Quantum algorithms for lattice phi^4 theory

Presenting Author: M. Sohaib Alam, NASA - Ames Research Center
Contributing Author(s): Andrew Hardy, Priyanka Mukhopadhyay, Layla Hormozi, Robert Konik, Eleanor Rieffel, Nathan Wiebe

We discuss methods for the quantum simulation of lattice phi^4 theory. We employ two different bases, one consisting of the Fock or occupation basis representation of the bosonic field, which is naturally discrete, and the other composed of the field amplitude basis in which the field operator is diagonal, but which must be discretized to enable mapping onto a digital quantum computer. We explore these bases with alternative simulation protocols, such as higher-order Trotter-Suzuki expansions and Linear Combination of Unitaries (LCU). The LCU primitives in turn can be employed as a block-encoding subroutine to perform Hamiltonian simulation or phase estimation, using qubitization based techniques. We tie these methods to the physical problem of computing scattering matrix elements for phi^4 theory in finite volume. Lastly, we provide a total resource estimate for a fault-tolerant implementation of these algorithms.

Efficient Quantum Algorithms for Testing Symmetry of Open Quantum Systems

Presenting Author: Rahul Bandyopadhyay, University of California, Davis
Contributing Author(s): Alex H. Rubin, Marina Radulaski, Mark M. Wilde

Symmetry is an important and unifying notion in many areas of physics. In quantum mechanics, one can eliminate degrees of freedom from a system by leveraging symmetry to identify the possible physical transitions. This allows us to simplify calculations and characterize the complicated dynamics of the system with relative ease. Previous works by LaBorde et al. have focused on devising quantum algorithms to ascertain symmetries by means of fidelity-based symmetry measures. In our present work, we develop alternative symmetry testing quantum algorithms that are efficiently implementable on quantum computers. Our approach estimates symmetry measures based on the Hilbert--Schmidt distance, which is considered significantly easier than using fidelity as a metric; more precisely, the problem of estimating the Hilbert-Schmidt distance has been shown to be BQP-complete. Additionally, the proposed algorithm does not rely on variational methods. The method is derived to measure symmetry of states, channels, and Lindbladians. We apply this method to a number of scenarios involving open quantum systems, including the amplitude damping channel and the spin chain, and test against symmetries within and outside the finite symmetry group of the Hamiltonian and loss operators.

Read this article online: https://rlab.engineering.ucdavis.edu/preprints

Resolving the Gravitational Redshift Within a Wannier-Stark Optical Lattice Clock

Presenting Author: Tobias Bothwell, National Institute of Standards and Technology, Boulder

Ever improving precision and accuracy in atomic clocks are inextricably linked to discovery, with each new decade exploring smaller energy scales. Towards this, we present our work engineering the latest generation of 1D optical lattice clocks. Shallow trap depth operation enables extended Wannier-Stark states, supporting half-minute atom-atom coherence times. By tuning the delocalization of atomic wavefunctions we engineer a clock free of atomic interaction induced frequency shifts. Combining these advances allows us to perform intra-clock comparisons, where we measure fractional frequency uncertainties of 4.4×10−18 at 1s and 8×10−21 at 90 hours. This stability allows us to rapidly evaluate field gradients across our millimeter length atomic sample, resolving to sub-millimeter the gravitational redshift within a single atomic ensemble.

Performance of Robust, High-Order Dynamical Decoupling Sequences on Superconducting Quantum Hardware

Presenting Author: Amy Brown, University of Southern California
Contributing Author(s): Vinay Tripathi, Bram Evert, Alex Hill, Xian Wu, Yuan Shi, Yujin Cho, Max Porter, Vasily Geyko, Ilon Joseph, Jonathan Dubois, Eyob Sete, Matthew Reagor, Daniel Lidar

The performance of today’s quantum hardware is limited by circuit depth and duration due to gate infidelity and decoherence, which adversely constrains the class of experiments achievable without error mitigation. Dynamical decoupling is an error-suppression technique that utilizes carefully timed sequences of pulses inserted during idle operation in order to cancel unwanted interactions with the environment, often allowing higher fidelity circuits to be run. A wide variety of dynamical decoupling sequences exists, ranging from simple first-order protection with uniform pulse intervals to robust, higher-order protection with non-uniform pulse interval sequences. Here, we explore and compare the performance of these various sequences on the Rigetti Aspen-M series of superconducting qubit chips. From this experimental data, we draw conclusions about the relative performance of various dynamical decoupling sequences and offer prognoses about near-term algorithmic capabilities enabled by the improvement in performance, paving the way toward performing deeper circuits with built-in environmental noise protection. *Supported by US DOE under Project SCW1736-1. Prepared by LLNL under Contract DE-AC52-07NA27344.

A unified graph-theoretic framework for free-fermion solvability

Presenting Author: Adrian Chapman, University of Oxford
Contributing Author(s): Samuel Elman, Ryan Mann

We show that a quantum spin system has an exact description by non-interacting fermions if its frustration graph is claw-free and contains a simplicial clique. The frustration graph of a spin model captures the pairwise anticommutation relations between Pauli terms of its Hamiltonian. This result captures a vast family of known free-fermion solutions. Previously, it was shown that a free-fermion solution exists if the frustration graph is either a line graph, or (even-hole, claw)-free. The former case generalizes the Jordan-Wigner transformation and includes the exact solution to the Kitaev honeycomb model. The latter case generalizes a non-local solution to the four-fermion model given by Fendley. Our characterization unifies these two approaches, extending generalized Jordan-Wigner solutions to the non-local setting and generalizing the four-fermion solution to models of arbitrary spatial dimension. Our key technical insight is the identification of a class of cycle symmetries for all models with claw-free frustration graphs. We prove that these symmetries commute, and this allows us to apply Fendley's solution method to each symmetric subspace independently. We give a physical description of the fermion modes in terms of operators generated by repeated commutation with the Hamiltonian. This connects our framework to the developing body of work on operator Krylov subspaces. Our results deepen the connection between many-body physics and the theory of claw-free graphs.

Read this article online: https://arxiv.org/abs/2305.15625

Beyond-classical quantum computing with superconducting circuits

Presenting Author: Yu Chen, Google

Quantum computers have the potential to solve problems that are intractable for classical computers. However, achieving such beyond-classical performance has been an outstanding challenge, requiring the development of quantum algorithms and capable hardware systems. I will give an overview on the research in the Google Quantum AI team in this area. I will discuss the technical advancements in our quantum hardware, which enabled the recent beyond-classical random circuit sampling result and its underlying physical mechanisms. I will conclude the talk by discussing the challenges and promises associated with measuring physical local observables in the beyond-classical regime.

Flexible but Robust: Advancing the Two-Qubit Gate on the Quantum Scientific Computing Open User Testbed (QSCOUT)

Presenting Author: Matthew Chow, Sandia National Laboratories
Contributing Author(s): Ashlyn D. Burch, Megan K. Ivory, Daniel Lobser, Melissa C. Revelle, Brandon P. Ruzic, Christopher G. Yale, Susan M. Clark

The Quantum Scientific Computing Open User Testbed (QSCOUT) is a highly-flexible ytterbium-ion based quantum processor designed and operated with the objectives of full transparency and low-level access for users targeting scientific applications. In this presentation, I will give a brief overview of QSCOUT’s current capabilities and then share an insider perspective on the internal research efforts that have improved QSCOUT’s two-qubit gate, the Mølmer-Sørensen gate. Specifically, I will share our novel approach to achieve broadly frequency-robust operation and our compatible set of calibrations that allow for continuously-parameterized entangling gates on fully-connected chains of four ions. Finally, I will highlight selected user projects that have taken advantage of the continuously-parameterized two-qubit gate and discuss outlook for extensions to multi-ion entangling operations. This work was funded by the US DOE, Office of Science, ASCR Quantum Testbed Program. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. Views expressed here do not necessarily represent the views of the DOE or the US Govt. SAND2023-07789A

Read this article online: https://arxiv.org/abs/2210.02372, https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9483669

Optimization of key building blocks for quantum information processing with trapped ions

Presenting Author: Craig Clark, Georgia Tech Research Institute

All of the fundamental elements needed to perform trapped-ion quantum information processing have been demonstrated, yet there remains room for improvement in terms of fidelity, speed, reproducibility, and flexibility. GTRI’s Quantum Systems Division has been working recently to develop methods for ion transport, gate operations, and cooling schemes that are more robust, are faster, and require fewer resources than many techniques commonly employed. In this talk I will give a high-level overview of these key building blocks for quantum information processing with trapped ions, focusing in particular on our group’s unique contributions. I will finish by highlighting recent work on a combination of cooling schemes form the Quantum Systems Division.

High temperature Gibbs state sampling via cluster expansions

Presenting Author: Jeffrey Cohn, IBM
Contributing Author(s): Norhan Eassa (Purdue University) Mahmoud Moustafa (Purdue University)

Gibbs states are useful for several applications such as quantum simulation, quantum machine learning, and quantum optimization. However, preparing or sampling from Gibbs states on a quantum computer is a notoriously difficult task requiring large overhead in resources and/or calibration even in classically tractable cases. We propose a cluster expansion type method based on sampling from a quasi-distribution over products of local mixed states. This results in reduction in circuit depth of state preparation at the cost of a larger sample complexity. Limiting the size of our local cluster limits us to states with short correlation lengths or high temperature. This limit it still of practical utility in 2 scenarios: 1) When calculating high temperature dynamical correlation functions of local Hamiltonians and 2) As a warm start to other methods such as imaginary time evolution. Finally, we demonstrate this method by calculating finite-temperature dynamic correlation functions of the 1-d XY-model on IBM Quantum hardware.

Quantum Science with Tweezer Arrays

Presenting Author: Manuel Endres, California Institute of Technology

I will give an update on experiments in our group for Rydberg arrays based on two valence electron atoms, specifically Strontium 88. The rich level structure of such atoms enables novel cooling, control, and read-out schemes. First, I will show our latest results on creating Bell states with ~0.9985 fidelity and associated error budget. Second, I will give an overview on benchmarking a 60-atom quantum simulation in terms of the fidelity for reaching maximum entanglement entropy states. I will also show new results for quantifying the amount of mixed state entanglement in this system. If time permits, I will show results on novel cooling, control and entanglement of motional states in tweezers.

Time-bin entangled GHZ-state generation

Presenting Author: Leili Esmaeilifar, University of Calgary
Contributing Author(s): Ashutosh Singh, Pascal Lefebvre, and Daniel Oblak

A pair of time-bin entangled states (|ψ⟩ = 1√2 (|ee⟩ + |ll⟩) are generated using the spontaneous parametric down-conversion (SPDC) process in two PPLN crystals. The signal and idler photons from each source are spatially separated using DWDM filters. One photon from either sources are made to interfere on an optical switch (2 x 2 Intensity Modulator). A Four-qubit GHZ-state (|GHZ⟩ = 1√2 [|eeee⟩ + |llll⟩]) is generated by post-selecting the photons at all four output ports and using the fourth photon as a trigger. The time-bin entanglement is verified using the interference visibility measurements on three qubits in Franson-like interferometers.

Have we seen a demonstration of experimental quantum advantage?

Presenting Author: William Fefferman, University of Chicago

In this talk we'll discuss the status quo regarding the latest experimental quantum advantage claims and the evidence for their classical hardness. In particular, we'll talk about the latest complexity theoretic results for believing that near-term, random quantum circuit experiments give rise to a provable quantum advantage over any efficient classical algorithm. We'll then talk about the current gaps between theory and experiment. A particular focus will be on understanding if uncorrected noise, which is a defining characteristic of any near-term quantum experiment, can be exploited by fast classical simulation algorithms.

Benchmarking a trapped-ion quantum computer with 29 algorithmic qubits

Presenting Author: John Gamble, IonQ
Contributing Author(s): Jwo-Sy Chen, Erik Nielsen, Matthew Ebert, Volkan Inlek, Kenneth Wright, Vandiver Chaplin, Andrii Maksymov, Eduardo Páez, Amrit Poudel, Peter Maunz

Quantum computers are rapidly becoming more capable, with dramatic increases in both qubit count and quality. Among different hardware approaches, trapped-ion quantum processors are a leading technology for quantum computing, with established high-fidelity operations and architectures with promising scaling. Here, we demonstrate and thoroughly benchmark the IonQ Forte system: configured here as a single-chain 30-qubit trapped-ion quantum computer with all-to-all operations. We assess the performance of our quantum computer operation at the component level via direct randomized benchmarking (DRB) across all 30 choose 2 = 435 gate pairs. We then show the results of application-oriented benchmarks, indicating that the system passes the suite of algorithmic qubit (AQ) benchmarks up to #AQ 29. Finally, we use our component-level benchmarking to build a system-level model to predict the application benchmarking data through direct simulation, including error mitigation. We find that the system-level model correlates well with the observations in many cases, though in some cases out-of-model errors lead to higher predicted performance than is observed. This highlights that as quantum computers move toward larger and higher-quality devices, characterization becomes more challenging, suggesting future work required to push performance further.

Read this article online: https://arxiv.org/abs/2308.05071

An atomic boson sampler

Presenting Author: Shawn Geller, National Institute of Standards and Technology, Boulder
Contributing Author(s): Aaron W. Young, William J. Eckner, Nathan Schine, Scott Glancy, Emanuel Knill, Adam M. Kaufman

A boson sampler implements a restricted model of quantum computing, defined by the ability to sample from the distribution resulting from the interference of identical bosons propagating according to programmable, noninteracting dynamics. Here, we demonstrate a new combination of tools for implementing boson sampling using ultracold atoms in a two-dimensional, tunnel-coupled optical lattice. These tools include fast and programmable preparation of large ensembles of nearly identical bosonic atoms (99.5+0.5−1.6% indistinguishability) by means of rearrangement with optical tweezers and high-fidelity optical cooling, propagation for variable evolution time in the lattice with low loss (5.0(2)%, independent of evolution time), and high fidelity detection of the atom positions after their evolution (typically 99.8(1)%). With this system, we study specific instances of boson sampling involving up to 180 atoms distributed among ∼1000 sites in the lattice. Direct verification of a given boson sampling distribution is not feasible in this regime. Instead, we introduce and perform targeted tests to determine the indistinguishability of the prepared atoms, to characterize the applied family of single particle unitaries, and to observe expected bunching features due to interference for a large range of atom numbers. When extended to interacting systems, our work demonstrates the capabilities required to directly assemble ground and excited states in simulations of various Hubbard models.

Read this article online: https://arxiv.org/abs/2307.06936

Single-shot decoding of good quantum LDPC codes

Presenting Author: Shouzhen Gu, California Institute of Technology
Contributing Author(s): Eugene Tang, Libor Caha, Shin Ho Choe, Zhiyang He (Sunny), Aleksander Kubica

Recently, there has been a lot of interest in good quantum low-density parity-check (QLDPC) codes (which have constant encoding rate and relative distance). Among them, quantum Tanner codes stand out due to a streamlined construction and admitting efficient decoders under ideal measurements. However, syndrome measurements are generally unreliable in practice and standard procedures require repeated rounds of measurements, typically resulting in large time or qubit overheads. In our work, we show that good QLDPC codes, such as quantum Tanner codes, facilitate single-shot quantum error correction (QEC): even in the presence of measurement errors, reliable QEC is possible given information from a single measurement round. We consider a variety of noise models, including adversarial and stochastic noise. The parallelized version of the single-shot decoder can be run in constant time while keeping the residual error small. The resulting constant-time overhead, combined with good code parameters and robustness to (possibly time-correlated) adversarial noise, makes quantum Tanner codes alluring from the perspective of quantum fault-tolerant protocols. Our results can be viewed as a step in making general QLDPC codes more practical. We believe that QLDPC codes, similar to classical LDPC codes, will constitute the gold standard for future quantum telecommunication technologies and form the backbone of resource-efficient quantum fault-tolerant protocols.

Read this article online: https://arxiv.org/abs/2306.12470

Measure and Forget Dynamics in Random Circuits

Presenting Author: Yucheng He, University of Southern California
Contributing Author(s): Todd A. Brun, Beni Yoshida

The study of the forgetful measurement is both of application interest in fault-tolerant quantum computing and fundamental interest in entanglement-pattern-related questions. This paper investigates the behavior of measurement-induced phase transitions (MIPT) in random Clifford circuits when measurement outcomes are partially forgotten. Our findings reveal a local thermalization rate, which remains constant regardless of system size. Meanwhile, the decay behavior at the turning points in the entropy diagram is also provided numerically. We also observe a counter-intuitive phenomenon where the entropy diagram reaches a threshold and stops evolving, even as the system size increases. This phenomenon can challenge people's intuition drawn from previous studies in noisy random circuits, where people believe the noise will cause the thermalization of the whole system. Additionally, we also identified the disappearance of the purification transition and leveraged the insights from entanglement dynamics to design quantum error correction codes.

Read this article online: https://static1.squarespace.com/static/63ae26de55da425349c48257/t/64d3d1fe45ab3d34848a1212/1691603455508/Measure_and_Forget_Dynamics_in_Random_Circuits+%286%29.pdf, https://www.ychephy.com/research

Integrated photonic modulators and scalable ion traps for quantum computing

Presenting Author: Craig Hogle, Sandia National Laboratories
Contributing Author(s): C. W. Hogle1, D. Dominguez1, J. Goldberg1, J. D. Hunker1, R. J. Law1, A. Leenheer1, B. K. McFarland1, H. J. McGuinness1, B. P. Ruzic1, W. J. Setzer1, J. D. Sterk1, J.W. Van Der Wall1, M. Eichenfield1,2, D. Stick1 [1] Sandia National Laboratories, Albuquerque, NM [2] Wyant College of Optical Sciences, University of Arizona, Tucson, AZ

Quantum information processors and atomic clocks based on trapped ions continue to scale towards greater I/O, size, and power requirements. These demands motivate the replacement of external optical conditioning elements, such as amplitude, phase, frequency modulators, with integrated versions. We present the design, fabrication, and implementation of a monolithically integrated piezo-optomechanical Mach-Zehnder modulator with a microfabricated surface ion trap. We have demonstrated single qubit gate fidelities better that 99.7% with this design[1]. In addition, we describe advances in fabrication and electrical control that have enabled us to realize larger and more capable ion traps for quantum computing[2]. [1] C.W. Hogle, et al., Nature Quantum Information 9, 74 (2023). [2] J.D. Sterk, et al. Nature Quantum Information 8, 68 (2022). This research was funded by the U.S. Department of Energy, Office of Science, Office of Advanced Scientific Computing Research. 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.

Hybrid Variational Classical-Quantum Computing: Ingredients to make it work

Presenting Author: Zoe Holmes, Ecole Polytechnique Federale de Lausanne

Parameterized quantum circuits serve as ansätze for solving variational problems and provide a flexible paradigm for programming near-term quantum computers. Here we discuss three fundamental criteria for this paradigm to be effective: expressibility, trainability and generalisability. We will introduce these concepts and present recent analytic progress quantifying to what extent these criteria can be achieved. While more generally applicable, the discussion will be framed around the example of trying to variationally learn an unknown quantum process. We will end with some more open-ended dreaming about the applications of these ideas for experimental quantum physics and quantum compilation.

Randomized benchmarking with mid-circuit measurements

Presenting Author: Daniel Hothem, Sandia National Laboratories
Contributing Author(s): Jordan Hines, Robin Blume-Kohout, Birgitta Whaley, Timothy Proctor

Building fault-tolerant quantum computers (FTQCs) will likely require new device characterization and benchmarking protocols. In particular, leading fault-tolerant schemes, such as surface codes, require mid-circuit measurements (MCMs). However, to date, there exists no efficient method for comprehensively assessing the performance of MCMs performed in parallel with quantum gates. We correct this by introducing the first randomized benchmarking (RB) protocol with MCMs. Our RB with MCMs protocol is a first-of-its-kind RB protocol for measuring the performance of the quantum operations required for FTQC as they occur in a circuit. As a holistic RB protocol, it allows experimentalists to understand how MCMs acting in conjunction with quantum gates perform. And as a scalable protocol, it allows experimentalists to efficiently understand how well their device operates at the scales required for FTQC. In this talk we will explain how our protocol modifies binary randomized benchmarking, an inversion-free RB protocol, to naturally allow for MCMs. We will then argue that our protocol measures an average error rate of mixed circuit layers of quantum gates and MCMs. Lastly, we will support our argument with a large-scale demonstration on a simulated quantum processor experiencing Pauli stochastic gate noise and realistic MCM noise. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Accelerating Quantum Algorithms with Precomputation

Presenting Author: William J. Huggins, Google
Contributing Author(s): Jarrod R. McClean

Real-world applications of computing can be extremely time-sensitive. It would be valuable if we could accelerate such tasks by performing some of the work ahead of time. Motivated by this, we propose a cost model for quantum algorithms that allows quantum precomputation; i.e., for a polynomial amount of "free" computation before the input to an algorithm is fully specified, and methods for taking advantage of it. We analyze two families of unitaries that are asymptotically more efficient to implement in this cost model than in the standard one. The first example of quantum precomputation, based on density matrix exponentiation, could offer an exponential advantage under certain conditions. The second example uses a variant of gate teleportation to achieve a quadratic advantage when compared with implementing the unitaries directly. These examples hint that quantum precomputation may offer a new arena in which to seek quantum advantage.

Read this article online: https://arxiv.org/abs/2305.09638

Benchmarking Quantinuum’s second-generation quantum processor

Presenting Author: Jacob Johansen, Quantinuum
Contributing Author(s): Steven Moses, Charlie Baldwin, Michael Mills, Joan Dreiling, John Gaebler, Mary Rowe, Anthony Ransford, Sarah Campbell, Juan Pino

One of the main challenges facing large-scale quantum computing is scaling systems to more qubits while maintaining high fidelity operations. In this talk, I will describe our efforts at Quantinuum in scaling trapped-ion quantum computers based on the quantum charge-coupled device architecture. We recently released our second-generation machine, which has a race-track shaped ion trap. The new system incorporates several technologies crucial to future scalability, including electrode broadcasting, multi-layer RF routing, and magneto-optical trap loading, while maintaining, and in some cases exceeding, the gate fidelities of our first-generation system. We initially released the system with 32 qubits, but future upgrades will allow for more. I will describe the thorough set of benchmarking experiments we performed to characterize the system, as well as present a selection of recent results of quantum circuits that have been run on the system.

Read this article online: https://arxiv.org/abs/2305.03828

Optimized experiment design and analysis for fully randomized benchmarking

Presenting Author: Alex Kwiatkowski, National Institute of Standards and Technology, Boulder
Contributing Author(s): Scott Glancy, Emanuel Knill

Randomized benchmarking (RB) is a widely used strategy to assess the quality of available quantum gates in a computational context. RB involves applying random sequences of gates to an initial state and determining the probability of an error in a final measurement. Current implementations of RB measure the probability of error by repeating each randomly chosen sequence many times. Here we investigate the advantages of fully randomized benchmarking, where each randomly chosen sequence is run only once. We find that full randomization offers several advantages including lower uncertainty measurement of the average gate fidelity, maximum likelihood analysis without heuristics, straightforward optimization of the sequence lengths, and ability to model and measure behaviors such as gate-position dependent errors beyond the basic randomized benchmarking model that is usually assumed. We provide concrete protocols to minimize the uncertainty of the estimated parameters given a bound on the time for the complete experiment, and implement a flexible maximum likelihood analysis. As one example, we consider a hypothetical RB experiment where the step error is drawn from a distribution independently for each trial and we show in simulation how this model can be statistically distinguished from the basic model by an optimized experiment. Furthermore, we consider several past RB experiments and determine the improvements that could have been obtained with optimized full randomization.

Towards networks of quantum processors

Presenting Author: Benjamin Lanyon, University of Innsbruck

I have a research group in Innsbruck that focuses on developing methods to entangle matter-based quantum systems in remote locations. Our quantum systems of choice are strings of trapped atomic ions that act as small-scale quantum processors. The strings are confined in linear Paul traps with an integrated optical cavity for the collection of 854 nm photons. In this talk I'll outline our main experimental methods and then give an overview of a recent paper [1] in which we entangled single ions in different buildings across our campus, several hundred meters apart. Finally, I'll show some recent results in which we demonstrate methods to entangle each ion in a string of ten to a different propagating photon. The underlying methods provide a route to entangling remote quantum processors.

[1] V. Krutyanskiy et al, Entanglement of Trapped-Ion Qubits Separated by 230 Meters, PRL 130, 050803 (2023)

The platypus of the quantum channel zoo

Presenting Author: Debbie Leung, University of Waterloo

The capacity of a noisy quantum channel N is the best rate of transmitting quantum data through many memoryless uses of N. In this talk, we examine the diverse and complex phenomena displayed by some simple families of quantum channels. The simplest example of the first family is obtained by gluing together a maximally useful and a completely useless qubit channel, and the resulting channel is unlike either of the constituent channels. In particular, it has additive quantum, private and classical capacity expressions, but the private capacity is significantly larger than the quantum capacity, and the channel has superadditive quantum capacity when used jointly with many other generically chosen channels. While part of the above results relies on a convincing conjecture, we construct a second related family of channels and prove similar results unconditionally.

Joint work with Felix Leditzky, Vikesh Siddhu, Graeme Smith, and John Smolin

State control and vibrational spectroscopy of a single molecular ion

Presenting Author: Yu Liu, National Institute of Standards and Technology, Boulder
Contributing Author(s): Zhimin Liu, April Reisenfeld, Julian Schmidt, Peter Chang, David Leibrandt, Scott Diddams, Dietrich Leibfried, Chin-wen Chou

In recent years, quantum-logic spectroscopy of single molecular ions (QLS-SMI) emerged as a novel platform for high-resolution spectroscopy, offering advantages including precise initial state preparation [1], minimal line broadening from collisional or motional effects, and long interrogation times [1,2,3]. The efficient control of the quantum state of a single molecule, however, remains difficult due to environmental perturbations, notably from thermal radiation (TR) emitted by surrounding surfaces. To mitigate this effect, we developed a QLS-based protocol to track and control the evolution of molecular population. By monitoring and reversing TR-induced quantum jumps from a rotational state with quantum number J = 1 to those with J = 0 and 2, we can confine the population in J = 1 to, on average, ~20 times its natural lifetime of 1.7 s, resulting in an improvement of our experimental duty cycle from 7% to 64%. Leveraging this improvement in state control, we performed vibrational overtone spectroscopy on CaH+ at the 10^-13 fractional uncertainty level. Using a single frequency comb source, we probe several rotational lines within the v = 0 -> 5 series. We discuss the generalizability of these state control and spectroscopy techniques to other molecular species which are of astrophysical interest or suitable for various precision measurement tasks. [1] Chou et al., Nature 545, 203 (2017) [2] Chou et al., Science 367, 6485 (2020) [3] Collopy et al., PRL 130, 223201 (2023)

Quantum state tomography: a machine learning perspective

Presenting Author: Sanjaya Lohani, University of Illinois Chicago
Contributing Author(s): Joseph M Lukens, Atiyya A Davis, Amirali Khannejad, Sangita Regmi, Daniel E Jones, Ryan T Glasser, Brian T Kirby, Thomas A Searles

We present our quantum state tomography framework where state reconstruction is performed using artificial intelligence (AI) directly from a set of measurements. Our hardware-aware data-centric AI techniques reconstruct quantum states of comparable fidelity to that of a typical reconstruction method with the advantage that costly computations are front-loaded with our reconstructing setup. AI has found broad applicability in quantum information science, where existing AI techniques are often applied without significant alterations to network architectures. In this presentation, we demonstrate physics-inspired data-centric heuristics for AI systems used in quantum information science and their efficacy for quantum state reconstruction. Moreover, we discuss methods for enhancing the accuracy of our systems reconstruction by developing custom data sets that reflect essential properties, such as mean purity, of quantum systems we expect to encounter in experiments. Finally, we present custom prior distributions that are automatically tuned and generally better conform to the physical properties of the underlying system than standard fixed prior distributions in Bayesian quantum state estimation. Using both simulated and experimental measurement results, we show that AI-defined prior distributions reduce net convergence times and provide a natural way to incorporate implicit and explicit information directly into the prior distribution.

Read this article online: https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.3.043145, https://iopscience.iop.org/article/10.1088/2632-2153/ac9036/meta, https://iopscience.iop.org/article/10.1088/1367-2630/ace6c8/meta

Towards a fieldable optical atomic clock using surface ion traps with integrated optical components

Presenting Author: Hayden McGuinness, Sandia National Laboratories
Contributing Author(s): Joonhyuk Kwon, William. J. Setzer, Megan Ivory, Michael Gehl, Nicholas Karl, Nicholas Boynton, Raymond Haltli, Eric Ou, Peter Schwindt, Dan L. Stick

As surface trap-based ion systems add more ions to allow for increasingly sophisticated quantum processing and sensing capabilities, the traditional optical-mechanical laboratory infrastructure that make such systems possible are in some cases the limiting factor in further growth of the systems. One promising solution is to integrate as many, if not all, optical components such as waveguides, diffractive gratings, and single-photon detectors into the ion traps themselves. We will highlight ongoing efforts at Sandia National Laboratories to produce these photonically integrated traps in the service of producing highly compact optical atomic clocks through our projects in the DARPA A-PhI and ROCkN programs.

Controlling trapped-ion motional modes for precision measurement*

Presenting Author: Jeremy Metzner, University of Oregon
Contributing Author(s): Alex Quinn, Sean Brudney, Shaun Burd, Dave Wineland, David Allcock

Motional modes of trapped ions have been shown to be a useful tool for quantum sensing, making use of time reversal protocols. This application requires the ability to prepare well-defined motional states with high fidelity. Many of these states can be generated from motional ground states without the use of laser fields. We report our results in generating one-mode and two-mode squeezed states using parametric excitation. These operations help to create motional state interferometers and can be used to achieve sensitivities approaching the Cramér-Rao bound. We present an implementation of an SU(1,1) interferometer using one and two motional modes of a 40Ca+ ion in a Paul trap, and compare the performance to the more traditional SU(2) Mach-Zender interferometer. To characterize the input and output motional states of the interferometers, the ions’ motion is coupled to internal ‘spin’ states, which are distinguishable through spin-dependent fluorescence. The calculation of the Fisher information from experimental data can be used to quantify the phase sensitivity that we can achieve in our setup. *We acknowledge support from NSF through the Q-SEnSE Quantum Leap Challenge Institute, Award #2016244.

Macrostates versus Microstates in the Classical Simulation of Critical Phenomena in Quench Dynamics of 1D Ising Models

Presenting Author: Anupam Mitra, University of New Mexico CQuIC
Contributing Author(s): Tameem Albash, Philip Daniel Blocher, Jun Takahashi, Akimasa Miyake, Grant W. Biedermann, Ivan H. Deutsch

We study critical phenomena in the quench dynamics of one-dimensional (1D) Ising models using truncated Matrix Product States (MPS). While accurate calculation of the full many-body state (microstate) is typically intractable due to the volume-law growth of entanglement, when simulating phases of matter associated with the macrostates of many-body systems, a precise specification of an exact microstate is rarely required. We simulate the critical behavior of a $\mathbb{Z}_2$ symmetry breaking dynamical quantum phase transition for a non-integrable transverse field Ising model with long-range interactions. We show that even when high-fidelity simulation of the full many-body state is intractable due to exponential scaling with system size, macroscopic quantities like order parameters, the critical point and critical exponents of a phase transition can be efficiently simulated. We also estimate long-time correlation lengths of the integrable 1D nearest-neighbor transverse field Ising model, finding that properties like long-time correlation lengths, that depend on the exact microstate can also be efficiently simulated if they can be extracted from the short duration behavior of the dynamics. We explain the tractability of simulation using truncated MPS based on quantum chaos and thermalization in the model. We find that local expectation values are most easily approximated for the most chaotic systems whose exact many-body state is most intractable.

Random quantum circuits on arbitrary architectures generate unitary designs

Presenting Author: Shivan Mittal, University of Texas, Austin
Contributing Author(s): Nicholas Hunter-Jones

We show that random quantum circuits (RQCs) on any connected graph generate approximate unitary designs. Consider local RQCs defined on an arbitrary connected graph, where the edges are the allowed 2-qudit interactions. Just as coin tosses generate a distribution over bit strings, local RQCs generate a distribution over the unitary group. Convergence of k-th moments of such distributions to the Haar measure on the unitary group is a notion of scrambling information called an approximate unitary k-design. It bears emphasis that the majority of the existing literature establishes such a convergence for RQCs on 1D architectures or else for higher dimensional architectures but with uncontrolled constants and high polynomial dependence on k. Given the prevalence of designs with RQCs in quantum information theory -- randomized benchmarking, tomography, decoupling and equilibration in many-body systems, and black hole information processing -- our results rigorously inform applications of near-term devices on a broader class of qudit architectures. Since proving the unitary-design property is related to lower bounding the spectral gaps of local Hamiltonians, we derive our results by proving gap lower bounds using the martingale and finite-size criteria methods due to Nachtergaele and Knabe, respectively. Moreover, we employ techniques from integrable systems to improve the spectral gap bounds and provide evidence for a stronger universal convergence to approximate unitary 2-designs.

Unveiling Quantum Complexity: Exploring Quantum Cellular Automata Dynamics and Network Measures

Presenting Author: Pratik Patnaik, Colorado School of Mines
Contributing Author(s): Logan E. Hillberry, Matthew Jones, Lincoln D. Carr

We present new insights into the interplay between Complexity and Many-body Quantum Systems through our study of Quantum Cellular Automata (QCA). Operating within three-site and five-site neighborhoods, we scrutinize QCA evolution using Totalistic rules for the latter. Our investigation encompasses Quenching on the initial states by the QCA Hamiltonian and contrasting Bond-entropy curves with that of Gaussian Orthogonal Ensemble (GOE) predictions, providing valuable understanding. We map the system to a complex network using Relative Entropy as edges, enabling calculations of Mutual Information and Clustering and Disparity in the temporal evolution. Notably, our findings reveal a distinctive dynamical behavior in clustering when mixed qubits serve as boundaries and an empty initial state. We observe intermittent spikes that gradually attenuate over extended time scales, unveiling captivating temporal characteristics. In contrast, the scenario with ground state boundary QCA exhibits an absence of dynamics when initiated from an empty state. Our discoveries not only contribute to an enhanced understanding of microscopic physical complexity but also hold implications for the practical implementation of such systems in hardware.

Short-Depth QAOA circuits and Quantum Annealing on Higher-Order Ising Models

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

We present a direct comparison between QAOA, one and two rounds, run on all 127 qubits of ibm washington and QA run on D-Wave Advantage system4.1 and Advantage system6.1. The problems which allow for this comparison are random Ising model problems whose connectivity matches the heavy hexagonal lattice topology of ibm washington and the Pegasus graph connectivity of the two D-Wave devices. We create two classes of problem instances for this comparison: one with higher order terms (ZZZ variable interactions), linear terms, and quadratic terms, and a separate problem type with only linear and quadratic terms. Our QAOA circuits are novel and extremely short depth, with a CNOT depth of 6 per round, which allows whole chip usage of ibm washington’s heavy hexagonal lattice and can be applied to future heavy-hex chips. We also test the effectiveness of the error suppression technique digital dynamical decoupling on the QAOA circuits, allowing one of the largest experimental evaluations of dynamical decoupling to date. The QAOA circuits compiled to ibm washington are composed of several thousand circuit instructions, approximately 3,000, making these some the largest quantum circuits ever executed on a digital quantum processor. QAOA and QA are compared against the classical heuristic algorithm of simulated annealing and all problem instances are exactly solved using CPLEX in order to evaluate which samplers, if any, correctly found the ground state solution(s) of the problem instance

Read this article online: https://arxiv.org/abs/2301.00520, https://link.springer.com/chapter/10.1007/978-3-031-32041-5_13, https://www.osti.gov/biblio/1985256/

Scalable Quantum Nanophotonics: From Nanofabrication to Quantum Circuit Mapping

Presenting Author: Marina Radulaski, University of California Davis

Photonic systems are the leading candidates for deterministic quantum sources, quantum repeaters, and other key devices for quantum information processing. Scalability of this technology depends on the stability, homogeneity and coherence properties of quantum emitters, which makes color centers in wide band gap semiconductors a highly desirable platform for applications [1,2]. Silicon carbide, in particular, has been an attractive commercial host of color centers featuring fiber-compatible single photon emission, long spin-coherence times and nonlinear optical properties [3]. Integration of color centers with nanophotonic devices has been a challenging task, but significant progress has been made with demonstrations up to 120-fold resonant emission enhancement of emitters embedded in photonic crystal cavities [4]. A novel direction in overcoming the integration challenge has been the development of triangular photonic devices, recently shown to preserve millisecond-scale spin-coherence in silicon carbide defects [5,6]. Triangular photonics has promising applications in quantum networks, integrated quantum circuits, and quantum simulation. Here, open quantum system modeling provides insights into polaritonic physics achievable with realistic device parameters through evaluation of cavity-protection, localization and phase transition effects [7]. The exponential computational demands of open quantum system modeling on classical computers motivate an exploration into quantum algorithms for nanophotonic phenomena with potentially advantageous scaling, including the mapping of cavity QED effects to quantum circuits [8,9], and symmetry testing of Lindbladians [10].

1. V. A. Norman, S. Majety, Z. Wang, W. H. Casey, N. Curro, M. Radulaski, "Novel color center platforms enabling fundamental scientific discovery," InfoMat, 1-24 (2020).

2. S. Majety, P. Saha, V. A. Norman, M. Radulaski, "Quantum Information Processing With Integrated Silicon Carbide Photonics," Journal of Applied Physics 131, 130901 (2022).

3. G. Moody, et al., "Roadmap on Integrated Quantum Photonics," Journal of Physics: Photonics, 4, 012501 (2022).

4. D. M. Lukin, C. Dory, M. A. Guidry, K. Y. Yang, S. D. Mishra, R. Trivedi, M. Radulaski, S. Sun, D. Vercruysse, G. H. Ahn, J. Vučković, "4H-Silicon-Carbide-on-Insulator for Integrated Quantum and Nonlinear Photonics," Nature Photonics 14, 330-334 (2020).

5. S. Majety, V. A. Norman, L. Li, M. Bell, P. Saha, M. Radulaski, "Quantum photonics in triangular-cross-section nanodevices in silicon carbide," J. Phys. Photonics 3, 034008 (2021).

6. C. Babin, R. Stöhr, N. Morioka, T. Linkewitz, T. Steidl, R. Wörnle, D. Liu, V. Vorobyov, A. Denisenko, M. Hentschel, G. Astakhov, W. Knolle, S. Majety, P. Saha, M. Radulaski, N.T. Son, J. Ul-Hassan, F. Kaiser, J. Wrachtrup, "Nanofabricated and integrated color centers in silicon carbide with high-coherence spin-optical properties," Nature Materials 21, 67-73 (2022).

7. J. Patton, V. A. Norman, R. T. Scalettar, M. Radulaski, "All-Photonic Quantum Simulators With Spectrally Disordered Emitters," arXiv:2112.15469.

8. M. K. Marinkovic, M. Radulaski, "Complexity reduction in resonant open quantum system Tavis-Cummings model with quantum circuit mapping," arXiv:2208.12029.

9. B. Marinelli, Brian, A. H. Rubin, V. A. Norman, Z. Rizvi, R. Naik, D. I. Santiago, C. Spitzer J. M. Kreikebaum, M. Krstic-Marinkovic, I. Siddiqi, M. Radulaski, "Digital Tavis- Cummings Simulation on Superconducting Quantum Hardware with Error Mitigation." In Quantum 2.0, pp. QM2A-3. Optica Publishing Group, 2023.

10. R. Bandyopadhyay, A. H. Rubin, M. Radulaski, M. Wilde, "Efficient quantum algorithms for testing symmetries of open quantum systems," arXiv:2309.02515.

Calculating the Correlation Energy in the Contextual Subspace

Presenting Author: Alexis Ralli, Tufts University
Contributing Author(s): Tim Weaving, William M Kirby, Andrew Tranter, Sauro Succi, Peter V Coveney, Peter J Love

The contextual subspace variational quantum eigensolver (CS-VQE) is a hybrid quantum-classical algorithm that approximates the ground-state energy of a given qubit Hamiltonian. It achieves this by separating the Hamiltonian into contextual and noncontextual parts. The ground-state energy is approximated by classically solving the noncontextual problem, followed by solving the contextual problem using VQE, constrained by the noncontextual solution. In general, computation of the contextual correction needs fewer qubits and measurements compared with solving the full Hamiltonian via traditional VQE. We showcase the CS-VQE algorithm by studying the ground state of HCl and benchmark different error mitigation schemes on different 27-qubit IBM Falcon quantum processors.

Read this article online: https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.5.013095, https://arxiv.org/abs/2303.00445

Dephasing in fluxonium qubits from coherent quantum phase slips

Presenting Author: Mallika Randeria, Lincoln Laboratory, Massachusetts Institute of Technology
Contributing Author(s): Thomas Hazard, Agustin Di Paolo, Kate Azar, Max Hays, Leon Ding, Junyoung An, Ilan Rosen, Michael Gingras, Bethany M. Niedzielski, Hannah Stickler, Jeffrey Grover, Jonilyn Yoder, Mollie E. Schwartz, William D Oliver, Kyle Serniak

Recent experiments on fluxonium qubits in planar circuit architectures have demonstrated near-record coherence times and gate fidelities. In order to improve these metrics further, we seek to develop a holistic model of decoherence in fluxonium qubits. Here we focus on one dephasing mechanism inherent to Josephson junction arrays which arises from the Aharonov-Casher effect, which can limit fluxonium coherence times in circuits with large superinductors. In this talk, we report coherence times in fluxonium qubits specifically engineered to be sensitive to this dephasing mechanism and compare with theoretical models. This material is based upon work supported under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. Air Force.

Automated detection of symmetry-protected subspaces in quantum simulations

Presenting Author: Caleb Rotello, National Renewable Energy Laboratory
Contributing Author(s): Eric Jones, Peter Graf, Eliot Kapit

The analysis of symmetry in quantum systems is of theoretical importance, is useful in applications and experimental settings, and is difficult to accomplish in general. Symmetries imply conservation laws, which partition Hilbert space into invariant subspaces of the time-evolution operator, each of which is demarcated by its conserved quantity. We show that, in a chosen basis, any invariant, symmetry-protected subspaces which are diagonal in that basis are discoverable using transitive closure on graphs representing state-to-state transitions under k-local unitary operations. We introduce two classical algorithms, which efficiently compute and elucidate features of these subspaces. The first algorithm explores the entire symmetry-protected subspace of an initial state in time complexity linear to the size of the subspace. The second algorithm determines if a given measurement outcome of a dynamically-generated state is within the symmetry-protected subspace of the state in which the dynamical system is initialized. We demonstrate the applicability of these algorithms by performing post-selection on data generated from emulated noisy quantum simulations of three different dynamical systems: the Heisenberg-XXX model and the T6 and F4 quantum cellular automata. These algorithms lend themselves to the post-selection of quantum computer data, optimized classical simulation of quantum systems, and the discovery of previously hidden symmetries in quantum mechanical systems.

Read this article online: https://arxiv.org/abs/2302.08586, https://journals.aps.org/prresearch/accepted/e9072D42Oeb1a003b0298a26a11ef0f5f072ad19d

All-photonic multiplexed quantum repeaters based on concatenated bosonic and discrete-variable quantum codes

Presenting Author: Filip Rozpedek, University of Massachusetts
Contributing Author(s): Kaushik P. Seshadreesan, Paul Polakos, Liang Jiang, Saikat Guha

Long distance quantum communication will require the use of quantum repeaters to overcome the exponential attenuation of signal with distance. One class of such repeaters utilizes quantum error correction to overcome losses in the communication channel. Here we propose a novel strategy of using the bosonic Gottesman-Kitaev-Preskill (GKP) code in a two-way repeater architecture with multiplexing. The crucial feature of the GKP code that we make use of is the fact that GKP qubits easily admit deterministic two-qubit gates, hence allowing for multiplexing without the need for generating large cluster states as required in previous all-photonic architectures based on discrete-variable codes. Moreover, thanks to the availability of the analog information generated during the measurements of the GKP qubits, we can design better entanglement swapping procedures in which we connect links based on their estimated quality. We find that our architecture allows for high-rate end-to-end entanglement generation and is resilient to imperfections arising from finite squeezing in the GKP state preparation and homodyne detection inefficiency. In particular we show that long-distance quantum communication over more than 1000 km is possible even with less than 13 dB of GKP squeezing. We also quantify the number of GKP qubits needed for the implementation of our scheme and find that for good hardware parameters our scheme requires around 10^3−10^4 GKP qubits per repeater per protocol run.

Read this article online: https://arxiv.org/abs/2303.14923

Quantum computation of stopping power for inertial fusion target design

Presenting Author: Nicholas Rubin, Google
Contributing Author(s): Dominic W. Berry, Alina Kononov, Fionn D. Malone, Tanuj Khattar, Alec White, Joonho Lee, Hartmut Neven, Ryan Babbush, Andrew D. Baczewski

As the popularity and investment in quantum computing grows the necessity of articulating computational speedups becomes increasingly important. In the physical simulation setting this entails precisely determining the quantum resources required to achieve an algorithmic goal such as ground state preparation or arbitrary time dynamics. In this work we describe the complete compilation of a new dynamics application of fault tolerant quantum computers for computing stopping power relevant to inertial confinement fusion reactor design. Stopping power is the rate at which a material absorbs the kinetic energy of a charged particle passing through it. Here, we describe a protocol for using a fault-tolerant quantum computer to calculate stopping power from a first-quantized plane-wave basis representation of the electrons and projectile. Our approach builds upon the electronic structure block encodings of Su et al. [PRX Quantum 2, 040332], adapting and optimizing those algorithms to estimate specific observables of interest from the non-Born-Oppenheimer dynamics of multiple particle species at finite temperature. Ultimately, we describe the costs of two time-evolution algorithms and two observable measurement protocols determining optimal leading-order Toffoli costs for computing the stopping power.

Quantum computing with neutral atom nuclear-spin qubits

Presenting Author: Albert Ryou, Atom Computing
Contributing Author(s): Atom Computing

We present progress toward a neutral-atom based quantum computer that uses the nuclear spin degree of freedom in the ground states of alkaline-earth and alkaline-earth-like atoms. The qubits exhibit long coherence times, and our architecture shows promise for scaling our qubit array. We discuss state preparation and readout, single-qubit and two-qubit operations, and midcircuit measurements.

Read this article online: https://arxiv.org/abs/2305.19119

The impact of microwave phase noise on diamond quantum sensing

Presenting Author: Maziar Saleh Ziabari, University of New Mexico CHTM
Contributing Author(s): Andris Berzins, Janis Smits, Yaser Silani, Ilja Fescenko, Joshua T. Damron, Andrey Jarmola, Pauli Kehayias, Bryan Richards, Victor Acosta

Precision measurements of the electron spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications, including nuclear magnetic resonance (NMR) spectroscopy. With the advancement of experimental sensitivity, the phase noise of the microwave generator controlling the electron spin becomes relevant [1], particularly for applications that require high microwave frequencies, such as NV NMR at high magnetic fields [2]. We characterize the impact of device phase noise and injected phase noise on the sensitivity of an NV-based AC magnetometer, and discuss an experimental mitigation method. [1] Harrison Ball, William D Oliver & Michael J Biercuk, "The role of master clock stability in quantum information processing", NPJ Quantum Information volume 2, Article number: 16033 (2016), DOI:10.1038/npjqi.2016.33 [2] J. Smits, J. Damron, P. Kehayias, A. F. McDowell, N. Mosavian, N. Ristoff, I. Fescenko. A. Laraoui, A. Jarmola, V. M. Acosta, "Two-dimensional nuclear magnetic resonance spectroscopy with a microfluidic diamond quantum sensor" Science Advances Vol 5, Issue 7 (2019), DOI: 10.1126/sciadv.aaw789

A brief prehistory of qubits

Presenting Author: Benjamin Schumacher, Kenyon College

Wernher von Braun is supposed to have said, "Research is what I'm doing when I don't know what I'm doing." The path to a new idea is seldom straight, and travelers seldom know what their actual destination will be. I intend to illustrate all this by describing the origin of the word "qubit" and the concept of quantum data compression in the early days of quantum information theory. The story will have some elements familiar to explorers: setting out, wandering around, getting lost, finding clues, getting stuck, giving up, changing course, reaching goals -- and finding oneself in a huge new territory that even now, thirty years on, is not fully understood.

Improved precision scaling for simulating coupled quantum-classical dynamics

Presenting Author: Sophia Simon, Other
Contributing Author(s): Raffaele Santagati, Matthias Degroote, Nikolaj Moll, Michael Streif, Nathan Wiebe

We present a super-polynomial improvement in the precision scaling of quantum simulations for coupled classical-quantum systems in this paper. Such systems are found, for example, in molecular dynamics simulations within the Born-Oppenheimer approximation. By employing a framework based on the Koopman-von Neumann formalism, we express the Liouville equation of motion as unitary dynamics and utilize phase kickback from a dynamical quantum simulation to calculate the quantum forces acting on classical particles. This approach allows us to simulate the dynamics of these particles without the overheads associated with measuring gradients and solving the equations of motion on a classical computer, resulting in a super-polynomial advantage at the price of increased space complexity. We demonstrate that these simulations can be performed in both microcanonical and canonical ensembles, enabling the estimation of thermodynamic properties from the prepared probability density.

Read this article online: https://arxiv.org/abs/2307.13033

Observation of anisotropic superfluid density in an artificial crystal

Presenting Author: Junheng Tao, University of Maryland Joint Quantum Institute
Contributing Author(s): Mingshu Zhao, Ian Spielman

We experimentally and theoretically investigate the anisotropic speed of sound of an atomic superfluid (SF) Bose-Einstein condensate in a 1D optical lattice. Because the speed of sound derives from the SF density, this implies that the SF density is itself anisotropic. We find that the speed of sound is decreased by the optical lattice, and the SF density is concomitantly reduced. This reduction is accompanied by the appearance of a zero entropy normal fluid in the purely Bose condensed phase. The reduction in SF density—first predicted [A.J.Leggett, Phys. Rev. Lett. 25 1543–1546 (1970)] in the context of supersolidity—results from the coexistence of superfluidity and density modulations, but is agnostic about the origin of the modulations. We additionally measure the moment of inertia of the system in a scissors mode experiment, demonstrating the existence of rotational flow. As such we shed light on some supersolid properties using imposed, rather than spontaneously formed, density-order.

Read this article online: https://journals.aps.org/prl/accepted/01076Y0cM1719078f6a809d79b393979633d02afb

Error suppression in a two-level system from non-Markovian environmental phase noise using light shifts

Presenting Author: Jason Twamley, Okinawa Institute of Science and Technology
Contributing Author(s): Fernando Quijandria, Anshuman Nayak

A ubiquitous decoherence mechanism in spin systems is that associated with small fluctuations in the spin’s energies: dephasing. Both homogenous (due to temporal fluctuations on an individual spin), and inhomogenous (due to spatial environmental inhomogeneities in a spin ensemble), dephasing are detrimental to any spin-based quantum technology. Dephasing can be suppressed by using dynamical decoupling and using this researchers have extended electronic spin lifetimes in NV defects in diamond to 1 sec, but this is challenging in spin ensembles which suffer large inhomogenous broadening. We describe an autonomous protection scheme [1], where we off-resonantly drive a transition to a third, more sensitive, quantum level and show that the resulting light shift can greatly suppress spatio-temporal dephasing noise. We demonstrate the efficacy of our scheme against two types of non-Markovian environments: Ornstein-Uhlenbeck noise (Gaussian) and Random Telegraph Noise (non-Gaussian). Through numerical simulations we show the enhancement by three orders of magnitude of the coherence time of a qubit in a fluctuating environment. This same scheme, using only two drives, can operate on a collection of qubits, providing temporal and spatial stabilization simultaneously thus yielding a collection of high quality near-identical qubits which can be useful for many quantum technologies such as quantum computing and sensing. [1] F. Quijandria, and J. Twamley, arXiv:2302.03827

Read this article online: https://arxiv.org/pdf/2302.03827.pdf

Finding a qubit in a haystack: identifying the best superconducting qubits by enumerating all possibilities

Presenting Author: Eli Weissler, University of Colorado
Contributing Author(s): Zhenxing Liu, Mohit Bhat, Josh Combes

The most popular superconducting qubits today, the transmon and fluxonium, are relatively small circuits consisting of a Josephson Junction in parallel with a capacitor or inductor respectively. Although these systems have shown incredible progress, they lack the intrinsic noise protection that is possible with larger, multi-mode circuits. While there already exist compelling proposals for these novel “nose-protected” qubits, the best circuits may yet remain undiscovered, due to the incredibly large search space of possible configurations. We present a technique for searching this metaphorical haystack to identify promising candidate qubits. The core of our approach is to explicitly enumerate all superconducting circuits up to 4 nodes in size and determine the set of unique Hamiltonians that can be realized from these systems. We then optimize over relevant parameter regimes to evaluate their performance as qubits. We will show preliminary results from this automated qubit discovery algorithm, as well as discussing tradeoffs in designing the procedure and the complications that arise when evaluating noise protected systems. References: A. Gyenis, et. al., PRX Quantum, Sep. 2021, 10.1103/PRXQuantum.2.030101 T. Menke, et. al., Npj Quantum Inf., Mar. 2021, 10.1038/s41534-021-00382-6 A. Gyenis, et. al., PRX Quantum, Mar. 2021, 10.1103/PRXQuantum.2.010339 D. R. Herber, et. al., J. Mech. Des., Apr. 2017, 10.1115/1.4036132.

Exponential quantum speedup in simulating coupled classical oscillators

Presenting Author: Nathan Wiebe, University of Toronto

We present a quantum algorithm for simulating the classical dynamics of coupled oscillators (e.g., masses coupled by springs). Our approach leverages a mapping between the Schrödinger equation and Newton's equations for harmonic potentials such that the amplitudes of the evolved quantum state encode the momenta and displacements of the classical oscillators. When individual masses and spring constants can be efficiently queried, and when the initial state can be efficiently prepared, the complexity of our quantum algorithm is polynomial in , almost linear in the evolution time, and sublinear in the sparsity. As an example application, we apply our quantum algorithm to efficiently estimate the kinetic energy of an oscillator at any time, for a specification of the problem that we prove is BQP-complete. Thus, our approach solves a potentially practical application with an exponential speedup over classical computers. Finally, we show that under similar conditions our approach can efficiently simulate more general classical harmonic systems with modes.

Read this article online: https://arxiv.org/abs/2303.13012

Applications of 3D printing technology in ion traps

Presenting Author: Shuqi Xu, University of California Berkeley
Contributing Author(s): Xiaoxing Xia, Qian Yu, Sumanta Khan, Eli Megidish, Bingran You, Boerge Hemmerling, Juergen Biener, Hartmut Häffner

We demonstrate miniaturized 3D-Paul traps fabricated using high-resolution 3D printing technology based on two-photon polymerization, which provides a route to drastically expand the geometric design-freedom of ion traps and fulfill the trap efficiency and scalability requirements for applications including quantum information processing. With such a 3D-printed trap, we confine single calcium ions in a quadrupole potential with radial trap frequencies ranging from 2 MHz to 24 MHz. The tight confinement eases the ion cooling requirements and allows us to demonstrate high-fidelity coherent operations on an optical qubit after Doppler cooling only. With 3D printing technology, ion trap geometries can be optimized for functionality rather than being limited by fabrication promising to support applications in mass spectroscopy, optical clock arrays, and scalable quantum information processing.

Group-theoretic error mitigation enabled by classical shadows and symmetries

Presenting Author: Andrew Zhao, University of New Mexico CQuIC
Contributing Author(s): Akimasa Miyake

Predicting local observables is a key subroutine for many quantum simulation algorithms. Implementations in the near term face two major challenges: learning a large collection of observables from a limited shot budget, and the accumulation of noise in devices without quantum error correction. In this paper, we build off recent advances in classical-shadow tomography to develop an error-mitigation scheme which addresses these challenges simultaneously. Our technique relies on the simulated quantum system obeying a symmetry which is “compatible” with the representation theory underlying the classical shadows protocol. As a concrete example, we consider U(1) symmetries, which commonly manifest in fermions as fixed particle number and in spins as conserved total magnetization. To reconcile the compatibility of such symmetries, we also present a unified group-theoretic framework of classical shadows for both fermions and qubits. For a class of measurement noise obeying minimal assumptions, we establish rigorous bounds on the prediction error and sample complexity of our technique. To probe its broader error-mitigation capabilities beyond measurement noise, we perform numerical experiments with a noise model derived from existing superconducting-qubit platforms. Our analytical and numerical results reveal our technique as a flexible and low-cost strategy to help mitigate errors from noisy quantum experiments.

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

Experimental observation of symmetry-protected signatures of N-body interactions

Presenting Author: Liudmila Zhukas, Duke University
Contributing Author(s): Or Katz, Qingfeng Wang, Marko Cetina, Iman Marvian, Christopher Monroe

The identification and characterization of higher-order interactions in quantum processes with unknown Hamiltonians remains a significant challenge. This is partly due to the fact that 2-body interactions can produce an arbitrary time evolution, or the universality of 2-local gates in the context of quantum computing. However, recent research has shown that when the unknown Hamiltonian respects a U(1) symmetry—e.g., corresponding to charge or number conservation—N-body interactions exhibit a distinct symmetry-protected signature known as the N-body phase, which cannot be mimicked by fewer-body interactions. In this study, we investigate the time evolution of quantum systems under U(1)-invariant Hamiltonians. Specifically, we develop and experimentally demonstrate an efficient technique that allows detection of N-body interactions despite the presence of unknown 2-body interactions. This technique requires probing unitary evolution and measuring its determinant in a small subspace that scales linearly with the system size, rendering it an efficient approach. This research has been supported by the IARPA LogiQ program, NSF Practical Fully Connected Quantum Computer STAQ program, DOE program on Quantum Computing in Chemical and Material Sciences, AFOSR MURI on Quantum Measurement and Verification, and AFOSR MURI on Interactive Quantum Computation and Communication Protocols.

Simulating quantum computation with magic states: how many "bits" for "it"?

Presenting Author: Michael Zurel, University of British Columbia
Contributing Author(s): Cihan Okay, Robert Raussendorf

A recently introduced classical simulation method for universal quantum computation with magic states operates by repeated sampling from probability functions [M. Zurel et al. PRL 260404 (2020)]. This method is closely related to sampling algorithms based on Wigner functions, with the important distinction that Wigner functions can take negative values obstructing the sampling. Indeed, negativity in Wigner functions has been identified as a precondition for a quantum speed-up. However, in the present method of classical simulation, negativity of quasiprobability functions never arises. This model remains probabilistic for all quantum computations. In this paper, we analyze the amount of classical data that the simulation procedure must track. We find that this amount is small. Specifically, for any number n of magic states, the number of bits that describe the quantum system at any given time is 2n^2+O(n).

Read this article online: https://arxiv.org/pdf/2305.17287.pdf