenting Author: Pragati Gupta, University of Calgary

We show that a coherent high-dimensional quantum system is enough to go beyond the standard quantum limit and reach the Heisenberg limit in quantum metrology, without using entanglement. To illustrate this, we propose an experimentally feasible parameter estimation scheme with a nuclear spin qudit prepared in a Schrödinger spin cat state, where the relative phase of the superposition state has an enhanced sensitivity to magnetic fields. We analytically calculate the achieved precision for parameter estimation and devise a quantum-control protocol that exploits this amplified phase for parameter encoding. Our scheme employs a generalized interferometry protocol to read out the relative phase and can perform better than entangled probes, even under Markovian noise.

Benchmarking multi-qubit gates

Presenting Author: Bharath Hebbe Madhusudhana, Los Alamos National Laboratory

Multi-qubit gates are unitary operators which act non-trivially on >2, if not all qubits. They can be implemented using a circuit or using continuous many-body Hamiltonian. Benchmarking such gates is met with two challenges: the unitaries corresponding to multi-qubit gates are exponentially large, and therefore the corresponding process tomography is not scalable. Moreover, this unitary in general cannot be computed classically, leaving no reference to benchmark experimental data. Here we develop a set of benchmarking techniques for such gates based on the mathematical properties of unitaries that are sensitive to errors. We classify errors in quantum operations into three categories. Errors due to coupling to a thermal bath results in the many-qubit gate being a Completely Postive (CP)-divisible, i.e., Markovian map, deviating from a unitary. The reduced Choi matrix of a unitary multi-qubit gate is doubly stochastic and this property is violated in the presence of markovian errors. We construct a benchmark using double-stochasticity violation and show that it is sensitive to coupling to any thermal bath at a finite temperature. Further, errors due to random, shot-to-shot fluctuations result in a non-markovian, i.e., CP-indivisible quantum process. A third category of errors comes from systematics in the implementation of a multi-qubit gate. We refer to this as unitary errors. We show that conserved quantities can be used to benchmark these errors.

Read this article online: https://arxiv.org/abs/2210.04330, https://arxiv.org/abs/2301.07109

Using symmetries to estimate experimental errors in the simulation of many-body Hamiltonians

Presenting Author: Bharath Hebbe Madhusudhana, Los Alamos National Laboratory
Contributing Author(s): Aditya Prakash

Quantum simulation of many-body Hamiltonians in, for example, trapped ultracold atoms constitutes several major advances in quantum control of large systems. An important next step is to characterize the accuracy of this quantum control in terms of a figure-of-merit. Based on the recent work on benchmarking multi-qubit quantum control [1, 2], we develop an experimental protocol to characterize the error in many-body Hamiltonians in trapped ultracold atom experiments. We focus on an Ising-type Hamiltonian, which is quintessential to Rydberg atoms trapped in a tweezer array. Two of the most experimentally probable errors stem from systematic deviations and random fluctuations of the Hamiltonian parameters.These errors do not share the same symmetries as the ideal Hamiltonian and therefore, they can be detected by measuring an observable that would be ideally conserved. The time variation of such observables is solely caused due to errors. We numerically study the sensitivity of the expectation value of the ideal Hamiltonian itself to errors modeled by a random fluctuation of the Hamiltonian parameters. We show that the expectation of the Hamiltonian, which is ideally time-independent, will show a linear time dependence due to systematic errors and a quadratic dependence due to random errors. Thus, we can independently quantify these two errors, by measuring the expectation value of the Hamiltonian. Finally, we discuss plausible experimental implementations.

Read this article online: https://arxiv.org/abs/2301.07109

A protocol based on reduced-process tomography for classical verification of few-qubit QEC circuits

Presenting Author: Bharath Hebbe Madhusudhana, Los Alamos National Laboratory
Contributing Author(s): Andrea Rodriguez-Blanco

The significance of Quantum Error Correction (QEC) circuits in establishing fault-tolerant quantum computation is paramount. Ensuring the robustness and accuracy of implementation these circuits becomes imperative. Therefore, benchmarking and validation of such QEC circuits is important. Here, we apply recently developed benchmarking techniques for multiqubit gates [1] to develop protocols for the classical verification of few-qubit QEC circuits. We consider the reduced Choi matrices corresponding to one and two-qubit systems for QEC circuits and show that they are sensitive to most, but not all locally generated errors. This includes single-qubit unitary errors, local depolarization, and decoherence. We develop experimental protocols specific to two and three-qubit circuits and discuss the possible implementations in various experimental platforms.

Read this article online: https://arxiv.org/abs/2301.07109

Scalable Full-Stack Benchmarks for Quantum Computers

Presenting Author: Jordan Hines, University of California Berkeley
Contributing Author(s): Stefan Seritan, Aidan Wilber-Gauthier, Timothy Proctor

Quantum processors are rapidly approaching the scale where performing interesting computational tasks may be possible, creating a need for benchmarks that assess performance of the full quantum hardware and software stack on algorithms of interest. However, it is computationally expensive to compute the outcomes of benchmarks based on tasks that cannot be done efficiently on classical computers, making it difficult to measure a processor's success rate on useful computational tasks. We introduce a general technique to create precise, computationally efficient full-stack benchmarks from any set of invertible circuits. Our benchmarks are built using mirror circuit fidelity estimation, an efficient fidelity estimation routine. They are scalable, because they do not require any expensive classical computations. We use our method to construct a computationally efficient version of the quantum volume benchmark, and we introduce a randomized benchmark that relaxes the all-to-all connectivity in quantum volume circuits. Furthermore, we introduce an algorithm-based benchmark that uses Hamiltonian simulation circuits, and we show how our method can be adapted to design benchmarks that measure progress towards running circuits beyond the capabilities of current processors. We perform our benchmarks on IBM Q devices and in simulations and compare our results to those of existing benchmarks. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Exact Average of Entanglement capacity of fermionic Gaussian states

Presenting Author: Youyi Huang, Texas Tech University
Contributing Author(s): Lu Wei

We study the capacity of entanglement as an alternative to entanglement entropies in estimating the degree of entanglement of quantum bipartite systems over fermionic Gaussian states. In particular, we derive the exact and asymptotic formulas of average capacity of two different cases -- with and without particle number constraints. For the later case, the obtained formulas generalize some partial results of average capacity in the literature. The key ingredient in deriving the results is a set of new tools for simplifying finite summations developed very recently in the study of entanglement entropy of fermionic Gaussian states.

Open Rabi system dynamics near avoided crossing quasi-degeneracies

Presenting Author: Yusuf Ismail, Carleton College
Contributing Author(s): Peter Rose, Arjendu Pattanayak

We explore the Rabi model by examining a single qubit weakly coupled to a harmonic oscillator and both coupled to separate thermal baths. Typically, the system behaves that can be understood in terms of an uncoupled spin and harmonic mode vector spaces. However, at narrow ranges of parameter values (coupling and decay rates of spin and harmonic modes) the system displays unusual and widely varied dynamics in their approach to equilibrium. These parameters correspond to regions of spectral avoided crossings for the closed quantum system. Thus these unusual behaviors can be understood as arising from the enhanced mixing between harmonic modes and spin modes in these situations and how this changes the state relaxation rates. We use entanglement entropy and the negativity of the Wigner function of reduced density matrix as quantum dynamical measures of the approach to equilibrium. We find that the negativity provides a more consistent and intuitive measure of the non-classical aspects of the behavior of the overall system than the entanglement entropy.

Levitated platforms for quantum sensing and computation

Presenting Author: Krishna Jadeja, Okinawa Institute of Science and Technology Graduate University, Japan
Contributing Author(s): James E. Downes, Glemarie C. Hermosa, Alexander H. Hodges, Daehee Kim, Ruvi Lecamwasam, Camila P. C. Padilla, Priscilla Romagnoli, Shilu Tian, Jason Twamley

Levitation has many potential applications in quantum science and technology as it can severely suppress coupling to the environment. These applications range from exploring fundamental questions relating quantum to gravity, through to novel sensors e.g., sensing exotic forces, dark matter, inertial forces and magnetometry. Mechanical modes have been proposed as a method to entangle many spins in solids (NVs). Unlike other levitation techniques, diamagnetic levitation requires no driving and should exhibit very low noise. Macroscopic resonators made of highly oriented pyrolytic graphite (HOPG) are easily levitated using magnet arrays. However, the high conductivity of graphite results in strong eddy current motional damping and this severely limits their applications. We demonstrate experimentally that the q-factor for HOPG resonators can be increased by carving slots into the material to interrupt the flow of eddy currents. We find we can increase the q-factor by two orders of magnitude [App. Phys. Lett. 122, 094192 (2023)]. As a further improvement we have developed a new electrically insulating, diamagnetic material by chemical means, achieving q-factors above 100000 in high vacuum. The exploration of fundamental quantum physics using a levitated mechanical system necessitates the preparation of the system close to its ground state of motion. We demonstrate feedback cooling of the centre of mass motion of the magnetically trapped macroscopic plate from room temperature.

Boson-Assisted Quantum Error-Correcting Codes

Presenting Author: Omid Khosravani, Other
Contributing Author(s): Or Katz, Kenneth R. Brown

To scale up quantum computations it is necessary to detect and correct errors that occur during the computation. Quantum error-correcting codes (QECC) were originally designed to encode information, and to detect and correct errors on two-level quantum systems, which were later extended to bosonic systems with the introduction of Gottesman-Kitaev-Preskill (GKP) code [1-2]. However, native qubit and bosonic systems are generally subject to noise models of different nature. Here, we present the first hybrid QECC that utilizes both spin and bosonic degrees of freedom to encode information, and to detect and correct errors, and discuss a fault-tolerant implementation of this code with trapped-ion quantum processors. [1] R. Calderbank and P. Shor (1996). "Good quantum error-correcting codes exist". Physical Review A. 54 (2): 1098-1105; P. W. Shor, "Fault-tolerant quantum computation," Proceedings of 37th Conference on Foundations of Computer Science, Burlington, VT, USA, 1996, pp. 56-65. [2] D. Gottesman, A. Kitaev, and J. Preskill, “Encoding a qubit in an oscillator”, Physical Review A 64, (2001).

Read this article online: TBA

Hamiltonian Quantum Generative Adversarial Networks

Presenting Author: Leeseok Kim, University of New Mexico
Contributing Author(s): Seth Lloyd, Milad Marvian

We propose Hamiltonian Quantum Generative Adversarial Networks (HQuGANs), to learn to generate unknown input quantum states using two competing quantum optimal controls. The game-theoretic framework of the algorithm is inspired by the success of classical generative adversarial networks in learning high-dimensional distributions. The quantum optimal control approach not only makes the algorithm naturally adaptable to the experimental constraints of near-term hardware, but also has the potential to provide a better convergence due to overparameterization compared to the circuit model implementations. We numerically demonstrate the capabilities of the proposed framework to learn various highly entangled many-body quantum states, using simple two-body Hamiltonians and under experimentally relevant constraints such as low-bandwidth controls. We discuss several applications of the framework, including learning unknown unitaries and encoding in decoherence-free subspaces.

Read this article online: https://arxiv.org/abs/2211.02584

Measurement back-action effects on density matrix dynamics

Presenting Author: Yuelin Kuang, Carleton College
Contributing Author(s): Maya Khesin, Yusuf Ismail (Carleton College), Sacha Greenfield (USC and Chapman University), Justin Dressel (Chapman University), and Arjendu K. Pattanayak (Carleton College)

Weak measurement affects quantum dynamics through backaction. Recent work using the nonlinear double well Duffing oscillator shows that a measurement choice (specifically the phase setting ϕ for a laser used for measurement in a quantum optics implementation) changes quantum state trajectories, including its energy absorption and dissipation. We use a semiclassical analysis to study how averages of dynamical properties such as Poincare sections and energy expectation values behave with increasing numbers of stochastic realizations to understand the density matrix behavior, including the recovery of ϕ-independent results appropriate for the full density matrix. We discuss simulation and analysis for a fluxonium superconducting circuit implementation.

Optimizing CV quantum network and transduction protocols for optically connected microwave systems

Presenting Author: Akira Kyle, University of Colorado
Contributing Author(s): Curtis L. Rau, Alex Kwiatkowski, John D. Teufel, Konrad W. Lehnert, Tasshi Dennis, Josh Combes

Achieving quantum operation of microwave-optical transduction is a challenging yet necessary goal for creating optical networks of superconducting qubits. While there are many possible networking schemes that can connect two microwave nodes in the ideal limit, it's not clear which networking schemes may be the most robust to imperfections. The many experimental sources of noise (e.g. nonzero thermal baths, losses, and bandwidth mismatches) result in many networking schemes being infeasible as they remain separable between the two nodes for realistic values of experimental sources of imperfection. Furthermore, given that Gaussian operations are not universal, non-Gaussian resources will be necessary in the network to overcome noise in transduction and transmission. However, it's not clear what experimentally realistic non-Gaussian resources should be used, and where to place them in the network. Thus the ability to quickly simulate various proposed network schemes will be essential to guiding ongoing experimental efforts. I will present progress towards simulating various network schemes along with evaluating their performance based on operational metrics which can then be used to further optimize them.

Two-Way Quantum Time Transfer: A Method for Daytime Space-Earth Links

Presenting Author: Randy Lafler, Airforce Research Lab
Contributing Author(s): Mark Eickhoff, Scott Newy, Yamil Nieves Gonzales, Kurt Stoltenberg, Frank Camacho, Mark Harris, Denis Oesch, R. Nicholas Lanning

High-precision remote clock synchronization is crucial for many classical and quantum network applications. Current state-of-the-art remote clock synchronization techniques achieve femtosecond- scale clock stability utilizing frequency combs, which are supplementary to quantum-networking hardware. Demonstrating an alternative, we synchronize two remote clocks across our freespace testbed using a method called two-way quantum time transfer (QTT). In one second we reach picosecond-scale timing precision under very lossy and noisy channel conditions representative of daytime space-Earth links with commercial off-the-shelf quantum-photon sources and detection equipment. This work demonstrates how QTT is potentially relevant for daytime space-Earth quantum networking and/or providing high-precision secure timing in GPS-denied environments.

Read this article online: https://arxiv.org/abs/2307.07371

Active steering into quantum stabilizer codespace

Presenting Author: Anirudh Lanka, University of Southern California
Contributing Author(s): Prithviraj Prabhu, Todd Brun

The quantum error correction protocol has been a practical problem in quantum computation, especially to measure high-weight stabilizers and decode the error syndrome to find recovery operators. We propose a technique to actively maintain a quantum stabilizer codestate in the codespace even under the influence of decoherence. Our protocol uses continuous measurements of operators from the stabilizer algebra to perform Hamiltonian corrections. The measurement operators and the correction strengths are provided by a reinforcement learning agent. We process the measurement data by first applying an exponential averaging filter and then stacking the previous measurement outcomes before sending them to a reinforcement learning agent. The agent then provides correction strengths and the subsequent measurement operators. We demonstrate that this protocol can evolve any unknown quantum state into a stabilizer code state, and also maintain it within the codespace. This technique is particularly useful since it is scalable to higher dimensional quantum stabilizer codes.

Feedback-based quantum algorithms for ground state preparation

Presenting Author: James Larsen, Sandia National Laboratories
Contributing Author(s): Matthew D Grace, Andrew D Baczewski, Alicia B Magann

The ground state properties of quantum many-body systems are a subject of interest across chemistry, materials science, and physics. Thus, algorithms for finding ground states can have broad impacts. To this end, variational quantum algorithms are one class of ground state algorithm that has received significant attention in recent years. These algorithms utilize a hybrid quantum-classical computing framework to prepare ground states on quantum computers. However, this requires solving a classical optimization problem that can become prohibitively expensive in high dimensions. Here, we develop formulations of feedback-based quantum algorithms for ground state preparation that can be used to address this challenge for two broad classes of Hamiltonians: Fermi-Hubbard Hamiltonians, and molecular Hamiltonians represented in second-quantization. Feedback-based quantum algorithms are optimization-free; in place of classical optimization, quantum circuit parameters are set according to a deterministic feedback law derived from quantum Lyapunov control principles. This feedback law guarantees a monotonic improvement in solution quality with respect to the depth of the quantum circuit. A variety of numerical illustrations will be presented that analyze the convergence and robustness of feedback-based quantum algorithms for these problem classes. Sandia National Labs is managed and operated by NTESS under DOE NNSA contract DENA0003525. SAND2023-07792A

Read this article online: https://arxiv.org/abs/2303.02917 (note: preliminary version not including more general analytical/numerical work targeting molecular Hamiltonians)

Numerical Recreation of the Coherent State Formalism for Fractional Quantum Mechanics

Presenting Author: Joshua Lewis, Colorado School of Mines
Contributing Author(s): Lincoln D. Carr

The fractional Schrödinger equation captures the unique behavior of quantum wavefunctions induced by multiscale media, manifesting through sub and super-dispersive effects. These properties have kindled significant interest in applied mathematics and quantum information sciences. Yet, the analytical techniques suitable for its integer-order counterpart often falter when applied to the fractional version. To address this, we introduce a sixth-order numerical method. This allows for high-precision extraction of eigenfunctions from the Fractional Schrödinger Equation (FSE). By applying this method to the quantum harmonic oscillator system, we discovered that although the coherent state formalism collapses when diverging from the regular integer order (2.0), it may be reconstructed. Adjusting the potential to counterbalance the fractional order deviation restores a linear eigenspectrum and, consequently, the coherent state formalism. Additionally, we validate a revival time in these systems. This suggests that many properties inherent to the coherent state with integer order may be mirrored in the FSE. We also succeed in numerically formulating approximate raising and lowering operators that exhibit heightened accuracy for higher-order eigenfunctions. Given the central role of the coherent state formalism in the integer-order Schrödinger equation, understanding fractional coherent states may lead to deeper insights into the physical implications of quantum multiscale materials.

Exploration of a novel entangling quantum logic gate in the presence of noise and laboratory constraints

Presenting Author: Bethany Little, Sandia National Laboratories
Contributing Author(s): Matthew Chow, Yuan-Yu Jau

Neutral atoms in tweezer arrays have rapidly advanced as a platform for quantum computing in the last decade. As the fidelities in single and two-qubit gates have improved and larger arrays are demonstrated, protocols and applications best suited for this maturing platform are being investigated. We report on the experimental implementation of a novel microwave-driven spin-flip logic gate, proposed earlier this year [Buchemmavari et al, arXiv:2307.16434] and evaluate its performance in the presence of real noise sources and practical considerations in the lab. This protocol maps the typical dipole-blockade physics onto the hyperfine states via Rydberg dressing and the spin-flip blockade. By taking advantage of the stability of the hyperfine states, this work contributes to the quest to develop gates which are robust to noise and therefore promise higher fidelity. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

Two qubit gate for the 0 − π qubit

Presenting Author: Zhenxing Liu, University of Colorado Boulder
Contributing Author(s): Eli Weissler, Joshua Combes

The 0 − π qubit is one of the most attractive candidates among superconducting qubits. By making a trade-off between device parameters requirements and the desired noise protection, the device of “soft 0 – π qubit” has been experimentally achieved. However, we still need a design of two-qubit gate for 0 – π qubit since two qubit gate realization is critical for quantum algorithms. In this talk we demonstrate a microwave-activated controlled-Z gate for the capacitively coupled soft 0 − π qubit. We show gate fidelities over 99.9% while the gate time is below 300ns. We find that off resonant drive can achieve higher fidelity than resonant drive. The gate analysis in terms of circuit parameter disorders and local noise operators is also provided.

Limits on nonlinear metrology and super-heisenberg scaling

Presenting Author: Noah Lordi, University of Colorado
Contributing Author(s): John Wilson, Murray Holland, Josh Combes,

In quantum metrology the precision to which one can estimate a parameter is limited by the amount of resources available in the probe. In an optical interferometer, for example, the parameter is a small phase shift and the resource is the number of photons in the probe state. Metrology puts limits on parameter estimation such as the standard quantum limit (SQL) or the lower Heisenberg limit which requires using more exotic probe states. Super-Heisenberg scaling is the holy grail of quantum metrology, and one way to achieve it is to take advantage of nonlinear parameter encodings that take better advantage of the available resources. A common belief in the metrology community is that many of these schemes, especially those that would produce an exponential improvement, are unphysical in practice. We put these claims on rigorous footing and establish limits on the effectiveness of nonlinear metrology when noise such as loss and dephasing are present in the probe system. We show that in the same way that the nonlinearity can boost parameter encoding, it can also boost your errors, not just making them large in size, but also making them potentially uncorrectable. Furthermore we provide a go/no-go result that can determine the largest nonlinearity given the amount of noise present, and provide several ways to still achieve super-heisenberg scaling even in the presence of noise.

Control over a shared phonon between three ions in a two-dimensional array

Presenting Author: Nathan Lysne, National Institute of Standards and Technology, Boulder
Contributing Author(s): Justin F Niedermeyer, Andrew C Wilson, Daniel H Slichter, Dietrich Leibfried

Two-dimensional arrays of ions trapped in individually tunable microtraps are promising systems for quantum simulation. In such arrays, by coherently controlling a single motional excitation (phonon) shared between the ions, one can generate multipartite entangled states of the ions and study many-body phenomena such as spin frustration and bosons evolving in synthetic magnetic fields. To realize a minimal two-dimensional array, we have developed a surface-electrode ‘triangle trap’ that creates an equilateral triangle of individual trapping sites spaced 30 µm apart. This device has sufficient degrees of freedom (dc electrodes) to independently control the potential field curvatures in each site. By tuning these field curvatures, we bring the motion of the ions in and out of resonance with one another. This hybridizes their motion into shared modes that exchange phonons via the Coulomb interaction. In this poster, we discuss the control of a single phonon shared between 9Be+ ions in our array. We present a demonstration of over 100 exchanges of a single phonon between pairs of ions in separate sites. We extend this control over the array of three ions, reintroducing a single phonon and comparing the observed dynamics of the hybridized modes to simulations.

Extensions of VQAs to optimize discrete parameters

Presenting Author: Lupe MacIntosh, Oregon State University
Contributing Author(s): David Craig

Variational quantum algorithms (VQAs) are a powerful way to leverage the computational ability of quantum computers while making use of efficient classical optimization algorithms. The study of the Consistent Histories formalism provides one application of VQAs. In this case, the cost function being optimized is the decoherence functional, determining consistent families of histories. This concept is extended to consider families with multiple discrete projectors at each time step in the history. The optimization process is thus applied to a discrete parameter instead of a single continuous parameter at each time step. This algorithm is then applied to a model of nested Mach-Zehnder interferometers to investigate potential counterfactual communication. Computational efficiency of the algorithm is evaluated and compared to the classical case.

Investigation of displaced photon counting for projections onto cat state basis

Presenting Author: Hariprasad Madathil, University of New Mexico CQuIC
Contributing Author(s): Francisco Elohim Becerra

Measurements based on photon counting and displacement operations for coherent-state discrimination and optical parameter estimation have demonstrated performances beyond the Gaussian measurement limits [1]. Recently, it was shown that displaced photon counting can be used to realize projections onto a basis formed by superpositions of coherent states, cat states, with low amplitudes [2]. However, the fidelities achieved for these projections were limited due to the noise and instability of the setup, and the measurements only worked for very low amplitude states (less than one photon on average). Here, we investigate how to overcome these limitations by using a polarization-based setup [3], which provides inherent phase stability and more robustness to noise. Moreover, we investigate how to use the information from multi-photon detection events to improve the overall fidelity, while allowing for projections onto higher-amplitude states. [1] R. S. Kennedy (1973). MIT Research Laboratory of Electronics Quarterly Progress Report 108: 219-225. [2] Izumi, Shuro et al. Scientific reports (2018) [3] M. T. DiMario et al. J. Opt. Soc. Am. B 35, 568-574 (2018)

Robust storage of topologically protected light in warm alkali vapor

Presenting Author: Benjamin Makias, Miami University
Contributing Author(s): Jianqiao Li, Reese Tyra, Julius Macbeth, Samir Bali

We investigate the storage properties of light with different transverse electric field profiles in warm Rb vapor via electromagnetically induced transparency. We first store Gaussian and Laguerre-Gaussian (LG) beams, and study the evolution of the stored intensity profile. We show that even though the LG beam vortex remains topologically protected, the intensity profile of either beam is significantly broadened owing to atomic diffusion. Next, we produce a Bessel beam with an axicon and show that the non-diffracting intensity profile is relatively immune to atomic diffusion and is preserved during storage. For comparison, we store an “imposter” Bessel beam – a Gaussian beam carrying dark rings to resemble a Bessel distribution – and show that the imposter degrades quickly. This work suggests the possibility of enhancing alkali atom based information storage by engineering the electric field profile of the light beam. We gratefully acknowledge funding by the US Army Research Office.

Programmable quantum simulations on a trapped-ions quantum computer with a global drive

Presenting Author: Jovan Markov, Weizmann Institute of Science
Contributing Author(s): Yotam Shapira Nitzan Akerman Ady Stern Roee Ozeri

NISQ devices, employed for quantum simulation tasks, are inherently error-prone. To overcome this challenge, it becomes imperative to employ methods capable of entangling multiple qubit pairs simultaneously in a single, programmable step. This approach facilitates the execution of simulations using shallow circuits, thereby minimizing error propagation. In this work, we focus on the simulation of Ising and XY models with a transverse field on a small-scale trapped ions quantum computer. Our approach centers on an entanglement scheme executed by a homogeneous global laser drive, which implements non-trivial qubit couplings by engineering the drive spectrum and utilizing all the motional modes of the ion-crystal. This method makes it possible to simulate systems with a wide range of geometries that are unconstrained by the physical one-dimensionality of the ion-crystal, such as nearest neighbor coupling with periodic or antiperiodic boundary conditions. Specifically, we simulate a 4-spin Ising quantum ring characterized by nearest-neighbor interactions and antiperiodic boundary conditions. We observe the dynamics under this Hamiltonian, as well as with the addition of a transverse field. We benchmark the simulation steps and are able to verify a high-quality of realization of the simulation by using our entanglement scheme.

Closed-loop RF magnetometry below the standard quantum limit with a collective spin ensemble

Presenting Author: Ian Marsh, University of Arizona
Contributing Author(s): David Melchior, Poul Jessen

Measurements with quantum limited resolution have important applications in metrology and sensing, including atomic clocks, atom interferometry, and magnetometry. In our work we perform a quantum-non-demolition measurement on the collective angular momentum of 106

. We estimate this sensitivity to be nearly 8dB below the SQL. We also report on progress in using our device to measure time-dependent magnetic fields.

Scaling Mølmer-Sørensen Sideband Detuning to Perform Zero Noise Extrapolation

Presenting Author: Oliver Maupin, Tufts University
Contributing Author(s): Ashlyn Burch, Christopher Yale, Brandon Ruzic, Antonio Russo, Daniel Lobser, Melissa Revelle, Matthew Chow, Susan Clark, Andrew Landahl, Peter Love

Error mitigation seeks to improve the accuracy and precision of NISQ algorithms without requiring fault-tolerant error correcting codes. Such techniques are essential to push the boundaries of current devices, but there remain questions as to best practices when it comes to implementation. We demonstrate and optimize zero-noise extrapolation (ZNE) on the QSCOUT trapped-ion device using a novel noise scaling technique. We performed preliminary scaling measurements by stretching the entangling gate duration, but ultimately found that in a related and more direct vein, we could increase the noise by shrinking our sideband detuning parameter which affects the entangling gate performance. We compare this result to other methods of scaling noise using a Variational Quantum Eigensolver (VQE) ansatz circuit for calculating the ground state energy of HeH+. We find that scaling the sideband detuning from the carrier reflects the particular type of noise present in our system, giving a more accurate energy expectation value than the unaltered circuit. This result emphasizes the need for careful understanding and modeling of a given quantum device's noise when attempting error mitigation in the NISQ era.

Predicting the fidelity of quantum circuits by propagating errors

Presenting Author: Ashe Miller, Sandia National Laboratories
Contributing Author(s): Kevin Young, Timothy Proctor

As quantum computers continue to develop and advance, the need to be able to predict their performance continues to be important. Current attempts work by one of two methods, the first assumes the error is simple and thus the total error of the circuit is given by the sum of each gate error. However, quantum errors propagate through circuits interacting with gates and other errors within the circuit, resulting in inaccurate estimations of circuit performance. The second method counteracts this problem by classifying each gate error as a process matrix which can then be propagated through the circuit but is numerically complex making it infeasible for estimating the performance of large quantum processors. Here, we propose a new method where each gate error can be approximated by the rates of a polynomial number of primitive error generators. These errors can be propagated through a Clifford circuit allowing for us to efficiently generate an error map for the entire circuit to high order. This method captures the transformations individual errors undergo as they interact with gates and other error generators while being scalable to large quantum processors. We demonstrate the accuracy of this new method by comparing the predicted end of circuit error generator with the true end of circuit error generator created by propagating process matrices. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

A Clifford approximation algorithm for MaxCut

Presenting Author: Manuel H. Munoz-Arias, Institute Quantique, University of Sherbrooke
Contributing Author(s): Alexandre Blais, Stefanos Kourtis

Here we present a Clifford approximated algorithm for the MaxCut problem. This algorithm results from a minimalistic set of properties extracted from the ADAPT-QAOA solution unitaries to the problem on weighted complete graphs. It is a local algorithm which starting from a seed builds an entangled state in a number of steps equal to the number of nodes. A measurement in the computational basis then returns the approximated maximal cut. The algorithm is polynomial both in time and space with worst case complexities O(N7)

, respectively. We implemented this algorithm and study its performance on different families of graphs. For positive edge weights we provide copious evidence that, for graphs up to 200 nodes, our algorithm produces a better approximated solution than the best classical algorithm, that of Goemans and Williamson. We hope our algorithm with aid in the identification of problem instances where quantum approximated optimization might offer an advantage over classical methods.

Assessing energy estimation algorithms for early fault-tolerant quantum computers

Presenting Author: Jacob Nelson, University of New Mexico CQuIC
Contributing Author(s): Shivesh Pathak, Andrew Baczewski

Early fault-tolerant quantum computers (FTQCs) are likely to be typified by not only a limited number of logical qubits, but modest logical error rates and the relatively significant overheads associated with implementing non-Clifford operations. Recent years have seen the development of numerous variations on quantum phase estimation (QPE) that promise to be well-adapted to the constraints of early FTQCs – but it is unclear which will achieve the best performance in energy estimation for even the smallest instances. In this contribution, we will compare models of several resource-limited approaches to QPE in terms of robustness to logical errors, the impact of algorithmic errors, and projected runtime as a function of accuracy. We will focus primarily on QPE applied to eigenvalue estimation for Trotterized Hamiltonian simulation, with the aim of staking out resource requirements for small classically simulable benchmark problems that might become viable on systems with thousands of physical qubits.

Tuning the molecular weight distributions of vinylketone-based polymers using RAFT photopolymerization and UV photodegradatio

Presenting Author: Tochukwu Nwoko, Miami University

The synergistic effect of photochemistry and Reversible Addition/Fragmentation Chain Transfer (RAFT) furnishes polymerization processes with both milder reaction conditions and the livingness of a conventional radical polymerization. Phenyl vinyl ketone (PVK) monomer has fascinating property due to the intrinsic photochemistry of the ketone group which allows for its photopolymerization and photodegradation without extraneous component. Beyond polymerization, the ability to tailor the molecular weight distributions (MWDs) and dispersity of this unique polymer is vital. In the RAFT polymerization of PVK, the choice and mixture of chain transfer agents (CTA) with differing activity was an essential tool in tuning the MWDs and the dispersity of the resulting polymer. A consistent trend in polymer dispersity was observed. Higher loadings of less active chain transfer agent yielded polymers with higher dispersities. Dispersity could be further modulated by utilizing photodegradation of vinyl ketone polymers under UV irradiation

Read this article online: https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py01129d/unauth

Propagation of cold trapped atoms in a precisely controlled arbitrary direction by weak quasiperiodic modulation of the confining lattice

Presenting Author: Stone Oliver, Miami University
Contributing Author(s): Danny Wingert, Luke Schmeltzer, Chanakya Pandya, Samir Bali

We investigate experimentally and theoretically the controlled transport of cold atoms randomly diffusing within a two-dimensional dissipative lattice formed by superposing three optical beams. We show that by weakly phase-modulating two of the lattice beams we generate forces in two perpendicular directions that rectify the Brownian motion, thus generating an atomic current within the lattice plane, potentially in any arbitrary direction. Such a two-dimensional ratchet has been demonstrated previously by biharmonic driving of the lattice [V. Lebedev and F. Renzoni, Phys. Rev. A 80, 023422 (2009)] – however, the direction of transport was difficult to control owing to inevitable cross-coupling between the x and y degrees of freedom. It has been predicted that the use of quasiperiodic driving can significantly suppress the coupling between the transverse degrees of freedom [D. Cubero and F. Renzoni, Phys. Rev. E 86, 056201 (2012)]. Here, we report on our progress in the lab toward implementing Cubero et al’s proposal. Our experiment, if successful, would permit atomic transport in a precisely controlled, arbitrary direction in two dimensions, without requiring a detailed knowledge of the lattice geometry. We gratefully acknowledge funding by the US Army Research Office.

Encoding a qubit in a qudit for fault tolerant quantum computation

Presenting Author: Sivaprasad Omanakuttan, University of New Mexico CQuIC
Contributing Author(s): Vikas Buchemmavari, Jonathan A. Gross, Ivan H Deutsch, Milad Marvian

Error-correcting codes tailored to dominant error sources exhibit superior thresholds and lower overheads when compared to unstructured noise models. In this study, we explore the potential of encoding a qubit in a qudit, a large spin system, to correct for the dominant error sources, namely linear and quadratic angular momentum operators. Employing a spin cat code, analogous to the continuous variable cat encoding, we investigate its applicability in fault-tolerant quantum computation. To preserve the dominant errors during gate operations, we identify a suitable universal gate set centered around the bias-preserving CNOT gate, which we implement for the qubit in a qudit using the Rydberg blockade. Categorizing the dominant errors as phase and amplitude errors, we demonstrate how phase errors, analogous to phase-flip errors for qubits, can be effectively corrected. Furthermore, we propose a novel measurement-free error correction scheme to address amplitude errors without relying on syndrome measurements. Through an in-depth analysis of logical CNOT gate errors, we establish that the fault-tolerant threshold for error correction in the spin cat encoding surpasses that of standard qubit-based encodings. These findings pave the way for encoding a qubit in a qudit with the potential to achieve fault tolerance, high threshold, and reduced resource overhead in quantum information processing.

Superstaq: Deep Optimization of Quantum Programs

Presenting Author: Victory Omole, Infleqtion

We describe Superstaq, a quantum software platform that optimizes the execution of quantum programs by tailoring to underlying hardware primitives. For standard benchmarks such as the Bernstein-Vazirani algorithm, we find that deep optimization can improve program execution performance by at least 10x compared to prevailing state-of-the-art optimizers. To highlight the versatility of our approach, we highlight results from several hardware platforms: superconducting qubits (AQT @ LBNL, IBM Quantum, Rigetti), trapped ion (QSCOUT), and neutral atom (Infleqtion). Across all platforms, we demonstrate new levels of performance and new capabilities that are enabled by deeper integration between quantum programs and the device physics of hardware.

Near-minimal gate set tomography experiment designs

Presenting Author: Corey Ostrove, Sandia National Laboratories
Contributing Author(s): Kenneth Rudinger, Stefan Seritan, Kevin Young, Robin Blume-Kohout

Gate set tomography (GST) provides precise, self-consistent estimates of the noise channels for all of a quantum processor’s logic gates. But GST carries nontrivial experimental cost, which has limited its use to characterizing systems of at most two qubits. We present a new protocol for streamlining GST experiment designs by stripping away nearly all of the redundancy, creating smaller and more scalable experiments without losing precision. We do so by determining how GST base circuits ("germs") are sensitive to variations in particular gate set parameters, and leverage this information to identify and strip out redundancy across a germ set as a whole. We apply this new protocol to experiment designs for two-qubit systems and show that we reliably produce experiment designs approaching information theoretic lower bounds. We demonstrate both in simulations and an analysis using the Fisher information that these streamlined experiment designs nonetheless maintain the Heisenberg-like precision scaling properties of traditional GST. Moreover, using this technique we achieve a final precision in our estimates comparable to those of much larger experiment designs using many times fewer circuits overall. We find that using this protocol the extension of GST to three-qubit systems is feasible, and demonstrate this by directly constructing and simulating an end-to-end three-qubit GST experiment. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Optical emulation of quantum Fourier transforms

Presenting Author: Prasad Padmanabha Iyer, Sandia National Laboratories
Contributing Author(s): Bharath Hebbe Madhusudhana

High-fidelity implementation of quantum Fourier transform (QFT) is a cornerstone for most quantum algorithms. State of the art implementations of QFT suffer greatly from errors (up-to 50% infidelity) for systems with O(10) qubits. Here, we develop a novel approach to emulate QFT using electromagnetic wave propagation. We show that high pixel density commercial spatial light modulators can be mapped to the full Hilbert space of 10 qubits where the exponential overhead is handled by mapping a pixel to each of the 2^10 quantum states. After a projection through an optical lens, the complex Fourier transformation imparted to the electromagnetic wave maps directly to a QFT of a 10 qubit quantum system. When realized through linear optical elements, we expect the fidelity to surpass the current quantum implementations. Further, we report on the experimental advances towards implementing it. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. DOE, Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOEs National Nuclear Security Administration under contract DE-NA0003525.

Towards adaptive measurements for efficient qubit phase estimation

Presenting Author: Sujeet Pani, University of New Mexico CQuIC
Contributing Author(s): Marco A. Rodríguez-García, Isaac Pérez Castillo, Pablo Barberis-Blostien and Francisco Elohim Becerra

Quantum phase estimation is a central problem in quantum metrology, where the goal is to estimate the phase encoded in a quantum state of a physical system. For a given parameter estimation problem, the quantum Cramer-Rao bound (QCRB) describes the limit in estimation precision, and an efficient estimation strategy would yield estimators with variance approaching the QCRB. Here, we consider the problem of phase estimation in a single qubit. Among different strategies for efficient single-qubit phase estimation, non-adaptive protocols are parameter independent, but rely on having optimal initial conditions, and require highly nontrivial measurement operations [1]. In an alternate approach, adaptive strategies [2] aim to take advantage of locally optimal measurements to achieve the QCRB. However, for the problem of phase estimation over the full range [0,2π), adaptive strategies can yield non-convex likelihood functions resulting in sub-optimal, inefficient estimators. We investigate an efficient adaptive phase estimation strategy [1] that can in principle solve this problem of non-identifiability of the parameter, enabling estimation in the entire parametric space of [0,2π). We are working towards the realization of these adaptive strategies based on single-photon polarization qubits using polarization optics for state preparation and measurement, and photon counting. [1] M. A. Rodríguez-García, et al., Quantum 5 (2021): 467 [2] R. Okamoto, et al., PRL 109.13 (2012): 130404

Novel architectures for resource-efficient all-photonic quantum repeater

Presenting Author: Ashlesha Patil, University of Arizona

In all-photonic quantum repeaters, multi-photon cluster states: (a) mimic holding a few logical qubits in quantum memories while photonic qubits entangled with them fly to nearest-neighbor network nodes to attempt entanglement across links, and (b) enable multiplexed entanglement swaps only via single qubits measurements, resulting in end-to-end entanglement rates that exceed the repeaterless bound [1]. The biggest challenge in realizing all-photonic quantum repeaters is that near-deterministic preparation of cluster states—even those of hundreds of photons—requires millions of single-photon sources at each repeater node, due to the need for a tiered multiplexed state preparation. This is because a linear-optical fusion measurement—which connects an n-photon cluster with an m-photon cluster to produce an n+m-2 photon cluster, if successful; and two disconnected n-1 and m-1 photon clusters; if it fails—is inherently probabilistic [2]. In this paper, we propose: (1) a method to recycle fusion-failure outcomes [3] that reduces resource requirements by a factor of two, (2) a biclique repeater state that uses tree-cluster augmentation of the outer-leaf photons in [1] that results in better rate with similar resource requirements as [1]. [1] M. Pant, H. Krovi, D. Englund, and S. Guha, Phys. Rev. A 95, 012304 (2017). [2] M. Pant, D. Towsley, D. Englund, and S. Guha,Nat. Commun. 10, 1070 (2019). [3] M. Gimeno-Segovia, H. Cable, G. J. Mendoza, P. Shadbolt, J. W. Silverstone, J. Carola

Mapping State Transition Susceptibility in Quantum Annealing

Presenting Author: Elijah Pelofske, Los Alamos National Laboratory

Quantum annealing in the transverse field Ising model, implemented on D-Wave devices, works by applying a time dependent transverse field, which puts all qubits into a uniform state of superposition, and then applying a Hamiltonian over time which describes a user programmed Ising problem. We present a method which utilizes two control features of D-Wave quantum annealers, reverse annealing and an h-gain schedule, to quantify the susceptibility, or the distance, between two classical states of an Ising problem. The starting state is encoded using reverse annealing, and the second state is encoded on the linear terms of problem Hamiltonian. An h-gain schedule is specified which incrementally increases the strength of the linear terms, thus allowing a quantification of the h-gain strength required to transition the anneal into a specific state at the final measurement. By the nature of quantum annealing, the state tends towards global minima and therefore we restrict the second classical state to a minimum solution of the given Ising problem. This susceptibility mapping, when enumerated across all initial states, shows in detail the behavior of the quantum annealer during reverse annealing. The procedure is experimentally demonstrated on three small test Ising's which were embedded in parallel on D-Wave Advantage_system4.1. Analysis of the state transition mapping shows detailed characteristics of the reverse annealing process including intermediate state transition paths.

Read this article online: https://arxiv.org/abs/2210.16513, https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.013224

High-Round QAOA for MAX k-SAT on Trapped Ion NISQ Devices

Presenting Author: Elijah Pelofske, Los Alamos National Laboratory
Contributing Author(s): Andreas Bärtschi, John Golden, Stephan Eidenbenz

The Quantum Alternating Operator Ansatz (QAOA) is a hybrid classical-quantum algorithm that aims to sample the optimal solution(s) of discrete combinatorial optimization problems. We present optimized QAOA circuit constructions for sampling MAX $k$-SAT problems, specifically for $k=3$ and $k=4$. The novel $4$-SAT QAOA circuit construction we present uses measurement based uncomputation, followed by classical feed forward conditional operations. The QAOA circuit parameters for $3$-SAT are optimized via exact classical (noise-free) simulation, using HPC resources to simulate up to $20$ rounds on $10$ qubits. In order to explore the limits of current NISQ devices we execute these optimized QAOA circuits for random $3$-SAT test instances with clause-to-variable ratio $4$ on four trapped ion quantum computers: Quantinuum H1-1 (20 qubits), IonQ Harmony (11 qubits), IonQ Aria 1 (25 qubits), and IonQ Forte (29 qubits). The QAOA circuits that are executed include $n=10$ up to $20$ rounds, and $n=22$ for $1$ and $2$ rounds. The high round circuits use upwards of 9,000 individual gate instructions, making these some of the largest QAOA circuits executed on NISQ devices. Our main finding is that current NISQ devices perform best at low round counts (i.e., $p = 1,\ldots, 5$) and then -- as expected due to noise -- gradually start returning satisfiability truth assignments that are no better than randomly picked solutions as the number of QAOA rounds are further increased.

Read this article online: https://arxiv.org/abs/2306.03238

An Exploration of Solution Curves in Feedback-Based Quantum Algorithms

Presenting Author: Vicente Pena Perez, Sandia National Laboratories
Contributing Author(s): Alicia B. Magann Matthew D. Grace

There is increasing interest in utilizing parameterized quantum circuits for tackling combinatorial optimization problems. Recently, the Feedback-based ALgorithm for Quantum OptimizatioN (FALQON) was introduced as an optimization-free framework for this. In FALQON, quantum circuit parameters are established layer-by-layer using a deterministic, measurement-oriented feedback law, creating what we refer to as FALQON solution curves. In this poster, I investigate the features and general applicability of these curves, with an emphasis on curves developed for addressing the Maximum Cut (MaxCut) problem on regular graphs, a widely-studied problem in computer science involving the partitioning of a graph. I explore the relationships between these solution curves and their corresponding MaxCut problem instances, with a specific focus on the observation that FALQON solution curves tend to demonstrate robustness, implying stability against variations in the problem instances. Inspired by this observation, I delve into the potential transferability of solution curves across differing MaxCut problem instances, and suggest possible examinations of these curves that reveal inherent similarities. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. SAND2022-14341 A

Efficient implementations of Riemannian gradient flows on quantum devices for ground state problems

Presenting Author: Mahum Pervez, Arizona State University
Contributing Author(s): Ariq Haqq, Christian Arenz

Adaptive quantum algorithms are promising approaches for preparing ground states on current quantum hardware. Instead of fixing a quantum circuit ansatz, in this approach a quantum circuit is successively grown based on data from qubit measurements, moving the actual state closer to the target ground state in each adaptive step. However, as the adaptive circuit growth can get stuck in local optima, adaptive approaches face similar challenges as traditional variational quantum algorithms. Here, we address this issue by first formulating adaptive quantum state preparation within the theory of gradient flows on the unitary group; an optimization framework where one directly optimizes over quantum circuits rather than circuit parameters. We show that despite the existence of saddle points, gradient flows converge to the ground state where the speed of convergence critically depends on the problem structure. While the full gradient flow is in general not efficiently implementable on a quantum device, we develop several approximation schemes that exhibit favorable convergence behavior but are implementable with quantum circuits of polynomial depth. We benchmark and test these schemes on IBM’s quantum devices.

Space Curve Quantum Control: A robust gate design framework

Presenting Author: Evangelos Piliouras, Virginia Tech
Contributing Author(s): Hunter Nelson, Kyle Connelly, Edwin Barnes

Precise quantum control is a fundamental requirement for most quantum technologies. Achieving this is made challenging by environmental noise and other sources of decoherence, which cause the evolution of the system to deviate from the ideal target evolution. In this talk, we will introduce a framework for designing noise-robust quantum evolutions or logic gates known as Space Curve Quantum Control (SCQC). By mapping the quantum evolution to a multi-dimensional geometric space curve, we uncover simple geometric conditions that must be satisfied in order to guarantee that noise errors self-cancel in the course of the evolution. The cancellation of different types of noise requires the corresponding space curves to possess specific, distinct geometric properties. In this talk, we will present the minimal conditions that must be satisfied by the space curves in order to cancel two of the most prevalent types of noise in existing quantum devices: transverse dephasing noise and multiplicative driving field noise. In both cases, we will describe the corresponding geometric constraints that must be obeyed to ensure noise cancellation, and we will present several different methods for solving these constraints to obtain explicit protocols for implementing dynamically corrected gates robust to these two noise sources.

Read this article online: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.108.012407, https://iopscience.iop.org/article/10.1088/2058-9565/ac4421

Errors in a time reversal based quantum state transfer protocol

Presenting Author: Kevin Randles, University of Oregon
Contributing Author(s): Steven van Enk

The transfer of quantum information, say the state of a qubit, from one system to another with potentially different spectral properties (resonance frequency and decay width) is important in the development of quantum networks as well as distributed and/or hybrid quantum systems. Protocols to achieve such quantum state transfer often use photons to carry the quantum information between the systems. When coupled systems have different spectral properties the photon wave packet emitted by one system must be modified and tailored so as to strongly interact with and be absorbed by another system. Accordingly, we analyze how unitarily transforming the photons time-frequency shape can increase the probability of the receiving system absorbing the photon to near one. We focus on systems comprised of a three-level atom in an optical cavity, however, the approach applies more generally provided the systems are controllable and the unitary transformation is realizable. Here we extend our previous work, which demonstrated how this quantum state transfer scheme works in ideal conditions, showing our protocols utility in the presence of realistic errors, provided we employ known error correction methods. Additionally, we show how the success of the state transfer can be determined by interpreting the protocol as a photodetection event in which the receiving system is a photodetector that projects onto a certain ideal wave packet.

Read this article online: doi.org/10.1103/PhysRevA.108.012421

Adaptive entanglement witnessing: neural network optimization and experimental realization

Presenting Author: Alec Roberson, Harvey Mudd College
Contributing Author(s): Oscar Scholin, Richard Zheng, Theresa W. Lynn

We present an adaptive entanglement witnessing protocol for qubit pairs that relies on local measurements in a limited set of bases. We use six witnesses defined by Riccardi et al. (Phys. Rev. A 101, 062319, 2020) which require measurements in only three bases, in addition to nine additional witnesses grouped into three triplets which each require measurements in two additional bases. For two-qubit states where the six original witnesses {W} do not certify entanglement, we use the existing measurements to predict which triplet of additional witnesses {W’} will be most likely to witness entanglement if it is present. For computationally generated random two-qubit states, we train a neural network to predict the best W’ witness triplet to test and thus the additional measurements to perform. The trained model predicts with up to 85% accuracy for entangled states not witnessed by any W but witnessed by at least one W’, leading the overall adaptive protocol to witness 72% of all entangled states – or 86% of entangled states with concurrence above 0.1 – using roughly half the measurements required for full tomography. Using polarization-entangled photons generated via spontaneous parametric down conversion, we demonstrate success of {W’} and our prediction strategy for a limited class of entangled pure states that {W} cannot witness.

Approaching the Quantum Precision Bounds in Adaptive Phase Estimation using Squeezed Vacuum States

Presenting Author: Marco Rodríguez, University of New Mexico CQuIC
Contributing Author(s): Francisco E. Becerra

Phase estimation plays a central role in experimental tests of physical theories and serves as a cornerstone for many sensing and metrology problems. Here, we propose a novel adaptive Gaussian measurement strategy for optical phase estimation with squeezed vacuum states to achieve precision close to the quantum limit. By constructing a comprehensive set of locally optimal positive operator-valued measures (POVMs) through rotations and homodyne measurements, our approach optimizes the adaptive measurement process based on the use of complete homodyne outcome records. This adaptive phase estimation strategy outperforms previous approaches for phase estimation and attains the quantum limit within the phase interval of $[0, \pi/2)$. Furthermore, we generalize this adaptive strategy to incorporate heterodyne measurements, enabling quantum-limited for phase estimation across the entire range of phases from $[0, \pi)$. Remarkably, our approach maintains an asymptotic quantum-optimal performance in this phase interval, which corresponds the maximum range of phases that can be encoded in squeezed vacuum. This quantum-optimal estimation strategy is a significant advancement forward in high-precision quantum metrology for optical phase estimation.

Progress towards a multi-ion clock with Al+

Presenting Author: Daniel Rodriguez Castillo, National Institute of Standards and Technology, Boulder
Contributing Author(s): Mason Marshall, David Hume

The 1S0-3P0 transition in 27Al+ is an attractive candidate for a frequency standard due to its low sensitivity to external perturbations and high intrinsic Q-factor (~8 mHz natural linewidth at a frequency of 1.121 PHz).In the Ion Storage Group at NIST, a previous version of the clock was demonstrated to have systematic uncertainty below 1e-18 and measurement instability approaching the quantum limit imposed by the excited state lifetime. The new version of this clock is aimed at improving upon this work by allowing for control of longer ion chains. Initial results on the characterization of this system with 25Mg+ will be presented. A 3D micromotion survey of the trap environment shows that a region where 16 ions with time-dilation shift below 5e-18 is possible. Upgrades in the design and build of the vacuum system have proven to be conducive to keeping a long ion chain. The reorder rate between two ions was observed to be ~1 event/hour and our estimated vacuum pressure to be bounded between 2e-12 and 6e-12 Torr. Currently the system is being prepared for 27Al+ spectroscopy and future steps include the cooling and preparation of long mixed-species chains of Mg+ and Al+ ions.

Physical Gate Set Tomography

Presenting Author: Brandon Ruzic, Sandia National Laboratories
Contributing Author(s): Kevin Young Stefan Seritan

In this poster, we develop a method for efficient, accurate interpolation of quantum process matrices over physical parameter space, and we apply our method to study the sensitivity of quantum applications at both the gate and circuit level. We also implement an extension of gate set tomography (GST) that provides a comprehensive view into the errors suffered by a quantum information processor. In its standard form, GST fits a general Markovian model to experimental data in terms of process matrices that are often difficult to interpret. Our extension to this method uses process matrices interpolated over physical parameters in the optimization loop of GST to directly estimate physical model parameters, providing a clear description of the errors in a quantum processor, while reducing the required number of GST sequences by orders of magnitude compared to its standard form. This material was funded in part by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research's Early Career Research Program. SNL is managed and operated by NTESS LLC, a subsidiary of Honeywell International, Inc., for the U.S. DOE's NNSA under contract DE-NA0003525. The views expressed here do not necessarily represent the views of the DOE or the U.S. Government.

Quantum metrology in a lossless Mach–Zehnder interferometer using entangled photon inputs for a sequence of non-adaptive and adaptive measurements

Presenting Author: Shreyas Sadugol, Tulane University
Contributing Author(s): Lev Kaplan 

Using multi-photon entangled input states, we estimate the phase uncertainty in a noiseless Mach–Zehnder interferometer using photon-counting detection. We assume a flat prior uncertainty and use Bayesian inference to construct a posterior uncertainty. By minimizing the posterior variance to get the optimal input states, we first devise an estimation and measurement strategy that yields the lowest phase uncertainty for a single measurement. N00N and Gaussian states are determined to be optimal in certain regimes. We then generalize to a sequence of repeated measurements, using non-adaptive and fully adaptive measurements. N00N and Gaussian input states are close to optimal in these cases as well, and optimal analytical formulae are developed. Using these formulae as inputs, a general scaling formula is obtained, which shows how many shots it would take on average to reduce phase uncertainty to a target level. Finally, these theoretical results are compared with a Monte Carlo simulation using frequentist inference. In both methods of inference, the local non-adaptive method is shown to be the most effective practical method to reduce phase uncertainty.

Read this article online: https://pubs.aip.org/avs/aqs/article-abstract/5/1/014407/2879134/Quantum-metrology-in-a-lossless-Mach-Zehnder?redirectedFrom=fulltext

Coherence-Preserving Ion Loss Detection Protocol

Presenting Author: Vikram Sandhu, GTRI
Contributing Author(s): Darian Hartsell, Craig Clark, Kenton Brown

Ion loss is inevitable in systems of trapped ions. Several common causes include collisions with background gas, imperfect trapping fields, and chemical reactions. In a future trapped ion quantum information processor, such loss errors will have a severely adverse impact on subsequent quantum operations. We propose a protocol to detect ion loss via fluorescence measurements which nonetheless preserves the coherence of the most important ions in the system. The protocol involves merging and separating a computational with another detection ion. The separation waveform is designed such that a single ion undergoing transport by itself always ends up in a well-defined “computational ion” location. Subsequent detection of fluorescence from the final location of the “detection ion” indicates that both ions remain in the trap, while absence of fluorescence there indicates that at least one ion has been lost. In this work, we present our experimental progress and challenges.

A model for circuit execution runtime and its implications for quantum kernels at practical data set sizes

Presenting Author: Travis Scholten, IBM
Contributing Author(s): Derrick Perry II, Joseph Washington, Jennifer R. Glick, Thomas Ward

Quantum machine learning (QML) is a fast-growing discipline within quantum computing. One popular QML algorithm, quantum kernel estimation, uses quantum circuits to estimate a similarity measure (kernel) between two classical feature vectors. Given a set of such circuits, we give a heuristic, predictive model for the total circuit execution time required, based on a recently-introduced measure of the speed of quantum computers. In doing so, we also introduce the notion of an "effective number of quantum volume layers of a circuit", which may be of independent interest. We validate the performance of this model using synthetic and real data by comparing the model's predictions to empirical runtime data collected from IBM Quantum computers through the use of the Qiskit Runtime service. At current speeds of today's quantum computers, our model predicts data sets consisting of on the order of hundreds of feature vectors can be processed in order a few hours. For a large-data workflow, our model's predictions for runtime imply further improvements in the speed of circuit execution -- as well as the algorithm itself -- are necessary.

Read this article online: https://arxiv.org/abs/2307.04980

Geometry of correlated quantum systems

Presenting Author: Johannes Seiler, Ulm University
Contributing Author(s): Thomas Strohm, Wolfgang P. Schleich

Correlated quantum systems have been extensively studied over the last few decades, as these correlations bear the potential to immensely outperform classically correlated systems. Indeed, quantum correlations and their ability to violate Bell inequalities have not only changed our understanding of the nature of physics, but nowadays even more important also paved the way for many new technological applications by improving existing classical standards in many different areas of quantum information. In this work, we present a geometrical interpretation of such correlated quantum systems, and demonstrate this picture on different examples, such as the famous Clauser-Horne-Shimony-Holt (CHSH) inequality, the Hardy nonlocality scenario, average correlation and even the privacy of random number generators. In particular, we show that our geometrical picture allows us to optimize measurement strategies in these scenarios for all possible pure and mixed qubit states, thus providing not only a clearer insight into quantum correlations but also a valuable tool for optimization tasks.

Read this article online: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.104.032218, https://journals.aps.org/pra/abstract/10.1103/PhysRevA.106.032211

Quantum optics in the time domain

Presenting Author: Denis Seletskiy, Polytechnique Montreal
Contributing Author(s): Patrick Cusson, Sho Onoe, Stephane Virally

In this talk I will discuss recent advances toward cycle quantum optics. Following experimental realizations of the electric field amplitude measurements localized in a confined region of space-time, I will discuss the geometric meaning of both quadratures of the quantum field together with our recent proposal of time-domain quantum state tomography. I will conclude with our recent proposal for optimal detection of non-Gaussian states on subcycle timescales. Together, these works are motivated by time-domain quantum spectroscopy.

Uncovering parallelism in block-encoded Hamiltonians for fault-tolerant qubitized quantum simulation

Presenting Author: Stefan Seritan, Sandia National Laboratories
Contributing Author(s): Antonio E. Russo, Shivesh Pathak, Max D. Porter, Andrew D. Baczewski

Qubitized quantum simulation algorithms implement time evolution operators with optimal query complexity with respect to some oracle for the Hamiltonian of interest; therefore, the algorithmic cost will be dominated by the Hamiltonian implementation. The key resources for fault-tolerant quantum computation expressed in a Clifford+T universal gate set are logical qubit count and T-count, or number of non-Clifford operations. “Unary iteration” is a well-known strategy that uses O(log(L)) qubits and O(L) T-count/T-depth for block-encoding a Hamiltonian with L terms. In this work, we demonstrate a K-fold parallel variant of unary iteration which trades O(K) qubits for O((1+log(K))N/K) T-depth and allows the application of K Hamiltonian terms simultaneously. In the fully parallel (i.e., K=L) limit, this enables O(log(L)) T-depth block-encoded Hamiltonian circuits with lower prefactors than existing logarithmic depth approaches. We also explore how this technique improves the resource estimates for systems such as the Fermi-Hubbard model in the low K limit that will be more relevant for early fault-tolerant quantum computation. This work was funded in part by the NNSA’s Advanced Simulation and Computing Physics and Engineering Models program and in part by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research Exploratory Research for Extreme-Scale Science Program. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Impact of a finite local oscillator strength on homodyne measurements, with applications to optical matrix multiplication

Presenting Author: Akshay Seshadri, University of Colorado Boulder
Contributing Author(s): Satoshi Kako, Edwin Ng

With the advent of large-scale computing, such as the training and deployment of large machine learning models, there has been considerable interest in specialized hardware accelerators for faster computation at higher energy efficiency. Some recent proposals include optical neural networks (ONNs) for machine learning and measurement-feedback-based coherent Ising machines (MFB-CIMs) for combinatorial optimization. Homodyne measurements, which enable access to phase-sensitive information encoded in the optical field, are central to the operation of such photonic computing platforms. However, current theoretical and numerical analysis of homodyne measurements in these proposals are either semi-classical, or the local oscillator (LO) amplitude is assumed to be large. To study quantum effects in the low-power regime, we derive exact and approximate expressions for the homodyne-measurement POVM to capture the behavior of the quantum measurement as a function of LO amplitude. We analyze how these nonidealities can affect a fast matrix-multiplication scheme for ONN accelerators. We also study how to model the quantum backaction induced by indirect homodyne measurements, which may have interesting implications for combinatorial optimization in MFB-CIMs in the low-power regime.

Quantum speedup on Simon’s problem with low Hamming weight

Presenting Author: Phattharaporn Singkanipa, University of Southern California
Contributing Author(s): Victor Kasatkin, Zeyuan Zhou, Gregory Quiroz, Daniel A. Lidar

Many quantum algorithms have been theoretically shown to outperform its classical counterparts in solving problems of increasing size. However, in today’s noisy intermediate-scale quantum (NISQ) devices, it is still challenging to demonstrate practical speedup on limited problem sizes due to noise which leads to computation errors on the devices. Previous works have shown algorithmic speedup that does not rely on complexity-theoretic conjectures, e.g., single-shot Bernstein-Vazirani algorithm. Here, we demonstrated algorithmic speedup on another quantum algorithm to Simon's problem for oracles with Hamming weight up to 6, using the metric number-of-oracle-calls-to-solution which scales with the problem size. Our results show that the performance of current NISQ devices doesn't allow quantum speed up for longer circuits, i.e., higher Hamming weight oracles. The experiments were performed on 127-qubit IBM Quantum (IBMQ) superconducting processors. The speedup is observed only when the quantum computation is protected by dynamical decoupling (DD).

Obtaining phase diagrams of Heisenberg models from semidefinite programming

Presenting Author: Jun Takahashi
Contributing Author(s): Chaithanya Rayudu, Cunlu Zhou, Robbie King, Kevin Thompson, Ojas Parekh

The Heisenberg model plays a central role in understanding condensed matter systems. Although conventional methods such as quantum Monte Carlo and tensor networks can be extremely powerful in some situations, the general problem of obtaining the ground state of a Heisenberg model is known to be QMA-complete, i.e. strongly believed to be intractable even with quantum computers (Quantum Max Cut). Here, we introduce a novel way to "approximate" the problem with semidefinite programming by considering the relaxation version of the problem. This approach is based on the proven optimality of the Goemans-Williamson algorithm for the Max Cut problem, which could be seen as the classical analogue. In this poster, I will focus on how this method could be used to practically obtain the phase diagram of condensed matter systems.

Read this article online: https://arxiv.org/abs/2307.15688

Improved scaling of variational quantum metrology with midcircuit adaptivity

Presenting Author: Tyler Thurtell, University of New Mexico CQuIC
Contributing Author(s): Shravan Shravan, Akimasa Miyake

The problem of phase estimation with only limited prior knowledge of the value of the phase has many applications. For example, atomic clocks are operated at the largest interrogation time at which phase wraps can still be neglected. Further, quantum algorithms based on phase estimation require the ability to estimate a phase of any value. For optimal strategies the average mean squared error scales as the one over the square of the number of times the unitary is used, i.e., Heisenberg scaling. Recently, it has been shown that variational metrology schemes based on global rotations and one-axis twists can achieve optimal performance provided many one-axis twists are used in the measurement stage. We show that when the number of twists used in the state preparation and measurement are small, these schemes achieve better scaling than analogous entanglement free schemes but do not achieve Heisenberg scaling or even the scaling suggested by the amount of available spin squeezing. Recently, midcircuit measurements have been demonstrated in neutral atom array, superconducting qubit, and trapped ion systems. Inspired by this, we show that with only a few rounds of adaptivity variational schemes with only a single state preparation twist and no measurement twists can achieve the 5/3 scaling expected from spin squeezing. We further investigate the robustness of these strategies to noise and the performance of adaptive strategies utilizing more entangling resources.

Speeding up the implementation of controlled phase gates in photonic systems through dynamical squeezing.

Presenting Author: Ankit Tiwari, Arizona State University
Contributing Author(s): Saikat Guha, Christian Arenz

Photonic systems are among the most promising platforms for realizing a universal quantum computer. However, the non-linear processes required to implement universal gate sets are typically weak compared to the characteristic energy scales of photonic systems, thereby making the (deterministic) implementation of universal quantum computations challenging in the laboratory. Here, we address this issue by utilizing a recently developed squeezing protocol [1,2] to speed up the implementation of controlled phase gates based on the cross-Kerr interaction in photonic systems. We show that through rapidly alternating between squeezing along different quadratures of a single mode the cross-Kerr interaction can be arbitrarily amplified, which in turn allows for implementing controlled phase gates several orders of magnitudes faster. We develop bounds on how fast squeezing must be amplified to achieve a desired speed up and identify parameter regimes where photon losses can be overcome. [1] C. Arenz et al., Quantum 4, 277 (2020) [2] S. C. Burd et al., arXiv: 2304.05529 (2023)

Average correlation as an indicator for nonclassicality

Presenting Author: Michael E. N. Tschaffon, Ulm University
Contributing Author(s): Johannes Seiler, Matthias Freyberger

Ever since their introduction, Bell inequalities such as the CHSH inequality have proven to be suitable to study nonclassicality for discrete variable quantum states. However, in practice, Bell inequalities require accurate measurements. As a solution to this problem, we introduce average correlation as a novel measure of nonclassicality. It is purely based on randomized measurements without the need of revisiting earlier measurements, making it simple to measure and evaluate theoretically. Further, it only depends on the correlations of the underlying state and not on the local Bloch vectors. Randomized measurements offer the advantage that they do not require good control of measurements, nor a shared reference frame between the subsystems. This is particularly useful in applications where aligning reference frames can be challenging. So far, quantities based on randomized measurements allow us to give necessary and sufficient conditions for inseparability but determining whether a state can violate a Bell inequality is a difficult task. This is what average correlation can help to solve. Based on average correlation, we derive a necessary and sufficient condition for nonclassicality, which can even be used to classify all bipartite qubit states. Moreover, we show how using average correlation, we can examine inseparability of a quantum state.

Read this article online: https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.023063

Mitigating Uncertainty with Supervised Distributed Quantum Computing

Presenting Author: Pedro Veloso, University of Colorado
Contributing Author(s): Jonathan Baker, Gokul Subramanian Ravi, Frederic T. Chong

The common process of executing quantum programs involves offline circuit writing and optimization, followed by submission for cloud-based execution on various hardware targets. This leads to uncertainties due to hardware variability, both internally and externally. The same program on different parts of a device or different devices can yield vastly different results. Temporal hardware errors, infrequent updates, and unreliable queue times exacerbate the issue. This study addresses these uncertainties by repeatedly executing programs on accessible quantum hardware, constructing a distributed model that minimizes uncertainties and maximizes quality while minimizing queue times. They employ a multi-objective optimization framework based on Bayesian inference, distributing circuits to available hardware to obtain consistent outcomes without sacrificing execution time or success rates. This approach improves signal-to-noise ratios and distinguishability indices by 10-30% and 20-90%, respectively, and speeds up program execution by 25-45x for a majority of benchmarks.

Quantum algorithm for the graph isomorphism problem

Presenting Author: Phuong Nam Vu, Yale University
Contributing Author(s): Steven Girvin, Victor Batista

The development of quantum algorithms for solving the molecular substructure search problem is a promising approach to a wide range of problems in chemistry. In this talk, we introduce a qubit-based algorithm for solving this problem. The algorithm is inspired by the well-known Bernstein – Vazirani algorithm and can be combined with efficient classical algorithms to sample all subgraphs. This hybrid quantum-classical approach offers several advantages over variational algorithms, including its deterministic nature and the ability to screen a large number of ligands without changing the circuit, or its parameters. We will also discuss ongoing developments in this area, including the exploration of algorithms for hybrid architectures containing both qubits and bosonic modes and implementations on near term devices.

Dynamical decoupling for suppressing crosstalk on superconducting qubit devices

Presenting Author: Zhihui Wang, NASA - Ames Research Center
Contributing Author(s): Bram Evert, Zoe Gonzalez, James Sud, Hong-Ye Hu, Shon Grabbe, Matt Reagor, Eleanor Rieffel.

Dynamical decoupling (DD) is a noise-mitigating strategy in which sequences of pulses are applied to single qubits to average out their interaction with the environment. DD has been extensively studied and demonstrated for suppressing single-qubit decoherence and can be tailored for different noise spectrum. We report another important adaptation of DD where crosstalk between qubits are suppressed. We demonstrate the efficiency of this procedure on quantum circuits on superconducting transmon-based quantum devices. We designed a family of syncopated DD sequences that effectively suppress ZZ coupling between qubit pairs, which is the dominating crosstalk form on the device. We insert DD to a quantum circuit whenever single qubits are idle (often during two-qubits gates on other qubits). While standard periodic DD suppress crosstalk between these qubits and their neighbors, the syncopated DD further decouples crosstalk between these qubits. We further designed short sequences that maximize the application of DD without adding time to the quantum circuit execution. Such DD sequences yield significant improvement of the performance of the algorithm on the hardware. The performance is further boosted by combining DD with another mitigation strategy, randomized compilation. Our work demonstrated that syncopated DD is effective and practical way to suppress crosstalk in quantum circuits and serves as a great probe to characterize the crosstalk and inform hardware design.

Dynamics of a nuclear spin bath within a quantum dot: short- and long-time behavior

Presenting Author: Wayne Witzel, Sandia National Laboratories
Contributing Author(s): Jesse Lutz

Advances in solid-state quantum information and quantum metrology are accelerated by the ability to predict and interpret the behavior of a bath of nuclear spins in the presence of electron spins. To these ends, the cluster-correlation expansion (CCE) technique [Phys. Rev. B 78, 085315 (2008)] has been a valuable tool for studying nuclear spin baths over shorter timescales, in which the propagation of information between spins is limited. Long-time simulations however, which can also be important in electron spin-qubit applications, are often intractable using CCE. Here we overcome this time-scale limitation by adapting the CCE technique to incorporate both short-time coherent behavior and long-time incoherent behavior for a smooth and reliable approximation over all relevant times. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

Temporal multiplexing for high-rate ion-photon entanglement

Presenting Author: Qiming Wu, University of California Berkeley
Contributing Author(s): Bingran You, Erhan Saglamyurek, Inder Monga and Hartmut Haeffner

Trapped atomic ions are a prime platform for large-scale distributed quantum information processing. The backbone to this application is to achieve high-rate, high-fidelity remote entanglement. Recent experimental studies have shown great progress on developing the quantum network of trapped-ion quantum processors over lab-scale distances using free-space photons. In addition, close-to-optimal photon extraction efficiency in conjunction with quantum frequency conversion to the telecom-band has been demonstrated using ion-cavity interfaces. While trapped ions is an excellent candidate for quantum networking, the communication latency imposed by the round-trip travel time of photons precludes practically useful entanglement rate over long-distances. To overcome this challenge, we present a scheme for temporal multiplexing of ion-photon entanglement. By implementing an adiabatic ion-chain transport and site-resolved single ion addressing, we hope to show that an N-ion array allows one to enhance the rate of ion-photon entanglement by factor of N through a single shared fiber channel. This contribution investigates several critical components of such a multiplexed trapped ion quantum repeater node: optimal ion chain transport with minimal motional excitation, negligible ion crosstalk and background noise for high-purity ion-photon entanglement.

high-dimensional semi-topological code and its efficient decoder under asymmetric noise

Presenting Author: Shixin Wu, University of Southern California
Contributing Author(s): Todd Brun

A semi-topological code is a two-dimensional quantum LDPC code with high stabilizer locality given by one hypergraph product. We present explicit constructions of the stabilizers and logical operators for an n-dimensional semi-topological code given by n-1 hypergraph products. It has been shown that the probabilistic-flip decoder works well with the 3D toric code under asymmetric noise. We propose a probabilistic-flip post-processor for the three-dimensional semi-topological code that rivals the commonly used OSD post-processor in decoding performance for quantum LDPC codes under asymmetric noise, while running asymptotically faster. We observe similar results for lifted-product codes with rather high parity-check matrix density.

Families of $d=2$ 2D subsystem stabilizer codes for universal adiabatic quantum computation with two-body interactions

Presenting Author: Zihan Xia, University of Southern California
Contributing Author(s): Phattharaporn Singkanipa, Daniel Lidar

Bravyi's A matrix offers an approach to devising quantum error correction codes (QECC) characterized by geometric constraints. Since two-body interactions are sufficient for universal adiabatic quantum computation (AQC), we focus on the quantum error detection code (QEDC) with $d=2$. We discovered a family of codes satisfying the maximum code rate, and by slightly relaxing the code rate, we uncovered an extended spectrum of codes within this framework. These codes present enhanced geometric locality, which amplifies their practical utility. Furthermore, we also map the requisite connectivity to alternative configurations so that the total Manhattan distance is minimized, providing valuable insights into hardware design. Lastly, we give a systematic framework for the assessment of codes within the context of AQC in terms of code rate, physical and geometrical locality, graph complexity, and Manhattan distances on the graph. This facilitates informed decision-making in code selection for specific quantum computing applications.

Propagation of Quantum Information in Tree Networks: Noise Thresholds for Infinite Propagation

Presenting Author: Shiv Akshar Yadavalli, Duke University
Contributing Author(s): Iman Marvian

We study quantum tree networks which propagate information from a root to leaves. At each node, the received qubit unitarily interacts with fresh ancilla qubits, after which each qubit is sent through a noisy channel to a different node in the next level. Therefore, as the tree depth grows, there is a competition between the effect of noise and the protection against such noise achieved by delocalization of information. In the classical setting, where each node copies the input bit into multiple output bits, this model has been studied as the tree broadcasting or reconstruction problem. In this work, we study its quantum version: consider a Clifford encoder at each node that encodes the input qubit in a stabilizer code. Such noisy quantum trees, for instance, provide a useful model for understanding the effect of noise within encoders of concatenated codes. We prove that above certain noise thresholds, which depend on properties of the code such as its distance, as well as properties of the encoder, information decays exponentially with tree depth. On the other hand, by studying certain efficient decoders, we prove that for codes with distance d>=2 and for sufficiently small (but non-zero) noise, classical information and entanglement propagate over noisy quantum trees with infinite depth. Indeed, we find that this remains true even for binary trees with certain 2-qubit encoders at each node, which encode the received qubit in the binary repetition code with distance d=1.

Read this article online: https://arxiv.org/abs/2306.14294

Quantum phase estimation by compressed sensing

Presenting Author: Changhao Yi, Fudan University, China
Contributing Author(s): Cunlu Zhou, Jun Takahashi

As a signal recovery algorithm, compressed sensing is particularly useful when the data has low-complexity and samples are rare, which perfectly matches the requirements of quantum eigenvalue estimation problem (QEEP). In this work we use the compressed sensing techniques to design Heisenberg-limited quantum eigenvalue estimation algorithms on early quantum computers. More specifically, given queries to evolution operators and many copies of an initial state, we are able to recovery the energy levels in accuracy level ϵ

samples. In particular, the on-grid compressed sensing algorithm works for the single eigenvalue situation, while the multiple eigenvalue problem is more suitable to be solved by the off-grid compressed sensing.

Read this article online: https://arxiv.org/abs/2306.07008

An efficient method for certifying quantum properties with non-i.i.d. spot-checking trials

Presenting Author: Yanbao Zhang, Oak Ridge National Lab
Contributing Author(s): Akshay Seshadri, Emanuel Knill

In practical situations, the reliability of quantum resources can be compromised due to complex generation processes or adversarial manipulations during transmission. Consequently, the trials generated repeatedly in an experiment may exhibit non-independent and non-identically distributed (non-i.i.d.) behavior. This non-i.i.d. behavior can introduce security concerns and result in overestimations when performing information tasks, such as quantum key distribution, self-testing, verifiable quantum computation, and resource allocation in quantum networks. To certify the performance of such tasks, one can make a random decision in each trial, either spot-checking some desired property or utilizing the quantum resource for the given task. Unfortunately, existing methods for certifying quantum performance through spot-checking are not suitable for non-i.i.d. repeated trials without additional assumptions. Here, we develop a novel method to tackle this challenge. Our method not only works efficiently with a finite number of non-i.i.d. trials but also can yield asymptotically tight results. Moreover, our approach can be adapted to estimate quantum properties in scenarios where the quantum resource is measured and destroyed during each non-i.i.d. repeated trial.

Noise-Resilient Quantum Simulation with Quantum Error Detection Code

Presenting Author: Dawei Zhong, University of Southern California
Contributing Author(s): Todd Brun

Quantum computing promises to offer substantial speed-up in simulating physical systems, but noise in near term quantum devices prohibits people from fully realizing its power. In this work, we propose a procedure to reduce uncertainty coming from noise for quantum simulation tasks based on quantum error detection code. Specifically, we extend the weakly fault-tolerant construction using [[n, n-2, 2]] code and develop a systematic method to construct a general logical exponential map. Together with mid-circuit measurement, this method can be used to detect errors in deep circuits consisting of sequential exponential maps, and thus can be used for dynamical or molecular system simulation after decomposing the overall exponential operator by Trotter–Suzuki formulas. Our work indicates the potential of using quantum error correction code to reduce noise in practical quantum computing tasks before the advent of completely fault-tolerant quantum computers.

Noise-Resilient Quantum Simulation with the Quantum Error Detection Code

Presenting Author: Dawei Zhong, University of Southern California
Contributing Author(s): Todd A. Brun

Quantum computing promises substantial speed-up in simulating physical systems, but noise in near-term quantum processors prevents us from fully realizing its power. In this work, we propose a procedure to reduce errors in quantum simulations on current and near-term quantum computers using the [[n,n-2,2]] quantum error detection code. We employ a weakly fault-tolerant construction and develop systematic methods to construct a logical exponential map for a general Pauli operator. Used together with mid-circuit syndrome measurements, this method can detect errors in deep circuits consisting of a sequence of exponential maps, as generally occur in dynamical or molecular system simulations using Trotter–Suzuki formulas. Our work suggests that one can use elements of fault-tolerance to reduce noise in practical near-term quantum computers.

An SU(2)-symmetric semidefinite programming hierarchy for quantum max cut

Presenting Author: Cunlu Zhou, University of New Mexico
Contributing Author(s): Jun Takahashi, Chaithanya Rayudu, Robbie King, Kevin Thompson, Ojas Parekh

Understanding and approximating extremal energy states of local Hamiltonians is a central problem in quantum physics and complexity theory. Recent work has focused on developing approximation algorithms for local Hamiltonians, and in particular the

Quantum Max Cut'' (QMaxCut) problem, which is closely related to the antiferromagnetic Heisenberg model. In this work, we introduce a family of semidefinite programming (SDP) relaxations based on the Navascues-Pironio-Acin (NPA) hierarchy which is tailored for QMaxCut by taking into account its SU(2) symmetry. We show that the hierarchy converges to the optimal QMaxCut value at a finite level, which is based on a new characterization of the algebra of SWAP operators. We give several analytic proofs and computational results showing exactness/inexactness of our hierarchy at the lowest level on several important families of graphs. We also discuss relationships between SDP approaches for QMaxCut and frustration-freeness in condensed matter physics and numerically demonstrate that the SDP-solvability practically becomes an efficiently-computable generalization of frustration-freeness. Furthermore, by numerical demonstration we show the potential of SDP algorithms to perform as an approximate method to compute physical quantities and capture physical features of some Heisenberg-type statistical mechanics models even away from the frustration-free regions.

Read this article online: https://arxiv.org/abs/2307.15688

Emergent Non-Markovian Dynamics in Logical Qubit Systems

Presenting Author: Jalan Ziyad, University of New Mexico CQuIC
Contributing Author(s): Kenneth Rudinger

Simulations of an error-corrected logical qubit have demonstrated that logical qubit dynamics can exhibit non-Markovian behavior, even when the underlying physical noise is Markovian. Such non-Markovian dynamics present challenges to holistic logical qubit characterization and can also non-trivially degrade the performance of error-corrected hardware. In order to understand (and hopefully mitigate) this emergent non-Markovianty, we construct a method for mapping arbitrary Markovian physical error dynamics to logical error dynamics. We use this method to investigate toy models in order to explain these non-Markovian phenomena.

Simulation of quantum computation with magic states via Jordan-Wigner transformations

Presenting Author: Michael Zurel, University of British Columbia
Contributing Author(s): Lawrence Z. Cohen, Robert Raussendorf

Negativity in certain quasiprobability representations is a necessary condition for a quantum computational advantage. Here we define a new quasiprobability representation exhibiting this property with respect to quantum computations in the magic state model. It is based on generalized Jordan-Wigner transformations and it has a close connection to the probability representation of universal quantum computation based on the Lambda polytopes. For each number of qubits it defines a polytope contained in the Lambda polytope with some shared vertices. It leads to an efficient classical simulation algorithm for magic state quantum circuits for which the input state is positively represented, and it outperforms previous representations in terms of the states that can be positively represented.

Read this article online: https://arxiv.org/pdf/2307.16034.pdf