# 2020 Talk Abstracts

### Quantifying non-Markovianity: a quantum resource-theoretic approach

**Presenting Author**: Namit Anand, University of Southern California **Contributing Author(s)**: Todd A. Brun

The quantification and characterization of non-Markovian dynamics in quantum systems is essential both for the theory of open quantum systems and for a deeper understanding of the effects of non-Markovian noise on quantum technologies. Here, we introduce the robustness of non-Markovianity, an operationally-motivated, optimization-free measure that quantifies the minimum amount of Markovian noise that can be mixed with a non-Markovian evolution before it becomes Markovian. We show that this quantity is a bonafide non-Markovianity measure since it is faithful, convex, and monotonic under composition with Markovian maps. A two-fold operational interpretation of this measure is provided, with the robustness measure quantifying an advantage in both state and channel discrimination tasks. Moreover, we connect the robustness measure to single-shot information theory by using it to upper bound the min-accessible information of a non-Markovian map. Furthermore, we provide a closed-form analytical expression for this measure and show that, quite remarkably, the robustness measure is exactly equal to half the Rivas-Huelga-Plenio (RHP) measure [Phys. Rev. Lett. 105, 050403 (2010)]. As a result, we provide a direct operational meaning to the RHP measure while endowing the robustness measure with the physical characterizations of the RHP measure.

Read this article online: https://arxiv.org/abs/1903.03880

### Speeding up number partitioning with Grover's algorithm

**Presenting Author**: Galit Anikeeva, Stanford University **Contributing Author(s)**: Emily Davis, Tori Borish, Ognjen Marković, Monika Scheier-Smith

A number of conceptually important quantum algorithms rely on the use of a black-box device known as an oracle, which is typically difficult to construct without knowing the answer to the problem that the quantum computer is intended to solve. A notable example is Grover's algorithm, which theoretically can offer a quadratic speed-up in search problems. Here we show how Grover's algorithm can be applied to a class of NP-complete decision problems---the subset sum problem and, as a special case, the number partitioning problem---in realistic experiments. Each instance of the problem is encoded in the strengths of couplings of a set of qubits to a central spin or boson, which mediates a collective phase gate constituting the quantum oracle. We propose and analyze implementations in cavity-QED and Rydberg-atom systems.

### Shot frugal optimization for variational quantum-classical algorithms

**Presenting Author**: Andrew Arrasmith, Los Alamos National Laboratory **Contributing Author(s)**: Jonas Kübler, Lukasz Cincio, Patrick Coles

Variational hybrid quantum-classical algorithms (VHQCAs) seem likely to be the first useful algorithms in the era of near-term quantum computing. There is however a justified concern that the number of measurements needed for these algorithms to converge might become prohibitive when scaling up to non-trivial problem sizes. We address this issue by adapting results from classical optimization to the problem of shot-frugal optimization of VHQCAs. Specifically, we present new techniques and compare them with standard methods to demonstrate the potential for improvement both with noiseless and noisy quantum devices

Read this article online: https://arxiv.org/pdf/1909.09083.pdf

### Arbitrary unitary transformation of temporal modes

**Presenting Author**: James Ashby, University of Oregon **Contributing Author(s)**: Brian Smith

Controlling the temporal mode shape of a quantum light pulses has wide ranging application to optical quantum technologies, including quantum key distribution with pulsed mode encoding, continuous-variable cluster state manipulation, linear-optics quantum computation, and enhanced quantum sensing. We propose a realistic linear optical system that can perform arbitrary unitary transformations on a set of temporal modes. First we show that any unitary transformation on pulsed modes can be decomposed into a sequence of temporal phase modulations and Fourier transforms on optical pulses. It is shown that this sequence of transformations can be performed on optical pulses using electro-optic phase modulators and dispersive optical elements. We show that photonic arbitrary waveform generation can be used to drive the phase modulators. Simulated results demonstrate that targeted transformations can be achieved with near unit fidelity and efficiency. Additionally, we provide preliminary experimental results.

### Traffic on the quantum highway: The direct path may not be the shortest

**Presenting Author**: Shahabeddin Aslamarand, Florida Atlantic University **Contributing Author(s)**: Razaei,Tahereh / Miller,Warner A. / Snyder,Robert / Khajavi,Behzad / Fanto,Michael / Alsing,Paul M. / Ahn,Doyeol

Quantum mechanics can produce correlations that are much stronger than classically allowed. Quantum entanglement is recognized as the key resource for quantum computers, and this stronger–than–classical correlation is the “fuel” for the quantum computing highway. Much attention has been placed on entanglement and in defining a proper measure of entanglement. This is still an active field of research. In 1991 Schumacher forwarded a beautiful geometric approach to this problem for a maximally entangled two-qubit state. His approach used a well–established information distance that depended on measurements made on an ensemble of identical singlet states. He calculated that for specific detector settings used to measure each of the two entangled states, that the resulting geometry violated a triangle inequality even though classically, this was not possible. This is an information geometric Bell inequality. Here we experimentally-reproduce his construction and demonstrate a definitive violation for a singlet state of two photons, |ψi = (|00i + |11i)/√2, based on coincidence counting of photons produced in the laboratory by the usual spontaneous parametric down-conversion in a paired BBO crystal configuration. The singlet states we produced have fidelities of ∼91 percent based on our tomographic measurements. We discussgeneralizations to higher dimensional multipartite quantum states.

### Quantum process tomography of linear optical devices

**Presenting Author**: Pratik Barge, Louisiana State University **Contributing Author(s)**: Kevin Jacob, Jonathan Dowling

Quantum process tomography is an indispensable tool for quantum information processing. In the realm of quantum optics, the task is to characterize the evolution of the mode creation and annihilation operators. Several methods to characterize a multi-mode passive linear optical unitary have been developed, but the precision of these methods is bound by the shot-noise limit. In our work, we propose a method to go beyond the shot-noise limit by using tools from quantum metrology. We demonstrate this by using non-classical states of light and by employing Gaussian and non-Gaussian measurement strategies.

### Maximally entangled bases for two four-state particles: Inequivalence under local operations

**Presenting Author**: Anna Barth, Harvey Mudd College **Contributing Author(s)**: Cathy Chang, Theresa Lynn

We investigate different sets of maximally entangled states and their equivalence or inequivalence under local operations. In particular, we focus on two maximally entangled bases for the space of two 4-dimensional particles. One basis is the d=4 qudit Bell basis; the other is the d=2 by d=2 hyperentangled Bell basis. We have shown that local operations cannot transform the complete set of 16 d=4 Bell states to the set of 16 2-by-2 hyperentangled Bell states. Because it is known that an LELM device can reliably distinguish 7 and no more than 7 of the 2-by-2 hyperentangled Bell states, but the equivalent problem has not been solved in the d=4 Bell basis, we seek to determine whether local operations can take a set of 7 d=4 Bell states to 7 distinguishable 2-by-2 hyperentangled Bell states.

### Nonlinear amplification models for single photon detection

**Presenting Author**: Saumya Biswas, University of Oregon **Contributing Author(s)**: Steven J van Enk

A quantum mechanical model describing the transduction and amplification of a single photon signal consisting of a sequence of operators must respect the commutation relationships of operators and causality. The amplification stage facilitates a projective measurement of the detector. We elucidate the operator evolutions in the Heisenberg picture that help us quantify the noise margins and fundamental limits of signal to noise ratios in specific models of amplification. Non-linear amplification mechanisms do not necessarily amplify thermal fluctuations in detector's internal degrees of freedom by the same factor as the amplification of the signal. Practical amplifiers in use do not show any substantial amplification of thermal noise. We explore a few example systems which demonstrate the nonlinear amplification of signals. We form the Positive Operator Valued Measurement(POVM) elements that represent the probability of a photon detection. Specifically, we present results on the Non-linear Kerr models and driven Jaynes-Cummings model.

### Quantum supremacy using a programmable superconducting processor

**Presenting Author **: Sergio Boixo, Google

In this talk I will cover the theoretical aspects of the quantum supremacy experiment at Google, such as: complexity theory foundation, cross entropy benchmarking, statistical analysis and classical simulation algorithms.

### Optimal protocols in quantum annealing and QAOA problems

**Presenting Author**: Lucas Brady, National Institute of Standards and Technology, Maryland **Contributing Author(s)**: Christopher Baldwin, Aniruddha Bapat, Alexey Gorshkov, Yaroslav Kharkov

Quantum Annealing and the Quantum Approximate Optimization Algorithm (QAOA) are both instances of the same control problem where two Hamiltonians can be applied in order to optimize the expectation value of an energy operator at the end of the procedure. Previous work has suggested that the bang-bang structure of QAOA is optimal but with significant caveats, leaving open the question of what procedure is optimal in practice. In this work, we formalize the optimal control arguments proving that a time-constrained procedure has a bang at the beginning and end but can take on an annealing-like form in-between. In numerics on transverse field Ising models, we show that bang-anneal-bang procedures are common with optimal time-constrained QAOA Trotterizing the annealing portion. We furthermore show an equivalence between different types of time-constraints in this style of problem. The optimal procedures we find are far removed from a monotonically changing adiabatic path in the short-time limit, but we show that as the allowed time increases, the adiabatic limit and intuition from adiabatic quantum computing apply.

### Towards analog quantum simulation of strongly correlated electron systems with lithographic quantum dots

**Presenting Author**: Mitchell Brickson, University of New Mexico CQuIC **Contributing Author(s)**: Q. Campbell, N. T. Jacobson, L. N. Maurer, A. D. Baczewski

Simulation of correlated quantum systems can be prohibitively computationally expensive on classical computers. However, fabrication and control of quantum systems has come to the point where we can emulate challenging target Hamiltonians without having to classically compute their properties – analog quantum simulation. Lithographic quantum dots (QDs) are one such promising platform which map naturally onto quantum impurity models (QIMs). QIMs can be used to study strongly correlated phenomenology, either directly (e.g., the Kondo effect) or indirectly as a component of embedding-based approaches to lattice problems (e.g., dynamical mean-field theory (DMFT)). We examine the viability of designing QD systems that can capture both types of phenomenology. Effective mass theory is applied to realistic device designs to extract QIM parameters. We assess the controllability of these parameters as a function of applied voltages and gate layouts. The possibility of achieving the strongly correlated regime for three QD technologies is then studied. We conclude by evaluating the possibility of using QDs as an analog quantum coprocessor for solving the Hubbard model within DMFT. 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-NA-0003525.

### A quantum algorithm for verifying first-order logic

**Presenting Author**: Daniel Briseno, Chapman University **Contributing Author(s)**: Justin Dressel, M. Andrew Moshier, Olivia Wissa

The verification of first-order-logic formulas is a question of particular importance in computer science, since an efficient algorithm for doing so would have important consequences for automated theorem proving, the 3-SAT problem and model checking. We take inspiration from a nested version of Grover's quantum database search algorithm and propose a similar nested algorithm to address first-order logic verification. Grover's search scales in time as O(√N) in the size N of the database, while we expect our proposed algorithm to scale similarly as O(〖√N〗^n) where n is the number of quantifiers in the logic formula with N terms.

### Quantum amplification of boson-mediated interactions

**Presenting Author**: Shaun Burd, National Institute of Standards and Technology, Boulder **Contributing Author(s)**: Raghavendra Srinivas, Hannah Knaack, Wenchao Ge, Andrew C. Wilson, David J. Wineland, Dietrich Leibfried, David T. Allcock, John J. Bollinger, Daniel H. Slichter

Strong and precisely controlled interactions between quantum objects are essential for emerging technologies such as quantum information processing, simulation, and sensing. A well-established paradigm for coupling otherwise weakly interacting quantum objects is to use auxiliary quantum particles, typically bosons, to mediate interactions, for example photon-mediated interactions between atoms or superconducting qubits, and phonon-mediated interactions between trapped ions. General methods for amplifying these interactions through parametric driving of the boson channel have been proposed for a variety of quantum platforms [1,2,3,4] but an experimental demonstration has yet to be realized. Here we experimentally demonstrate the amplification of a boson-mediated interaction between two trapped-ion qubits by parametrically modulating the confining potential of the trap. The stronger interaction enables a 3.3-fold reduction in the time required to implement an entangling gate between the two qubits. Our method can be applied wherever parametric modulation of the boson channel is possible, enabling its use in a variety of quantum platforms to explore new parameter regimes and for enhanced quantum information processing. [1] W. Ge. et al. PRL 122, 030501 (2019) [2] W. Qin et al. PRL 120, 093601 (2018) [3] M-A. Lemonde. Nat Commun 7, 11338 (2016) [4] C. Arenz et al. arXiv:1806.00444 (2018)

### Contextuality from quantum reference frames

**Presenting Author**: Lucas Burns, Chapman University **Contributing Author(s)**: Lorenzo Catani, Tomáš Gonda, Thomas Galley, Justin Dressel

We show that multiparticle states can be viewed algebraically as specifying relations between reference frames associated with particle degrees of freedom. Building on work by Gour and Spekkens on quantum reference frames, we propose a characterization of contextuality as information compression related to identifying reference frames of particular degrees of freedom. As a simple example, we show how bipartite entanglement arises from product states subject to an identification of rotational reference frames.

### Qubit models for quantum simulation of quantum field theory

**Presenting Author**: Alex Buser, California Institute of Technology **Contributing Author(s)**: Tanmoy Bhattacharya, Shailesh Chandrasekharan, Hersh Singh, Rajan Gupta

Quantum computers are expected to outperform classical methods in the simulation of strongly-coupled quantum field theories, as they permit the calculation of dynamic quantities in real-time and avoid the notorious sign-problem. We consider a class of qubit models which can be simulated efficiently on a fault-tolerant quantum computer, and present evidence that these models possess a rich phase diagram. One of the quantum critical points in the phase diagram may help define the traditional asymptotically free O(3) non-linear sigma model. We discuss implementation of these qubit models on both NISQ and fault-tolerant quantum computers, and provide numerical results on adiabatic ground state preparation for the O(3) sigma model. This work serves as a stepping stone towards simulating non-Abelian Kogut-Susskind type gauge theories with quantum devices.

### Flag fault-tolerant error correction for arbitrary stabilizer codes

**Presenting Author**: Rui Chao, University of Southern California **Contributing Author(s)**: Ben Reichardt

Quantum fault tolerance generally incurs a large qubit overhead. Conventional fault-tolerant error-correction schemes based on ancilla verification or decoding tricks needs as many ancillas as the maximum stabilizer generator weight, whereas a scheme using the flag paradigm requires only two ancillas for common distance-three codes. Recently, Chamberland and Beverland (Quantum 2, 53 (2018)) have provided a framework for flag error correction of arbitrary-distance codes. However, their construction requires certain conditions, which only a few code families are known to satisfy. In this paper, we describe a detailed flag fault-tolerant error-correction scheme that applies unconditionally to arbitrary stabilizer codes. In particular, the circuit construction and correction procedure only depend on the code distance and the weights of the stabilizers. For a code with distance d, it uses d+1 ancilla qubits.

### Optimal recognition of exact free-fermion solutions for spin models

**Presenting Author**: Adrian Chapman, University of Sydney **Contributing Author(s)**: Steven T. Flammia

Finding exact solutions to spin models is a fundamental problem of many-body physics. A workhorse technique for many such exact solution methods is mapping to an effective description of noninteracting particles. The paradigmatic example of this method is the exact solution of the one dimensional XY model by mapping to free fermions via the Jordan-Wigner transformation. We connect the general problem of recognizing models which can be exactly solved in this way to the graph-theoretic problem of recognizing line graphs, which has been solved optimally. Our solution method captures an entire class of spin models which can be described by dynamical fermions coupled to Pauli symmetries. We give an example of a previously unsolved spin model and demonstrate its exact solution by our method. We close by showing how these techniques can be used to design new fermion-to-qubit mappings.

### Demonstration of robust one-way EPR steering with polarization entangled photons

**Presenting Author**: Ivy Chen, Harvey Mudd College **Contributing Author(s)**: Ava Sherry, Helen Chaffee, Nick Koskelo, Lorenzo Calvano,Theresa Lynn

EPR steering is a signature of a class of two-qubit states for which one party, who holds one qubit of the pair, can prove to the possessor of the other qubit that their states are entangled. The protocol requires many copies of the two-qubit state, and involves the first party demonstrating that their measurement choices and results alter the one-qubit state held by the second party, thus “steering” the second party’s state. EPR steerable states form a strict superset of Bell nonlocal states, since they include states with too little entanglement to be Bell nonlocal. Surprisingly, given the mutual nature of bipartite entanglement, certain two-qubit states are actually one-way steerable, with Alice being able to “steer” Bob, but not vice versa. For a large set of mixed states, we show experimental results for one-way EPR steering by certifying that no local hidden state model can produce the measured correlations across multiple bases. EPR steering, both mutual and one-way, could be useful as a signature of partial entanglement in a variety of quantum communication or distributed quantum computing schemes. One-way steering has the potential for further application in communication protocols where the level of trust is asymmetric between the parties.

### A theoretical analysis of the power of pausing

**Presenting Author**: Huo Chen, University of Southern California **Contributing Author(s)**: Daniel Lidar

Recent experimental results have shown that adding a pause during quantum annealing can significantly improve the success probability for certain hard optimization problems. An optimal pausing position, where the maximum performance improvement compared to the unpaused case is achieved, has also been observed. In this work, we present a theoretical analysis that explains these observations. We identify the key features of examples known empirically to benefit from pausing. Using these features as building blocks, we then construct a toy model with a simple analytic structure. Using this model, we derive, in an open quantum system setting, a set of sufficient conditions for which an optimal pausing position exists.

### Quantum dynamical complexity and reliability of analog quantum simulation

**Presenting Author**: Karthik Chinni, University of New Mexico CQuIC **Contributing Author(s)**: Pablo Poggi, Ivan Deutsch

The NISQ era is characterized by the absence of fully fault-tolerant quantum simulators, which raises a question about the reliability of such devices. To address this, we seek to quantify the reliability of an analog quantum simulator, which doesn’t have access to error correction, in the presence of perturbations that make the dynamics quantum chaotic. In doing so we seek to identify the relationship between the robustness of the quantities that we seek to extract from the simulator and the dynamical complexity of the analog evolution. As one measure, we quantify the complexity by the number of variables that one must track to approximately yield the output within a desired accuracy. We address these questions by studying the basic paradigms such as the ground state and the excited state quantum phase transitions in the Lipkin-Meshkov-Glick (LMG) model[1]. References:[1]Marco Távora, and Francisco Pérez-Bernal. "Excited-state quantum phase transitions in many-body systems with infinite-range interaction: Localization, dynamics, and bifurcation." Physical Review A 94.1 (2016): 012113

### Approximating finite-temperature spectral functions on quantum computers

**Presenting Author**: Jeffrey Cohn, IBM **Contributing Author(s)**: Khadijeh Sona Najafi, Barbara Jones, James Freericks

Dynamic correlation functions such as single particle Green's functions, linear response functions, or dynamical susceptibilities serve as a foundational tool kit for studying strongly correlated quantum systems. Ideally, a quantum computer will be able to extract these functions for systems sizes that are intractable on classical computers. When it comes to studying these functions at finite temperature the main bottleneck comes from the resource overhead and circuit complexities required in preparing each Gibbs sample. We present a framework aimed at alleviating this bottleneck by optimizing a series of approximations. Specifically, we sample from a series of time averaged embedded clusters initially in their respective local Gibbs states. After extracting each approximate dynamic correlation function we employ Richardson extrapolation where the error expanded in the series is determined by the total number of sub-clusters used in each approximation. We obtain higher order estimates for each distinct path of approximations. We can optimize even further by weighting each distinct path by how closely each path fits the proper fluctuation theorem. We demonstrate this toolbox numerically using exact diagonalization of the Hubbard model on small clusters. Our results show strong evidence that this framework will be a desirable tool as quantum computers begin to scale.

### The quantum alternating operator Ansatz on max-k vertex cover

**Presenting Author**: Jeremy Cook, Los Alamos National Laboratory **Contributing Author(s)**: Stephan Eidenbenz, Andreas Bärtschi

We study the performance of the Quantum Alternating Operator Ansatz (a generalization of the QAOA for problems with hard constraints) on the problem of Max-k Vertex Cover due to its modest complexity, while still being more complex than the well studied problems of Max-Cut and Max-E3LIN2. Our approach includes (i) a performance comparison between easy-to-prepare classical states and Dicke states, (ii) a performance comparison between two XY-Hamiltonian mixing operators: the ring mixer and the complete graph mixer, (iii) an analysis of the distribution of solutions via Monte Carlo sampling, and (iv) the exploration of efficient angle selection strategies. Our results are: (i) Dicke states improve performance compared to easy-to-prepare classical states, (ii) an upper bound on the simulation of the complete graph mixer, (iii) the complete graph mixer improves performance relative to the ring mixer, (iv) the standard deviation on the distribution of solutions decreases exponentially in p (the number of rounds in the algorithm), requiring an exponential number of random samples find a better solution in the next round, and (iv) a correlation of angle parameters which exhibit high quality solutions that behave similarly to a discretized version of the Quantum Adiabatic Algorithm.

### Environment noise analysis and real-time decoupling feedback control for an NV center

**Presenting Author**: Arshag Danageozian, Louisiana State University **Contributing Author(s)**: Nathaniel Miller, Pratik Barge, Narayan Bhusal, Jonathan Dowling

Nitrogen Vacancy (NV) centers in diamond have been seeing increasing attention in applications of quantum theory and especially in quantum information processing. To understand the effects of the diamond lattice on the NV center, we analyze the noise due to the nuclear spins of the Carbon 13 atoms (that's about 1% of the Carbon atoms in the lattice) in the diamond lattice using the phenomenon of Coherent Population Trapping and then we find the optimum dynamical decoupling pulse sequence that prolongs the decoherence of a computational qubit made out of the nuclear spin degree of freedom of the Nitrogen 15 atom in the NV center.

### Symmetric Bell tests witnessing quantum advantage

**Presenting Author**: Austin Daniel, University of New Mexico CQuIC **Contributing Author(s)**: Sri Datta Vikas Buchemmavari, Akimasa Miyake

A major benchmark for near-term quantum computers is the demonstration of an advantage over their classical counterparts. This is a daunting task due to the imperfect nature of NISQ devices, but not all is lost. Recent light has been shed on provable, unconditional separations between the power of constant-depth quantum and classical circuits. These rest on the inability of shallow classical circuits to mimic non-local quantum correlations present in quantum games. In this regard, we leverage symmetry properties of graph states to design a family of quantum games with strategies that are robust to certain noise inherent to ion-trap computers making this an interesting application to test on a real device; namely the STAQ device, a surface trap-based quantum computer. To each game we attribute a Bell-type inequality whose violation witnesses the quantum advantage and study the robustness of the violation to noise.

### Single molecule magnets for spintronics and quantum information processing

**Presenting Author**: Richard Escalante, Universidad Catolica de Chile **Contributing Author(s)**: Enrique Rodriguez, Ivan Gonzalez, Eduardo Ortega, Thomas Moller, Pablo Fuentealba, Jeronimo Maze

We are investigating single molecule magnets to see if we can manipulate and detect their spin by monitoring their luminescence. This is similar to the nitrogen vacancy center defect in diamond, which can be thought of as an artificial molecule. We would like to find a single molecule which exhibits similar behavior. The other line of work we would like to take is use the NV center as a means of detecting molecules which do not exhibit this spin dependent luminescence. This would be done by measuring the longitudinal relaxation rate of the NV in the presence of these molecules while under an externally applied magnetic field. This relaxation would increase when the energy transitions in the molecule and NV are on resonance from the Zeeman splitting.

### Charge state instabilities in shallow NV centers for quantum sensing

**Presenting Author**: Mattias Fitzpatrick, Princeton University **Contributing Author(s)**: Zhiyang Yuan, Nathalie P. de Leon

Nitrogen Vacancy (NV) centers in diamond are a promising platform for nanoscale sensing, quantum information processing, and quantum networks. For most sensing applications, due to the decay of target signal outside the diamond, it is advantageous to have NV centers as close as possible to the diamond surface. However, it has been observed that for shallow NV centers, the measurement contrast for Rabi experiments and optically detected magnetic resonance (ODMR) is worse than that for bulk NV centers. Here we demonstrate that the degradation of shallow NV centers Rabi and ODMR contrasts can be associated with dynamics between the two charge states of the NV center (NV$^0$ and NV$^-$). We validate this claim by comparing two distinctly different diamond samples with shallow NVs, one which has charge-state stable NVs and the other with measurably less stable NVs. We measure NV spectra to compare the equilibrium charge state population for NVs in these two samples. Charge state conversion measurements are performed to extract the ionization and recombination rates in the dark and under both green (\unit{532}{\nano\meter}) and orange laser (\unit{590}{\nano\meter}) illumination. Finally, to understand how the charge state population and dynamics are influencing the ESR contrasts, we use time-resolved measurement of NV fluorescence and develop a model of the spin states of NV$^-$ and the NV$^0$ charge state. By fitting the measured fluorescence as function of time to our model, we deduce that the primary cause for lower ESR and Rabi contrast is an increase in the NV$^0$ population and rapid spin-nonconserving charge state conversion at higher laser powers. This research was supported by an appointment to the Intelligence Community Postdoctoral Research Fellowship Program at the Princeton University by Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the U.S. Department of Energy and the Office of the Director of National Intelligence (ODNI).

### Reducing ion measurement errors with Bayesian techniques

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

We describe a measurement protocol for trapped ions using repeated measurements. By employing the non-demolition nature of measurement of a fluorescence-based qubit, we can make repeated measurements to extract more information out of the system. Ideally, one would perform a long measurement where we can collect many photons, and then perform a hypothesis test to determine the state of the ion. However, there are spurious depumping and repumping effects that take the qubit into and out of the many other states in the hyperfine manifold during measurement. By using a Hidden Markov Model, we can efficiently estimate the probability that there was an unwanted state transition during measurement. We furthermore leverage the microwave control of the qubit to induce state transitions that will minimize the probability of error, which we calculate using Bayesian techniques.

### Probing quantum statistics with coherent control of rotating ion crystals

**Presenting Author**: Neil Glikin, University of California Berkeley **Contributing Author(s)**: Erik Urban, Sara Mouradian, Hartmut Haeffner

The symmetrization of identical quantum particles has been elegantly demonstrated in many ways, including by the creation of degenerate quantum gases, by direct spectroscopy of systems of molecules, and by Hong-Ou-Mandel-type interference. I will outline a unique demonstration of the symmetrization postulate by way of direct, coherent exchange of a single pair of trapped ions, and describe experiments and progress towards this goal. Such an exchange can be performed by preparing a coherent superposition of many-body rotational states and waiting for an appropriate exchange time. We create a planar quantum rotor with a pair of calcium-40 ions in a circularly symmetric surface-electrode Paul trap, and demonstrate coherent control of the state of this rotor. We furthermore study the decoherence of such states in the presence of environmental noise, finding agreement with recent theory of open quantum rotor systems. By moving into a regime of long rotational coherence times, we aim to use this control to perform direct particle exchange, allowing us to study the emergence of indistinguishability even for particles which are always separated by many micrometers.

Read this article online: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.133202

### Towards low motional heating of surface trap with in-situ argon treatment

**Presenting Author**: Nicole Greene, University of California Berkeley **Contributing Author(s)**: Eli Megidish Sara Mouradian

Surface Paul traps suffer from excessive electric field noise which limits single and multi qubit gate fidelities. While the physical mechanism for this heating is still under extensive research it has been shown that surface treatment using ion bombardment can reduce this effect by two orders of magnitude. In this work we have built a vacuum setup that is compatible with the HOA-2.1 surface trap from Sandia National Laboratory and have characterized the trap at room temperature with Ca ions. In addition, we have the ability for *in-situ* treatment of the surface. We believe that this treatment capability will enable manipulating larger quantum systems in room temperature surface traps.

### Efficient simulation of non-Markovian qubit trajectories

**Presenting Author**: Sacha Greenfield, Chapman University **Contributing Author(s)**: Shiva L. Barzili, Justin Dressel

Superconducting transmon qubits are measured by coupling them to a resonator and monitoring the leaked microwave field continuously in time. In the bad-cavity regime, we can treat the resonator as a steady-state bath that produces Markovian quantum trajectories of the qubit. However, when not at steady-state, such as during ring-up and ring-down transients of the resonator, this measurement process generally allows information to flow back from the resonator to the qubit, which causes non-Markovian dynamics at the level of the qubit. This more general situation has now become relevant for the experimental community, so we consider how to efficiently simulate such non-Markovian trajectories for comparison with experimental data.

### Error mitigation of superconducting qubits in generative modeling benchmark tasks

**Presenting Author**: Kathleen Hamilton, Oak Ridge National Laboratory **Contributing Author(s)**: Raphael C. Pooser

Application-based benchmarks for near-term quantum devices rely on multi-component computational tasks to evaluate hardware capabilities. We have used the gradient-based training of parameterized circuit models to quantify the performance of noisy superconducting qubits for generative modeling tasks [1]. A classical gradient-based optimizer (e.g. Adam) is used to train the circuit parameters and the quantum hardware is used to evaluate the loss function gradient. The difficulty of this benchmark is partially due to the number of noisy quantum circuit executions required to evaluate the gradient function. The incorporation of error mitigation into a gradient-based training workflow is a non-trivial task. Error mitigation has the potential to speed up gradient-based training by reducing spurious information passed to a classical optimization method; but the loss of relevant information can impair training. In this talk we will present results for matrix-based error mitigation used inside gradient-based training of circuits trained on superconducting qubits, highlighting specific use cases and discussing several challenges. [1] Hamilton, Kathleen E., Eugene F. Dumitrescu, and Raphael C. Pooser. "Generative model benchmarks for superconducting qubits." Physical Review A 99.6 (2019): 062323. This work was supported as part of the ASCR Testbed Pathfinder Program at Oak Ridge National Laboratory under FWP #ERKJ332

### Many-body entanglement and truncated moment problems

**Presenting Author**: Bharath Hebbe Madhusudhana, Max Planck Institute for Quantum Optics **Contributing Author(s)**: Grigoriy Blekherman

Preparing, measuring and controlling many-body entangled states is a central challenge in quantum technologies. The problem of deciding whether a quantum state is entangled, in general, is known to be NP hard. Nevertheless, realistically, tomography of large many-body systems are infeasible. Therefore, the mathematical challenge in this problem is to develop criteria to decide whether a many-body state prepared in the lab is entangled based on a small number of observables that could be measured in the lab. We show that this problem is related to the so-called truncated moment problem, well known in convex algebraic geometry. The space of unentangled mixed states is convex and so is the space of the corresponding observable values. Therefore, the problem of determining whether a set of observable values could correspond to an unentangled state is tantamount to checking for membership in a convex set, also known as a moment cone, of a point with coordinates given by the set of observable values. The latter is an instance of the truncated moment problem. Here, we adapt techniques from convex algebraic geometry to develop necessary and sufficient criteria for entanglement in a many-body system of bosonic atoms with a non-zero spin [1]. We also show that these criteria are asymptotically tight, in the number of atoms. [1] G. Blekherman and H. M. Bharath, “Quantum entanglement, nonnegative polynomials and moment problems ”, arXiv: 1904.00072

Read this article online: https://arxiv.org/abs/1904.00072

### Deterministic squeezing and closed-loop magnetometry with a collective atomic spin

**Presenting Author**: Daniel Hemmer, University of Arizona **Contributing Author(s)**: David Melchior, Ezad Shojaee, Ivan Deutsch, 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 from a million spin-4 Cs atoms. Through quantum backaction this measurement generates upwards of 4.5 dB metrologically relevant spin squeezing. By introducing real-time feedback, we can use the collective spin to perform precision magnetometry with resolution below the standard quantum limit. In the past our ability to leverage squeezing and quantum feedback has been limited by a noisy field environment. We have therefore rebuilt our experiment inside a multi-layered magnetic shield of mu-metal and aluminum, where background fields are reduced by a factor >10,000 at frequencies up to tens of kHz. We report on progress towards composite-pulse control, which will allow us to detect and correct classical control errors such as noisy rotations of the collective spin. The shielded environment will also allow us to better evaluate and optimize the performance of our closed-loop magnetometer.

### Quantum algorithms for physicists

**Presenting Author**: Anya Houk, California Polytechnic State University, San Luis Obispo **Contributing Author(s)**: Katharina Gillen

Quantum computing is a cross-disciplinary field, but members of different disciplines often hold different paradigms. There are not a lot of resources designed for translating concepts from one discipline to another. For example, with the direction the world of quantum computing is going physicists need to gain a better understanding of quantum algorithms in order to implement them on their devices. This can be especially important for undergraduate students as emerging careers in quantum computing open. In this project we, an undergraduate computer engineering student and a physics professor, started with a high-level description of Grover’s quantum search algorithm and learned how to break it into its major operations and step by step decomposed each into its single and two qubits gate level circuits. At this level of gate decomposition, physicist know how to physically implement these operations on existing systems like neutral atoms trapped by light and addressed by laser beams. We hope that our experience will be educational for others.

### Sparsity of the stabilizer projector decomposition of a density matrix and robustness of magic

**Presenting Author**: Yifei Huang, Tufts University **Contributing Author(s)**: Peter Love

We extend the stabilizer rank of state vectors to mixed states and define the rank(minimal l_0 norm) for stabilizer projector decomposition of a density matrix and show its advantage over the rank of Pauli decomposition. Both improvements on the scaling over standard orthonomal basis(computational basis for state vector and Pauli basis for density matrix) come from the fact that stabilizer states form a densely overcomplete basis. In comparison with Monte Carlo simulation that scales with Robustness of Magic(minimal l_1 norm), we analyse the strong simulation cost of noisy Clifford+T circuits with respect to the rank. Using results from compressed sensing, we explore the sparsity condition where the minimal l_0 and l_1 norm are reached at the same decomposition.

### Dynamics of silicon-vacancy color center in nanodiamonds

**Presenting Author**: Chunjing Jia, Stanford University **Contributing Author(s)**: Yan-Kai Tzeng, Yu Lin

Abstract: Diamond-based color centers have emerged for a variety of applications in quantum communication and quantum photonics, etc. Optical color center of SiV formed by one silicon atom sited in the middle of two vacancies shows great promise as a single-photon source. Unlike the NV center, which can be improved sufficiently for quantum applications by high-temperature annealing to mobilize only vacancies, SiV center may become unstable in diamond lattice under annealing. Upon annealing to temperatures up to 800 °C, both SiVs and vacancies would become mobile to the surface of diamond. It is unclear during the annealing process whether SiV or vacancy impurity migration dominates and the relative time-scale of the two processes. To unravel this problem, in this poster we will present our ab initio DFT calculations to quantify the dynamics of two impurities. Our theoretical results show that SiV centers can indeed be stabilized associated with the crystalline quality during the production of artificial SiV photonic centers. This is consistent with what have been found in the laser heating and annealing experiments conducted by our collaborators.

### Measurement of Dicke-narrowed optical transitions in warm alkali vapor for different buffer gas pressures

**Presenting Author**: Kefeng Jiang, Miami University **Contributing Author(s)**: Jianqiao Li, Linzhao Zhuo, Ken DeRose, Samir Bali

We demonstrate the quadratic dependence on the relative pump-probe beam angle of the electromagnetically induced transparency narrowed transition linewidth - a defining signature of Dicke narrowing of the optical transition linewidth. We vary the buffer gas pressure thus varying the atomic spatial localization and hence the size of the “quantization box” causing the Dicke narrowing. By carefully defining the zero-value for the relative angle where the linewidth is measured to be a minimum, we find that our data agrees with the theory better than ever before, with no fit-parameters. Funded by Army Research Office.

### Variational preparation of quantum Hall states on a lattice

**Presenting Author**: Eric Jones, Colorado School of Mines **Contributing Author(s)**: Eliot Kapit

Simulation of many-body quantum systems is one of the most promising applications of near-term quantum computers. The fractional quantum Hall states display fascinating many-body physics such as topological order and strong correlations and so are interesting candidates for quantum simulation experiments. We classically diagonalize for the low-energy spectrum of the Kapit-Mueller Hamiltonian for hardcore bosons on a lattice. The Laughlin state is an exact ground state of this long-range Hamiltonian for appropriate magnetic flux densities. In addition, we study the low-lying spectrum of a shorter-range proxy Hamiltonian and tune its hopping and interaction parameters in order to optimize the associated topological degeneracy and many-body gap. We then demonstrate a scheme for variational preparation of the Laughlin state on the lattice through a Trotterization of adiabatic state preparation with defect-pinned particles as the reference state. Such calculations suggest a way forward in the simulation of fractional quantum Hall states on quantum hardware.

### Dimensional reduction without symmetries: an algorithm for finding irreducible representations of operators and its applications

**Presenting Author**: Oleg Kabernik, University of British Columbia **Contributing Author(s)**: Jason Pollack, Ashmeet Singh

We propose a general, not inherently numeric, algorithm for finding irreducible representations of operator algebras in applications where symmetry considerations may fall short. This algorithm is motivated by applications where, given a set of operators that is not necessarily characterized by any known symmetries, it is essential to identify the invariant subspaces on which all these operators act irreducibly. One such application is the problem of identifying how the quantum state reduces given a restriction on observables that can be measured. Such state reductions can account for limited resolution measurements, inability to individually address the particles, inaccessible external environments, lack of reference frames or any other scenario where operational constraints can be specified as a restriction of the allowed observables. Another potential application is reduction of dynamics that does not rely on symmetries to identify the simplifying structure. With this approach we can reduce the dynamics by considering the algebras generated by the operators from which the Hamiltonian or the quantum channel is constructed. In particular, we will show how the action of parameterized Hamiltonians with “tunable” terms can be reduced to lower dimensional (virtual) subsystems if the algebra generated by the terms is not trivial.

Read this article online: https://arxiv.org/abs/1909.12851

### Real-time characterization of time-dependent noise with deep reinforcement learning

**Presenting Author**: Danial Khosravani, Georgia Institute of Technology **Contributing Author(s)**: Kenneth R. Brown

NISQ-era quantum processors are afflicted by both time-dependent Markovian and non-Markovian noise. Characterizing the time-dependent noise on a quantum computer can help with designing optimal 1,2-qubit gates, improving quantum error-correction codes and increasing the depth of quantum circuits on near-term quantum devices. Common methods for noise characterization are not designed for a general time-dependent noise and the existing quantum spectrum analyzer methods are not suitable for real-time characterization. Here we formulate the problem as a partially observed Markov decision processes (POMDP) and utilize deep reinforcement learning to construct a qubit which can learn a general time-dependent noise model by real-time adaptation of dynamical decoupling and Ramsey measurements. We show that our model is capable of real-time learning of Lindblad equation. Finally we extend this to two qubits and show that the method can be used to obtain spatially correlated time-dependent noise in real-time. We conduct simulations for various scenarios and provide optimality bounds for the specific case of time-dependent noise with bounded two-point correlation.

### Noncontextuality as classicality in variational quantum eigensolvers

**Presenting Author**: William Kirby, Tufts University **Contributing Author(s)**: Peter Love

In this talk we show how to use contextuality, an indicator of non-classicality in quantum systems, to evaluate the variational quantum eigensolver (VQE), a promising tool for near-term quantum simulation. We present an efficiently computable test to determine whether or not the Hamiltonian in a VQE procedure is contextual. We then show that we may construct a simple, global unitary mapping that diagonalizes a noncontextual Hamiltonian. The diagonal Hamiltonian resulting from this mapping is efficiently classically calculable, which proves that the noncontextual Hamiltonian problem is NP-complete. We also give a quasi-quantized model for variational quantum eigensolvers whose Hamiltonians are noncontextual. This provides a second sense in which noncontextual Hamiltonians are essentially classical. These results support the notion of noncontextuality as classicality in quantum systems.

Read this article online: https://arxiv.org/abs/1904.02260

### Solving 3-SAT through simulated quantum evolution

**Presenting Author**: Trevor Kling, Chapman University **Contributing Author(s)**: Dr. Justin Dressel

In 2008, Aharonov et al. proved that the field of adiabatic quantum computation is equivalent to standard quantum computation. We present an explicit analysis of how such adiabatic computation can be used to solve a particular NP-complete problem: the 3-SAT problem.The boolean satisfiability problem of 3 variables, known as 3-SAT, is a simple NP-complete problem with applications in bounded model checking and product configurations. We analyze a quantum algorithm that computes a maximally correct set of boolean values via Hamiltonian evolution, both in an analog quantum device and in a simulated quantum circuit model.

### Cost of classical strong simulation of the T-gate magic state

**Presenting Author**: Lucas Kocia, Sandia National Laboratories **Contributing Author(s)**: Peter Love, Mohan Sarovar

The stabilizer rank of qubit T-gate magic state has been postulated to grow slowest with increasing number of qubits, suggesting that the T-gate is in this sense the most efficient state outside the Clifford subtheory that can be simulated by classical strong simulation that nevertheless extends this subtheory to quantum universality. Unfortunately, the T-gate magic state’s stabilizer rank scaling is not formally known and has only been found numerically for up to seven qubits. We examine this problem from the perspective of the cost of strong classical simulation of discrete Wigner functions in systems with odd dimension and compare with the known results for qubits. To accomplish this, we develop and exploit relationships between the number of critical points of quantum states’ Wigner functions in a periodized stationary phase approximation and spanning decompositions of stabilizer states that are closely related to the stabilizer rank. We report on the trends we observe. Disclaimer: 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-NA-0003525.

### Reproducing the performance enhancement of adiabatic reverse annealing

**Presenting Author**: Matthew Kowalsky, University of Southern California **Contributing Author(s)**: Tameem Albash

We propose a conventional (forward) quantum annealing protocol with a diagonal catalyst of programmable strength λ. By adjusting λ we demonstrate an exponential improvement in the efficiency of quantum annealing for solving the p-spin model, reproducing the performance improvements of adiabatic reverse annealing(1). Our protocol allows us to identify the enhancement mechanism of such approaches: balancing Hamiltonian terms in a way that mitigates discontinuous shifts in the global minimum of the semiclassical potential. This observation allows us to identify and solve other problems amenable to such protocols. 1.Ohkuwa, M., Nishimori, H. & Lidar, D. A. Reverse annealing for the fully connected $p$-spin model. Phys. Rev. A 98, 022314 (2018).

### Constraints on photon loss for continuous-variable entanglement swapping using Gaussian resources

**Presenting Author**: Alex Kwiatkowski, University of Colorado Boulder, NIST **Contributing Author(s)**: Ezad Shojaee, Scott Glancy, Emanuel Knill

In many procedures for entanglement swapping in the continuous variable domain the entangling measurement is quadrature measurement of two modes combined on a beamsplitter, where the two modes are sent by the two parties that wish to swap entanglement. In the presence of sufficient photon loss however, we show that such measurements cannot be used to swap entanglement. We show analytically that if the modes sent by the two parties suffer losses that average between them to 50% or more, then any linear optical circuit followed by quadrature measurement has a separable POVM on those modes and therefore cannot be used to perform entanglement swapping. We also show that the result holds even if the measurement circuit includes ancillas prepared in arbitrary Gaussian states, and conclude that the average loss threshold of 50% must be met in order to perform entanglement swapping with Gaussian resources alone.

### High-fidelity quantum state estimation via autoencoder tomography

**Presenting Author**: Shiva L. Barzili, Chapman University **Contributing Author(s)**: Noah Stevenson, Brad Mitchell, Razieh Mohseninia, Irfan Siddiqi, Justin Dressel

We investigate the use of supervised machine learning, in the form of a denoising autoencoder, to simultaneously remove experimental noise while encoding one- and two-qubit quantum state estimates into a minimum number of nodes within the latent layer of the neural network. We decode these latent representations into positive density matrices and compare them to similar estimates obtained via linear inversion and maximum likelihood estimation. Using a superconducting multiqubit chip we experimentally verify that the neural network estimates the quantum state with greater fidelity than either traditional method. Furthermore, we show that the network can be trained using only product states and still achieve high fidelity for entangled states. This simplification of the training overhead permits the network to aid experimental calibration, such as the diagnosis of multi-qubit crosstalk.

### Diagnosing gate faults in quantum circuits using machine learning

**Presenting Author**: Margarite LaBorde, Louisiana State University **Contributing Author(s)**: Allee Rogers, Jonathan Dowling

We propose a procedure to diagnose where gate faults occur in a given circuit using a hybridize quantum-and-classical K-Nearest-Neighbors (KNN) machine learning technique. This is accomplished using a diagnostic circuit and selected input qubits to obtain the fidelity between output states of the altered circuit and a set of given reference states, providing a quantum analogy to the Euclidean distances used for KNN classification algorithms. The outcomes of the quantum circuit can then be stored to be used for a classical KNN algorithm. We demonstrate numerically an ability to locate a faulty gate in circuits with over 30 gates and up to 9 qubits with over 90% accuracy

### Bounds on relative entropy decay

**Presenting Author**: Nicholas LaRacuente, University of Illinois at Urbana-Champaign **Contributing Author(s)**: Li Gao, Marius Junge, Haojian Li

I will summarize our recent progress bounding the decay rates of relative entropy under different classes of quantum channels. These include modified logarithmic Sobolev inequalities for quantum Markov semigroups, their tensor-stable complete versions, state-independent comparisons of the relative entropy of channel outputs, and variations on subadditivity. The relative entropy underpins many resource measures and common entropy expressions, including coherence, asymmetry, and a variety of communication rates. Relative entropy generalizes naturally to infinite-dimensional settings, such as field theories in which the ordinary von Neumann entropy diverges. We use techniques from operator algebras and noncommutative geoemetry, as well as more familiar information-theoretic and variational approaches. Finally, I will discuss how the common form of relative entropy with respect to subalgebras or subspaces implies that particular classes of results automatically apply across a wide variety of resources and operational scenarios.

Read this article online: https://arxiv.org/abs/1909.01906, https://arxiv.org/abs/1807.08838

### Variational quantum linear solver: A hybrid algorithm for linear systems

**Presenting Author**: Ryan LaRose, Michigan State University, NASA - Ames Research Center **Contributing Author(s)**: Carlos Bravo-Prieto, M. Cerezo, Yigit Subasi, Lukasz Cincio, Patrick J. Coles

Solving linear systems of equations is central to many engineering and scientific fields. Several quantum algorithms have been proposed for linear systems, where the goal is to prepare $\ket{x}$ such that $A\ket{x} \propto \ket{b}$. While these algorithms are promising, the time horizon for their implementation is long due to the required quantum circuit depth. In this work, we propose a variational hybrid quantum-classical algorithm for solving linear systems, with the aim of reducing the circuit depth and doing much of the computation classically. We propose a cost function based on the overlap between $\ket{b}$ and $A\ket{x}$, and we derive an operational meaning for this cost in terms of the solution precision $\epsilon$. We also introduce a quantum circuit to estimate this cost, while showing that this cost cannot be efficiently estimated classically. Using Rigetti's quantum computer, we successfully implement our algorithm up to a problem size of $32 \times 32$. Furthermore, we numerically find that the complexity of our algorithm scales efficiently in both $1/\epsilon$ and $\kappa$, with $\kappa$ the condition number of $A$. Our algorithm provides a heuristic for quantum linear systems that could make this application more near term.

Read this article online: https://arxiv.org/abs/1909.05820

### Interferometric extraction of the exchange symmetry of a biphoton joint spectral amplitude

**Presenting Author**: Cody Leary, The College of Wooster **Contributing Author(s)**: Michael Raymer, Brian Smith, Andrew Marcus

We calculate output coincidence rates for a biphoton state propagating through a Mach-Zehnder interferometer, with delay stages present both before and within the device. A time-dependent relative phase between interferometer paths, supplied by the presence of two acousto-optical modulators, gives rise to a time-dependent coincidence rate containing multiple oscillation frequencies. We show that frequency decomposition of the coincidence signal allows for the extraction of amplitude and relative phase information concerning the symmetric and antisymmetric components of the joint spectral amplitude of the biphoton input source.

### Plasmonic-assisted ultrabright and fast emission of SiV in diamond for quantum applications

**Presenting Author**: Shuo Li, Stanford University **Contributing Author(s)**: Alison Rugar, Dan Kohler, Shuo Sun, Zachary Jones, Jelena Vučković, Nicholas Melosh, Robert Hamers

Color centers in diamond are promising candidates for emerging quantum sensing and quantum information processing applications due to their exceptional properties. Coupling diamond color centers with metal plasmonic nanostructures is reported to enhance the optical properties of color centers effectively, but the plasmonic metal structures of previous studies are vulnerable to the environment, making the diamond-metal system unstable. Here, we demonstrate a novel and highly-tunable structure of Ag nanospheres coated with a layer of diamond containing SiV centers. The Ag, protected by the stable diamond shell, enhances the emission rate and brightness of SiV in diamond drastically. The structure also shows great stability to extreme corrosive and high-temperature conditions, which is promising for applications in ultrabright quantum sensing and high-efficiency magnetometry.

### Bounds on classical simulation of simple quantum models from quantum supremacy.

**Presenting Author**: Peter Love, Tufts University

Quantum simulation is a central application of quantum computing. Quantum supremacy defines systems whose efficient classical simulation is unlikely given complexity-theoretic assumptions. We consider simple physically motivated quantum models that can display quantum supremacy and hence whose efficient simulation by classical means is unlikely. We consider quantum extensions of classical hydrodynamic lattice gas models. We find that the existence of local conserved quantities strongly constrains such extensions. We find the only extensions that retain local conserved quantities correspond to changing the local encoding of a subset of the bits. These models maintain separability of the state throughout the evolution and are thus efficiently classically simulable. We then consider evolution of these models in the case where any of the bits can be encoded and measured in one of two local bases. For quantum extensions of classical models that are computationally universal such quantum extensions can encode Simon’s algorithm and demonstrate quantum supremacy, thus presenting an obstacle to efficient classical simulation.

Read this article online: https://doi.org/10.3390/condmat4020048

### Observation of a non-local reality in local post-selected systems via variable strength measurements

**Presenting Author**: Noah Lupu-Gladstein, University of Toronto **Contributing Author(s)**: Ou Teen Arthur Pang, Hugo Ferretti, Weng-Kian Tham, Kent Bonsma-Fisher, Aharon Brodutch, Aephraim M. Steinberg

We implement a variant of the quantum pigeonhole paradox experiment to study the relation between measurement results and physical reality. Our results highlight the subtleties of quantum measurements on pre- and post-selected systems (i.e., those with a fixed past and future). By implementing two different strategies to measure the same observable at different strengths we show that weak values, the ensemble averaged results of weak measurements, can be treated as consistent objective properties of the system. Furthermore, we observe how the procedure that leads to a complex weak value can (and should) be extended to the strong regime, albeit with an unclear interpretation. Finally, we show that for fixed pre and post selections, some operators can be replaced by real numbers when used in a Hamiltonian. Surprisingly, these can be non-local even when the pre and post selected states are completely local. To measure such non-local observables we implement the first variable strength non-local measurements using a technique that can be extended to other optical scenarios that require effective three body Hamiltonians.

### The impact of errors in the operation of a small, highly-accurate quantum simulator

**Presenting Author**: Nathan Lysne, University of Arizona **Contributing Author(s)**: Kevin Kuper, Pablo Poggi, Ivan Deutsch, Poul Jessen

Quantum simulation is widely considered a promising application of NISQ processors. Recently, these devices have reached a scale where their operation cannot be validated by direct comparison against classical computation. At the same time, the accuracy of a quantum simulation running on NISQ-era hardware is likely to degrade after only a few time steps, especially when applied to complex systems whose dynamics exhibit quantum chaos and hypersensitivity to errors. This leaves open the question whether the output of a non-trivial quantum simulation on such a device will ever be useful. We have developed a small, highly-accurate, and fully programmable quantum processor capable of simulating >100 time steps with an average fidelity >0.99 per time step. Using this as our test bed, we investigate the interplay between native and Trotter errors in quantum simulation. Our preliminary findings shows that the optimal Trotter step is not the smallest possible, but instead represents a device-dependent compromise between Trotter and native errors. We also examine how sensitive different observables are to the presence of native errors and find quantitative differences based on their structure. Our results support the conjecture that “bulk” observables, (e. g., total magnetization) can be robust in the presence of errors, while observables tied to a specific quantum state (e. g., the fidelity) are not.

### Simulating a quantum heat engine on transmon qubits

**Presenting Author**: Nicholas Materise, Colorado School of Mines **Contributing Author(s)**: Eliot Kapit, Colorado School of Mines

We devise a scheme to simulate a quantum heat engine using in an array of flux-tunable transmon qubits. This device provides a one-to-one mapping to a fixed nearest-neighbor coupled, disordered Bose-Hubbard model, with precision control over transitions between the many body localized (MBL) phase and the thermal or superfluid phase [Nature 566, 51 (2019)]. Our approach expands on a previous MBL engine proposal [Phys.Rev.B 99, 024203 (2019)], where the adiabatic strokes involve transitions between the two nondegenerate ground states and first excited states of the MBL and thermal phases. We expect that this choice of energy levels reduces the likelihood of excursions to higher levels, especially in the case of the ground states, serving as a prototype for future investigations into studies of MBL as a thermodynamic resource.

### Variationally scheduled adiabatic quantum computing

**Presenting Author**: Shunji Matsuura, 1QBit

Solving quantum and classical optimization problems are one of the major applications of quantum computing. Some important limitations NISQ devices are the absence of error correction. Therefore, shortening the time taken to perform a single run of a quantum algorithm is essential for successfully obtaining accurate results. In this work, we consider AQC approach. In AQC, computational results highly depend on the annealing time and the annealing path. While a long annealing time causes accumulation of environment-induced errors, a small annealing time may cause harmful diabatic transitions. In order to avoid those problems, we propose to find optimal annealing paths variationally. We consider two variational methods. The first method introduces a new Hamiltonian which is characterized by variational parameters and has a finite coefficient only in the middle of annealing. The second method uses independent schedule functions for terms in the Hamiltonian. These schedule functions are determined variationally. In both methods, the required annealing time per single run can be reduced by some orders of magnitude. The major findings are: 1) Our variational methods provide accurate results even in the presence of noise, while the standard AQC fails to do so. 2) Our variational methods provide accurate results even when the final Hamiltonian on a quantum device has inaccurate couplings. 3) It can reduce the total annealing time compared to the standard AQC.

### Toward efficient LiYb molecule formation in an optical lattice

**Presenting Author**: Katherine McCormick, University of Washington **Contributing Author(s)**: Alaina Green, Jun Hui See Toh, Xinxin Tang, Yifei Bai, Subhadeep Gupta

Because of their potential for tunable, long-range interactions and rich energy-level structure, cold molecules are promising platforms for quantum computing, simulation, and metrology. In contrast to many cold molecule experiments, which use bi-alkali systems where the ground state is $^1\Sigma$, the LiYb molecule has a $2\Sigma$ ground state; this introduces a spin degree of freedom, which could prove useful for quantum information applications or for studies of spin-controlled chemistry. I will discuss recent experiments performed to identify and study the magnetic Feshbach resonances between $^{6}Li$ and $^{173}Yb$ and updates to the apparatus, including building a three-dimensional optical lattice and stabilizing the magnetic field, to efficiently and coherently form cold LiYb molecules.

### Mean field approximation for identical bosons on the complete graph

**Presenting Author**: Alexander Meill, Wentworth Institute of Technology **Contributing Author(s)**: David A. Meyer

Non-linear dynamics in the quantum random walk setting have been shown to enable conditional speedup of Grover's algorithm. We examine the mean field approximation required for the use of the Gross-Pitaevskii equation on identical bosons evolving on the complete graph. We show that the states of such systems are parameterized by the basis of Young diagrams and determine their one- and two-party marginals. We find that isolated particles are required for good agreement with the mean field approximation, proving that without isolated particles the matrix fidelity agreement is bounded from above by 1/2. This is the completion of work presented at SQuInT last year.

Read this article online: https://arxiv.org/abs/1910.14521

### The role of spectral entanglement in two- and four-photon interference

**Presenting Author**: Sofiane Merkouche, University of Oregon **Contributing Author(s)**: Valerian Thiel, Alex O. C. Davis, Brian J. Smith

We investigate theoretically and experimentally the role of spectral entanglement in two- and four-photon interference. Using a double-passed parametric down-conversion source, we observe several different interference effects in two-fold and four-fold coincidences, and identify which of these correspond to non-classical effects and non-local correlations. We use this scheme to implement an entanglement-swapping experiment in the spectral domain, and verify entanglement in the swapped state through Hong-Ou-Mandel interference. We show how these results are relevant to quantum communication and metrology protocols relying on information encoding in the time-frequency domain of light.

### Realizing quantum computation using continuous variables in trapped ions

**Presenting Author**: Jeremy Metzner, University of Oregon **Contributing Author(s)**: Alex Quinn, Daniel Moore, Vikram Sandhu, Dave Wineland, and David Allcock

Instead of encoding quantum information in the internal electronic “spin” states of trapped ions, it is possible to encode solely within the ions’ harmonic vibrational modes. Like spin qubits, this system is capable of universal quantum computation given an appropriate set of operations. Operations such as phase shift, displacement, squeezing[1], and “beam splitting”[2] have been performed quickly and with high fidelity, using only electric fields acting on the ions’ charge. Part of our investigation will be to put this set of operations together and demonstrate a basic computation algorithm. However, to have full realization of universal continuous variable quantum computing, there is a need for a non-gaussian operation. In order to produce such an operation, a Hamiltonian that is at least third order in the bosonic-mode raising and lowering operators, is needed. In principle higher order potentials can create this type of operation, for example, generation of a quartic potential will create a non-linear phase shift, but typical ion traps are inefficient at generating these potentials. We will be working on design and detailed simulations of microfabricated surface-electrode traps to see if this approach is feasible. [1]- S. C. Burd et al. Science Vol. 364, Issue 6446, pp. 1163 [2]- D. J. Gorman et al. Phys. Rev. A 89, 062332 (2014)

### Quantum-classical transition in analog quantum supremacy subject to Markovian decoherence

**Presenting Author**: Razieh Mohseninia, University of Southern California **Contributing Author(s)**: Milad Marvian, Daniel Lidar

Instantaneous Quantum Polynomial time circuits are a promising way to demonstrate quantum supremacy. We study the robustness of quantum supremacy in an analog Hamiltonian version of such circuits in the presence of a Markovian environment whose noise operators commute with the system Hamiltonian. We find a transition from a regime of quantum supremacy to classical simulability that occurs at a finite critical decoherence rate, that depends on the system size.

( Session 5 : Saturday from 5:00pm - 7:00pm)

### Metastable qubits in trapped Calcium-43 ions

**Presenting Author**: Isam Moore, University of Oregon **Contributing Author(s)**: Alexander Quinn, Jeremy Metzner, Vikram Sandhu, David Wineland, David Allcock

While all of the basic primitives required for universal quantum computing (QC) have been demonstrated in trapped-ion qubits with high fidelity, it is currently not possible to simultaneously realize the highest achieved fidelities in a single ion species. This can be a serious impediment to the development of practical quantum computers. However, there are possibilities for achieving high-fidelity and full functionality in a single species with the use of multiple internal levels: augmenting existing species with new functionality. Specifically, essential dual-species capabilities can be developed in the Calcium-43+ ion through novel encoding schemes in metastable states, allowing user-selectable, ion-specific activation of the necessary functions on demand (e.g. storage, coupling to motion, cooling, and state preparation and measurement). I will present simulation results and progress towards experimental implementation of high-fidelity preparation and readout procedures in metastable states of Calcium-43+.

### Markovian model of dynamical decoupling and other open system methods

**Presenting Author**: Evgeny Mozgunov, University of Southern California **Contributing Author(s)**: Daniel Lidar

The control of the qubits in the intermediate scale quantum devices is often done via microwave pulses. Qubits also interact with the environment that causes decoherence. We present a universal description of the interplay between the control and the decoherence. Our description is applicable for a range of realistic environments, including weak spin bath and bosonic bath with the Ohmic spectral density. A single markovian master equation is capable of describing error correction by encoding into a quantum code and by dynamical decoupling. This master equation is valid for an arbitrary control of the qubits.

Read this article online: https://arxiv.org/abs/1908.01095

### Quantum annealing with boundary canceling schedules

**Presenting Author**: Humberto Munoz Bauza, University of Southern California **Contributing Author(s)**: Lorenzo Campos Venuti, Daniel Lidar

The boundary cancellation theorem (BCT) for open systems bounds the distance between the final equilibrium state and the evolved state of an ergodic dissipative Liouvillian, where the scaling with anneal time depends on the number of vanishing derivatives of the annealing schedule at the end of the evolution. We test the thermal scaling of small gadgets up to 8 qubits on the D-Wave quantum annealer by controlling the vanishing derivatives of the schedule during the thermalization phase. While the theoretical BCT bounds are not accessible experimentally, we find that such schedules improve thermal scaling during the anneal and consequentially enhance the ground state probability of the gadgets.

### Quantum simulation of p-spin models using continuous measurements and feedback

**Presenting Author**: Manuel Munoz-Arias, University of New Mexico CQuIC **Contributing Author(s)**: Karthik Chinni, Tameem Albash, Pablo Poggi, Poul Jessen, Ivan Deutsch

Ferromagnetic p-spin models, which describe the collective dynamics of systems of spin-1/2 particles under p-body interactions, have attracted attention recently as a useful system for studying the performance of quantum annealing [1,2]. In this context, different values of p imply different levels of complexity for finding the ground state of the system via adiabatic evolution. In this work we use a scheme involving continuous quantum measurements of the collective spin of a system of spin-1/2 particles and a simple feedback loop, to realize quantum simulations of the dynamics of mean-field p-spin models [3]. This allows us to investigate how the intrinsic stochasticity of the quantum measurement induces spontaneous symmetry breaking for different values of p. Furthermore, we use measurement-based feedback to explore protocols for extracting the location of critical point from the quantum dynamics. Finally, we study the nonlinear dynamics, both regular and chaotic, emerging in the case of a kicked p-spin model, and present a realization of their dynamics within the scope of our measurement and feedback scheme [1] T. Jörg, et.al., 2010 EPL 89, 40004 [2] S. Matsuura, et.al. PRA 95, 022308 (2017) [3] M. Muñoz-Arias, P. Poggi, P. Jessen, I. Deutsch, arXiv:1907.12606

### Sampling complexity of interacting bosonic random walkers on a lattice

**Presenting Author**: Gopikrishnan Muraleedharan, University of New Mexico CQuIC **Contributing Author(s)**: Adrian Chapman, Sayonee Ray, Akimasa Miyake, Ivan Deutsch

A central goal for modern quantum information science is to demonstrate computational speedup for experimentally feasible architectures in the noisy intermediate-scale regime. In a previous work, we studied this problem in the context of simulating boson sampling by noninteracting bosonic atoms on a one-dimensional lattice [1]. We extend these results to include Bose-Hubbard-type interactions. In the presence of weak interactions, we show that the output-sampling distribution is close to that of a free-boson sampler in the total variational distance. We calculate the scaling of the interaction-strength such that this total variational distance is bounded by a constant, demonstrating a regime where the sampling complexity is equivalent to that of the corresponding boson sampler. Finally, we consider the possibility of applying worst-to-average-case reduction tools to extend these results beyond the perturbative regime. [1] G. Muraleedharan et al. NJP, 21(5), 055003.

### Characterizing quantum detectors by Wigner functions

**Presenting Author**: Rajveer Nehra, University of Virginia **Contributing Author(s)**: Kevin Valson Jacob

We propose a method for characterizing a photodetector by directly reconstructing the Wigner functions of the detector's Positive-Operator-Value-Measure (POVM) elements. This method extends the works of S. Wallentowitz and Vogel [Phys. Rev. A 53, 4528 (1996)] and Banaszek and Wódkiewicz [Phys. Rev. Lett. 76, 4344 (1996)] for quantum state tomography via weak-field homodyne technique to characterize quantum detectors. The scheme uses displaced thermal mixtures as probes to the detector and reconstructs the Wigner function of the photodetector POVM elements from its outcome statistics. In order to make the reconstruction robust to the experimental noise, we use techniques from quadratic convex optimization.

Read this article online: https://arxiv.org/abs/1909.10628

### Donor qubits in ZnO

**Presenting Author**: Vasileios Niaouris, University of Washington **Contributing Author(s)**: Xiayu Linpeng, Maria L.K. Viitaniemi, Aswin Vishnuradhan, Y. Kozuka, Cameron Johnson, M. Kawasaki, Kai-Mei C. Fu

A new spin qubit system for quantum network applications is an electron bound to a neutral donor (D0) in the direct bandgap semiconductor ZnO. In this system we seek to combine promising spin properties with strong radiative efficiency to an excited state. In the past two years, our group has demonstrated optical initialization and has measured the spin properties of Ga donor ensembles in ZnO: T1 = 140ms (X T) at B=2.25T and T=1.5K & T2 = 50+-13μs (Y T) at B=5T and T=5.5K [ref: Xiayu Linpeng et al., Coherence Properties of Shallow Donor Qubits in ZnO, Phys. Rev. Applied 10, 064061 (2018)]). We have also observed a B^-4 dependence on T1 which is inconsistent with our current understanding of spin-orbit coupling in the material. In this work we show that we are able to extend T1 to more than 800 ms and report on progress toward (1) understanding the longitudinal spin-relaxation mechanism and (2) integrating microwave control of the qubit ensembles.

### Homotopical approach to quantum contextuality

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

We consider the phenomenon of quantum mechanical contextuality, and specifically parity-based proofs thereof. Mermin’s square and star are representative examples. Part of the information invoked in such contextuality proofs is the commutativity structure among the pertaining observables. We investigate to which extent this commutativity structure alone determines the viability of a parity-based contextuality proof. We establish a topological criterion for this, generalizing an earlier result by Arkhipov.

Read this article online: https://arxiv.org/pdf/1905.03822.pdf

### Quantum walk with deterministic dynamical systems

**Presenting Author**: Sivaprasad Omanakuttan, University of New Mexico **Contributing Author(s)**: Prof. Arul Lakshminarayan

We studied the discrete-time quantum walks using simple deterministic dynamical systems like coins. The dynamical system under consideration, in the classical limit, shows a range of behavior from the integrable to chaotic, or deterministically random. For the integrable case, the Fourier coin that generalizes the Hadamard coin was studied and was found that the overall nature of the walk was similar to the well understood two-dimensional coin. It shows the famous ballistic growth of standard deviation, and hence the coin dimensionality has very little significance on the quantum walk. Using kicked Harper map as the deterministic model of random walk, we studied the effect of coin chaos on the quantum walk. We found that the walk becomes classical in the chaotic regime of the kicked Harper and hence the ballistic growth paves way to the diffusive one. The probability distribution in the chaotic regime was studied using coins from the Gaussian unitary ensemble (GUE) and we obtained an approximate expression that resembles the Gaussian distribution for the classical random walk. For a finite-dimensional walker with chaotic coin we obtained a classical to quantum transition in the behavior of the walker. The diffusive growth becomes ballistic after some time of unitary evolution and the time at which the transition occurs depends on the coin dimension but not on the walker dimension.

### Discrete and continuous distributions of magnetic fluxes in quantum mechanics

**Presenting Author**: Ismael Lucas Paiva, Chapman University **Contributing Author(s)**: Yakir Aharonov, Jeff Tollaksen, Mordecai Waegell

A study of the dynamics of quantum charges in the presence of magnetic fluxes in different configurations is presented. Then, it is shown that there is a sense in which the quantum treatment of discrete distributions of magnetic fluxes is analogous to continuous distributions of magnetic fields in classical physics. Finally, practical and theoretical consequences are discussed.

### Experimental comparison of Bohm-like theories with different ontologies

**Presenting Author**: Arthur Pang, University of Toronto **Contributing Author(s)**: Hugo Ferretti, Noah Lupu-Gladstein, Weng-Kian Tham, AharonBrodutch, Kent Bonsma-Fisher, J. E. Sipe, and Aephraim M. Steinberg

The de Broglie-Bohm theory is a hidden variable interpretation of quantum mechanics which involves particles moving through space with definite trajectories. This theory singles out position as the primary ontological variable. Mathematically, it is possible to construct a similar theory where particles are moving through momentum space, and momentum is singled out as the primary ontological variable. In this paper we experimentally show how the two theories lead to different ontological descriptions. We construct the putative particle trajectories for a two-slit experiment in both the position and momentum space theories by simulating particle dynamics with coherent light. Using a method for constructing trajectories through the primary and derived (i.e. non-primary) spaces, we compare the ontological pictures offered by the two theories and show that they do not agree. This contradictory behaviour brings into question which ontology for Bohmian mechanics is to be preferred.

Read this article online: https://arxiv.org/abs/1910.13405

### Robust preparation of many-body states in Jaynes-Cummings Lattices

**Presenting Author**: Prabin Parajuli, University of California, Merced **Contributing Author(s)**: Kang Cai, Gui Lu Long, C. W. Wong, and Lin Tian

Strongly-correlated polaritons in Jaynes-Cummings (JC) lattices at integer fillings can exhibit novel quantum phase transitions. However, it is often challenging to prepare such many-body ground states near the quantum critical points where the energy gap vanishes. Here we study the robust preparation of the many-body ground states of polaritons in a finite-sized JC lattice by combining the techniques of quantum state engineering and adiabatic evolution. In the deep Mott-insulating or deep superfluid regimes, the many-body ground states can be generated with high fidelity via quantum-engineered pulse sequences. Using these states as the initial state and tuning the system parameters adiabatically, the many-body ground states in the intermediate regime can be reached. We employ a nonlinear ramping scheme during the adiabatic evolution and find the optimal nonlinear index for achieving high fidelity. This study provides insights into the preparation of many-body states in artificial quantum systems, such as quantum simulators.

### Using measurement back action to change nonlinear open system dynamics

**Presenting Author**: Arjendu Pattanayak, Carleton College **Contributing Author(s)**: April Anderson, Sacha Greenfield, Yueheng Shi

Back-action induced by weak measurement can dramatically affect the quantum state dynamics of a nonlinear quantum system. We devise a measurement control protocol using the local oscillator phase $\phi$ for a homodyne measurement of a signal from the system. We use the the percentage of non-classical energy in the system $Q$ as a measure of quantumness, and find that adaptive control of $\phi$ increases $Q$ by several orders of magnitude. We explore using sinusoidally oscillating homodyne phases to drive the system, and discuss thermal engines and work extraction via measurement.

### Cavity QED of silicon vacancy centers with a silica optical microresonator

**Presenting Author**: Abigail Pauls, University of Oregon **Contributing Author(s)**: Ignas Lekavicius, Hailin Wang

Negatively charged silicon-vacancy (SiV) centers in diamond feature an inversion symmetry, making the SiV optical transitions robust against charge fluctuations in the surrounding environment. SiV centers have been successfully integrated into photonic crystals for cavity QED studies and for the development of optical quantum networks. Diamond photonic crystals, however, feature a relatively broad optical linewidth (~ 50 GHz), limiting this platform to the bad cavity limit in cavity QED. We report the development of an experimental platform combining a thin diamond membrane with a thickness of about 110 nm and a tunable silica optical microresonator with a diameter near 50 micrometers. This composite system can feature an optical cavity linewidth as narrow as 40 MHz, enabling the achievement of the good cavity limit in cavity QED. The diamond membranes were fabricated with reactive ion etching from a bulk film of electronic grade diamond. Photoluminescence excitation spectra of SiV centers at 10 K show linewidths ranging from 200 to 300 MHz for membranes as thin as 100 nm. For cavity QED studies, a 110 nm thick diamond membrane is in contact with a silica microsphere. Whispering gallery mode (WGM) linewidths near 40 MHz have been observed for the composite membrane-microsphere system. This composite cavity QED system provides a highly promising platform for pursuing cavity QED of SiV centers in the good cavity limit. This work is supported by NSF.

### Controlling phase diagram of finite spin-1/2 chains by tuning boundary interactions

**Presenting Author**: Cheng Peng, Stanford University

We propose a construct of finite size quantum spin-1/2 system, referred to as controllable spin wire (CSW), in which only Ising-type spin-spin interactions with a transverse field in the bulk, and multiple spin couplings as well as canted fields on the boundaries. The tunable Hamiltonians on the boundaries, we call them tuning Hamiltonians (TH’s), are used for determing physical properties in the bulk, including energy/entanglement spectrums, entanglement entropy, and magnetization. The CSW contains only nearest-neighboring interactions, which could be easily realized in artificial quantum circuits. Last but not least, we provides a possible scheme of studying infinite spin systems with only moderate CSW system.

Read this article online: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.085111

### Tunneling and entanglement oscillation dynamics in the (few) many-body kicked top

**Presenting Author**: Noah Pinkney, Carleton College **Contributing Author(s)**: Alex Kiral, Sudheesh Srivastava, and Arjendu K Pattanayak

Initially pure coherent spin states evolving in a many-body spin system behaving collectively as a quantum kicked top become periodically highly entangled before recohering — this is arguably generalized tunneling. We present techniques for determining the existence of such tunneling using measures of localization and linear entropy given an initial coherent state. We explore these dynamics in detail and in particular show that the locations of boundaries between different tunneling regimes reflect phase space structures (visualized through the Husimi representations) arising from symmetries of the kicked top Hamiltonian but are not directly connected to classical chaos. We apply spectral techniques to determine rates of tunneling as well as to explore the slowing of entanglement oscillations at specific values of a dynamical parameter. Our analysis thus expands upon prior work in many-body entanglement while clarifying the correlation between the structures of classical chaotic systems in phase space and our non-classical dynamics.

### Robustness and sensitivity to imperfections in the dynamics of general observables within near-term quantum simulators

**Presenting Author**: Pablo Poggi, University of New Mexico CQuIC **Contributing Author(s)**: Nathan Lysne, Kevin Kuper, Poul Jessen, Ivan Deutsch

We analyze how robust the output of analog quantum simulators is in the presence of weak perturbations. To this end, we focus on the dynamics of expectation values of general observables under the simulated dynamics and study the error arising from the imperfect operation of the device. We show that the properties of the error depend crucially on the observable considered as output of the simulator. and that even a low-fidelity quantum simulation is still able to reproduce the dynamics of other observables with a small relative error. We define the observable purity, which quantitatively characterizes the magnitude of the support of the observable in Hilbert space, and analytically show that, in general, these imperfect devices are able to reproduce the dynamics of low-purity observables (such as collective operators like the magnetization in a spin system) accurately, while the error in the expectation value of high-purity observables (such as projectors onto pure states) are much larger on average. We demonstrate the universality of these features by experimentally simulating various physical models of interest in a state-of- the-art 16-dimensional quantum simulator, which is universal and fully programmable by optimal control methods. Without assuming any particular error model, we show that the predicted behavior is generic for a highly accurate device where we expect imperfections to be small.

### Exploring phase transitions in a quantum dimer using a quantum annealer

**Presenting Author**: Bibek Pokharel, University of Southern California **Contributing Author(s)**: Kabuki Takada, Hidetoshi Nishimori, Daniel Lidar

It is well known that problems with exponentially decreasing gaps can cause adiabatic quantum computation (AQC) to fail. However, this does not imply that open-system quantum annealing (QA) will also fail for these problems. We consider an archetypical problem that is classically trivial, but difficult for AQC: an Ising ladder exhibiting a first-order phase transition with an exponentially closing gap, and a second-order phase transition with a polynomially closing gap. We investigate the performance of QA at these quantum phase transitions/ gap scalings theoretically using open quantum systems theory and experimentally using the D-Wave quantum annealer. === This material is based upon work supported by the Intelligence Advanced Research Projects Activity (IARPA) and the Army Research Office (ARO) under Contract No. W911NF-17-C-0050.

### Error suppression for quantum subsytem codes with weak measurement induced Zeno effect

**Presenting Author**: Bibek Pokharel, University of Southern California **Contributing Author(s)**: Gerardo Paz-Silva, Jason Dominy, Daniel Lidar

The goal of quantum error mitigation is to dampen and possibly eliminate, errors from quantum computation. While active quantum error correction involves a recovery step dependent on the errors identified, passive error mitigation is agnostic to the exact errors that occur during the computation. Weak measurement quantum Zeno effect (WMQZE) is a passive error mitigation method. Paz Silva et. al. [PRL, 108(8), p.080501] proved that weak measurement of stabilizers of a subspace code can induce a Zeno effect and correspondingly protect arbitrary encoded states to arbitrary accuracy while at the same time allowing for universal quantum computation. We extend their results to subsystem codes by proving that weak measurement of gauge operators, which form a non-Abelian group and are smaller in weight than the corresponding stabilizers of that code, can also protect arbitrary encoded states. We also numerically verify that for WMQZE-based protection works not just in the limit of infinitely many strong measurements, but also in experimentally accessible regimes. For the Bacon-Shor [[4,1,2]] code, we measure the gauge operators with varying measurement frequencies and strengths and report that in all cases attempted the protected state was more resistant to error than their unprotected counterparts.

### Robustly decorrelating errors with mixed quantum gates

**Presenting Author**: Anthony Polloreno, University of Colorado **Contributing Author(s)**: Kevin Young

Coherent errors in quantum operations are ubiquitous. Whether arising from spurious environmental couplings or errors in control fields, such errors can accumulate rapidly and degrade the performance of a quantum circuit significantly more than an average gate fidelity may indicate. Furthermore, coherent errors are considerably more difficult to model than stochastic errors, and understanding their impact on a generic quantum circuit or algorithm can be challenging. In this talk, we discuss using robust optimal control techniques to construct many different implementations of a target gate, each with a different coherent error. As Hastings and Campbell have recently shown, randomly sampling over that ensemble yields an effective quantum channel that well approximates the target, but with dramatically suppressed coherent error. Our results extend those of Hastings and Campbell to include robustness to drifting external control parameters. We have implemented these constructions using a superconducting qubit and will discuss randomized benchmarking results consistent with a marked reduction in coherent error.

### Ion-trapping lab setup for quantum information experiments

**Presenting Author**: Alex Quinn, University of Oregon **Contributing Author(s)**: Jeremy Metzner, Daniel Moore, Vikram Sandhu, Dave Wineland, David Allcock

The Allcock group at the University of Oregon is in the process of setting up a new ion trap lab. The broad purpose of our setup is to trap +Ca43 ions and use them for quantum information experiments. We are currently building and integrating: a macroscopic, linear Paul trap that will operate at room temperature; an ultra-high vacuum system; and a compact, rack-mounted laser system for ion cooling, state preparation, state readout, and logic gates. The apparatus includes an imaging system for collecting light from trapped ions for either counting photons or imaging individual ions, and a control system, based on ARTIQ hardware, gateware, and software, for managing and analyzing experiments.

### Accessing different topological classes and types of Majorana edge states in 1D superconductors using perturbations.

**Presenting Author**: Sayonee Ray, University of New Mexico CQuIC **Contributing Author(s)**: Subroto Mukerjee, Nayana Shah

Classification and realization of various classes of topological superconductors with different types of Majorana bound states (MBS) is of ongoing interest. The standard platform of these studies have been the conventional 1D Kitaev wire and its realizations. Here, we present the edge states of different types of p-wave SC in 1D in the presence of an additional Zeeman field and s-wave SC component. Within the framework of the tenfold classification scheme, we study the transition between different topological classes caused by such perturbations and analyze the nature of the corresponding MBS. Further, we study the junctions between different classes of topological superconductors and explore transport properties to probe the mid-gap states.

### Benchmarking VQE with exactly solvable models

**Presenting Author**: Ken Robbins, Tufts University

Perhaps the most promising application of Noisy Intermediate Scale Quantum (NISQ) computers is the Variational Quantum Eigensolver (VQE). Due to their namesake noise, NISQ computers performing VQE will need benchmarks to interpret their results. Exactly solvable models such as the Lipkin-Meshkov-Glick (LMG) model, a simple nuclear model of N fermions, can provide such benchmarks. We give circuits that produce low-N LMG eigenstates on a quantum computer with gate and qubit costs suited to the NISQ era. Further, we discuss how we might generalize the circuits for simulations of a higher number of particles.

### Optimal phase estimation for qubits states

**Presenting Author**: Marco Antonio Rodríguez García, Universidad Nacional Autonoma de Mexico **Contributing Author(s)**: Pablo Barberis Blostein

In this work, we will develop optimal strategies for phase estimation in pure qubits states, for multiple copies of qubits prepared in the same state. We find different types of optimal setups in three regimes, for one observation, multiple finite observations and the asymptotic regime. In the asymptotic regime, the setup saturates the Crámer-Rao bound.

### Time-varying dissipation for passive error correction in small logical qubit architectures

**Presenting Author**: David Rodriguez Perez, Colorado School of Mines **Contributing Author(s)**: Eliot Kapit

A very common approach in designing superconducting qubit architectures focuses on increasing the quantum state coherence by reducing noise. However, an increasing body of research has shown that engineered noise can be used for the stabilization of quantum states. This work focuses on the use of time-varying, engineered noise via lossy qubits or resonators, coupled to high-coherence qubits to aid in prolonging their lifetimes. We demonstrate this with the most basic case in a single high coherence qubit coupled to a single lossy qubit. We then demonstrate this technique with an idealized three qubit bit-flip code as a show case for error correction. And finally, we discuss the future direction in implementing this technique with the Very Small Logical Qubit.

### Energy gap calculation on near-term quantum hardware with robust phase estimation

**Presenting Author**: Antonio Russo, Sandia National Laboratories **Contributing Author(s)**: Andrew Baczewski, Benjamin Morrison, Kenneth Rudinger

Can alternative approaches to phase estimation yield better results for chemical simulation on NISQ hardware? Robust phase estimation (RPE) calculates the difference in phases between two eigenstates of a unitary, provided oracular access to that unitary and the relevant eigenstates. In contrast to conventional phase estimation, it does not require a controlled-U and is naturally resistant to state preparation and measurement errors. We evaluate the suitability of RPE on near-term hardware. In particular, we calculate of the energy landscapes of H_2 and H_3 on extant quantum hardware (pre-compiling to emulate hardware with significantly higher fidelities). Resource requirements for larger molecules are considered. Sandia National Labs is managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a subsidiary of Honeywell International, Inc., for the U.S. Dept. of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in this presentation do not necessarily represent the views of the DOE or the U.S. Government.

### A continuous-variable quantum repeater based on quantum scissors and switched mode-multiplexing

**Presenting Author**: Kaushik Seshadreesan, University of Arizona **Contributing Author(s)**: Hari Krovi Saikat Guha

Quantum repeaters are indispensable for high-rate, long-distance quantum communications. The vision of a future quantum internet strongly hinges on realizing quantum repeaters in practice. Numerous repeaters have been proposed for discrete-variable (DV) single-photon-based quantum communications. Continuous variable (CV) encodings over the quadrature degrees of freedom of the electromagnetic field mode offer an attractive alternative. For example, CV transmission systems are easier to integrate with existing optical telecom systems compared to their DV counterparts. Yet, repeaters for CV have remained elusive. We present a novel repeater scheme for entanglement distribution over a lossy bosonic channel based on CV entanglement sources that beats the direct transmission exponential rate-loss tradeoff. The repeater nodes consist of a) two-mode squeezed vacuum (TMSV) state sources for entanglement generation between adjacent nodes, b) the so-called quantum scissors operation to perform nondeterministic noiseless linear amplification of lossy TMSV states for entanglement distillation, c) a layer of switched mode-multiplexing inspired by second-generation DV repeaters—which is the key ingredient, apart from probabilistic entanglement purification that makes DV repeaters work, and d) a non-Gaussian entanglement swap operation to connect adjacent repeater nodes. We report our exact results on the rate-loss envelope achieved by the scheme.

Read this article online: https://arxiv.org/abs/1811.12393, https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.022315

### Experimentally-motivated benchmarking protocols for analog quantum simulators

**Presenting Author**: Ryan Shaffer, University of California Berkeley **Contributing Author(s)**: Eli Megidish, Wei-Ting Chen, Joseph Broz, Hartmut Häffner

Analog quantum simulation is expected to be an important application of near-term quantum devices. Verification of these devices without comparison to known simulation results will be an important task as the system size grows beyond the classically-simulable regime. We introduce a set of experimentally-motivated benchmarking protocols for analog quantum simulators that are sensitive to a variety of error sources and are scalable to larger system sizes. We discuss the demonstration of these protocols on a two-site Ising model implemented in a trapped-ion device and on a numerically-simulated five-site Heisenberg model.

### Noise resilience of variational quantum compiling

**Presenting Author**: Kunal Sharma, Louisiana State University **Contributing Author(s)**: Sumeet Khatri, Marco Cerezo, Patrick J Coles

Variational hybrid quantum-classical algorithms (VHQCAs) are near-term algorithms that leverage classical optimization to minimize a cost function, which is efficiently evaluated on a quantum computer. Recently VHQCAs have been proposed for quantum compiling, where a target unitary U is compiled into a short-depth gate sequence V. In this work, we report on a surprising form of noise resilience for these algorithms. Namely, we find one often learns the correct gate sequence V (i.e., the correct variational parameters) despite various sources of incoherent noise acting during the cost-evaluation circuit. Our main results are rigorous theorems stating that the optimal variational parameters are unaffected by a broad class of noise models, such as measurement noise, gate noise, and Pauli channel noise. Furthermore, our numerical implementations on IBM's noisy simulator demonstrate resilience when compiling the quantum Fourier transform, Toffoli gate, and W-state preparation. Hence, variational quantum compiling, due to its robustness, could be practically useful for noisy intermediate-scale quantum devices. Finally, we speculate that this noise resilience may be a general phenomenon that applies to other VHQCAs such as the variational quantum eigensolver.

Read this article online: https://arxiv.org/abs/1908.04416

### Thermodynamic dissipation bounds on classical and quantum reversible information processing

**Presenting Author**: Karpur Shukla, Flame University **Contributing Author(s)**: Michael P. Frank (Sandia National Laboratories)

We describe how to examine systems supporting classical and quantum reversible information processing, and provide bounds on the energy dissipation of reversible information processing operations, by exploiting the thermodynamic uncertainty relations (TURs) for non-equilibrium steady states (NESSs) and the four-corners partition of the Lindbladian with multiple steady states. We briefly review the motivation for reversible computing, and describe the additional constraints that reversible information processing specifically imposes on state Hilbert spaces. We then describe how the dynamics of a system encoding classical and quantum bits with reversible operations, possibly including some sort of dissipative dynamics, can be described using the four-corners partition of Lindbladians. We examine how the requirements of logical reversibility gives rise to a specific class of Lindbladian dynamics, and discuss the dissipation-delay product (DDP), a quantity of interest in reversible computing which characterizes the average energy costs over time of reversible operations. Finally, I discuss TURs for a single NESS, and discuss how this can be extended to TURs for systems with several NESSs by exploiting the geometric properties of Lindbladians with multiple steady states. From this, we provide a preliminary bound on the DDP.

### Superadditivity of coherent information for a class of simple channels

**Presenting Author**: Vikesh Siddhu, Carnegie-Mellon University

The qubit dephasing channel followed by the erasure channel, introduced recently by Leditzky, Leung and Smith, provides a useful laboratory for exploring the question of superadditivity of coherent information. It can be shown that a concatenated channel composed of an arbitrary channel followed by an erasure channel, can also be obtained by first applying a suitable erasure channel and then following it with a slight modification of the arbitrary channel. Placing the erasure channel first provides an alternative understanding of the concatenated channel, and simplifies the discussion of the coherent information. The dephasing channel belongs to the class of qubit channels with a two dimensional environment. Such channels, up to local unitaries, can be described by two parameters. We study the concatenation of such channels with the erasure channel. This three parameter family of channels, with the erasure probability q being the third parameter, has various interesting properties. Unlike a general channel, its coherent information can be computed by optimizing over a single parameter. For q > 0, its complement has positive capacity. For a large region of the parameter space, the coherent information is superadditive at the two letter level. The channel in a significant portion of the parameter space, exhibits a different kind of superadditivity, where its coherent information is boosted by using it in parallel with a symmetric channel with a qubit output (symmetric channels map the input space to a symmetric subspace of the output and the environment thus have zero coherent information and zero capacity).

### Optimizing bidirectional quantum teleportation

**Presenting Author**: Aliza Siddiqui, Louisiana State University **Contributing Author(s)**: Sumeet Khatri, Mark M. Wilde

The goal of bidirectional teleportation is to exchange qubits between spatially separated parties by means of local operations and classical communication (LOCC) and using as little entanglement as possible. The ideal outcome of such a protocol is that the two parties simulate a SWAP gate. Previous implementations of bidirectional teleportation have been proposed in prior work, including that of Kiktenko et al. [arXiv:1602.01420]. In this work, we calculate how well these previous proposals perform at the bidirectional teleportation task. We also show how to interpolate between perfect bidirectional teleportation and the scheme of Kiktenko et al. As another contribution, we find a method for simulating the protocol of Kiktenko et al. that consumes less entanglement than that consumed in their scheme. Finally, we show that optimizing the error when simulating a SWAP gate from an arbitrary bipartite resource state and PPT operations is a semi-definite program, which we can use to estimate the performance of bidirectional teleportation for a variety of example bipartite states.

### Updates to magneto-optical trap for neutral atom quantum computing

**Presenting Author**: Jacob Siderman, California Polytechnic State University **Contributing Author(s)**: Eric Bettencourt, Jordan Churi, Anya Houk, Katharina Gillen, Glen Gillen

Several different physical systems show promise in bringing quantum computing into reality. Our goal is to identify and realize light patterns to hold atoms for neutral atom quantum computing. We hope to use a pinhole diffraction pattern [1] to trap rubidium atoms that were pre-cooled in a magneto-optical trap (MOT). Time is a limited resource for any research project, especially at an undergraduate institution where student turnover is high and training in the tuning techniques is required every year. A MOT requires precise tuning of two diode lasers to the correct hyperfine transitions. This is currently the most time-consuming task students need to perform in the lab. Our newest team is updating and streamlining this process to make transitions between groups easier while reducing the amount of time needed to tune. We identified several changes to achieve faster tuning and more consistent functioning: changes to the optical setup to improve beam shape and intensity; rewriting the code running the tuning process to improve performance and user interface; combining and streamlining tasks currently carried out with separate programs; and eventually automating more of the tuning process. With these updates, we hope to achieve an instrument that will efficiently and consistently test and analyze different light patterns as atom traps for quantum computing. [1] K. Gillen-Christandl, et al., Phys. Rev. A 83, 023408 (2011).

### Classical simulation of high temperature quantum Ising models

**Presenting Author**: Sam Slezak, University of New Mexico CQuIC **Contributing Author(s)**: Elizabeth Crosson

We consider generalized quantum Ising models, including those which could describe disordered materials or quantum annealing devices, and we rigorously prove that the path integral Monte Carlo method yields an efficient algorithm for approximating the partition function above some system-size independent threshold temperature. This threshold temperature depends on the local interaction energy of each qubit, and in one formulation of the bound it suffices for the temperature to be greater than two + the maximum interaction degree (valence) over all qubits, measured in units of the local coupling constant. For example, this implies that the classical simulation of the thermal state of a superconducting device modeling a quantum Ising model with maximum valence of 6 and coupling strengths of 1 GHz is possible at all temperatures above 400 mK. Our proof is based on a probabilistic method called coupling which allows us to analyze the relaxation time of the worldline update PIMC Markov chain, showing that it equilibrates in the shortest possible time of O(n log n) worldline updates, where n is the number of qubits. In contrast, most previous rigorous simulations of systems without a sign problem only work when all of the couplings in the system are ferromagnetic. While the kind of quantum-to-classical transitions we identify were already known experimentally and from heuristic arguments, this result rigorously justifies the empirical success of PIMC in simulating such syst

### Velocity-sorting and stochastic resonances in a dissipative optical lattice

**Presenting Author**: Alexander Staron, Miami University **Contributing Author(s)**: Kefeng Jiang, Ajithamithra Dharmasiri, Anthony Rapp, and Samir Bali

We present detailed pump-probe spectra depicting Zeeman light-shifts, Raman vibrational modes, and velocity-selective Brillouin propagation modes in a dissipative optical lattice which agree with predictions from a simple F = 1/2 --> F’ = 3/2 atomic model. Depending on the incident angle of the probe beam, specific velocity classes of atoms are ratcheted in different directions. For the first time via direct pump-probe spectroscopy, we explore the possibility of observing a classical stochastic resonance in an optical lattice, where environmental fluctuations in the form of random spontaneous emission recoils are coupled to the atomic system to yield enhanced ratcheting. Further, we discuss prospects for inserting sharp, sub-wavelength potential barriers into the lattice potentials using recent dark state wavefunction engineering methods [M. Lacki, et al., Phys. Rev. Lett. 117, 233001 (2016); Y. Wang, et al., Phys. Rev. Lett. 120, 083601 (2018)]. By inducing quantum tunneling across these narrow barriers we seek to explore the possibility of observing quantum stochastic resonances in optical lattices.

### Quantum digital cooling in NISQ devices

**Presenting Author**: Jaimie S. Stephens, Sandia National Laboratories **Contributing Author(s)**: Kevin Young; Robin Blume-Kohout; Craig W. Hogle; Peter Maunz; Susan Clark

Quantum Digital Cooling (QDC) is a method of preparing the ground state of a Hamiltonian proposed by Polla \textit{et. al.} 2019. In QDC, the system is coupled to a single ancila qubit which is referred to as the fridge qubit. The fridge qubit is cooled to then indirectly cool the system down to the ground state. This technique mitigates errors via cooling rather than relying on fault-tolerant quantum computing. This makes it a good candidate for NISQ devices. We look at the performance of QDC when we add a noise. We look at two error models in particular. The first is iid Pauli errors caused by the environment reheating the system. The second is cross talk from operating on the fridge qubit. We experimentally demonstrate our theoretical predictions on trapped ions. Sandia National Labs is managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA0003525.

### Laser-free trapped-ion entangling gates with simultaneous insensitivity to qubit and motional decoherence

**Presenting Author**: R. T. (Tyler) Sutherland, Lawrence Livermore National Laboratory **Contributing Author(s)**: Raghu Srinivas, Shaun Burd, Hannah Knaack, Andrew Wilson, David Wineland, Dietrich Leibfried, David Allcock, Daniel Slichter, Stephen Libby

Dominant error sources for state-of-the-art laser-free trapped-ion entangling gates are decoherence of the qubit state and motion. The effect of these decoherence mechanisms can be suppressed with additional control fields, or through other techniques that reduce gate speed. Here, we propose using a near-motional-frequency magnetic field gradient to make a laser-free gate that is simultaneously resilient to both types of decoherence, does not require additional control fields, and has a relatively smaller cost in gate speed.

Read this article online: https://arxiv.org/pdf/1910.14178.pdf

### qubit-ADAPT-VQE: An adaptive algorithm for constructing hardware-efficient ansätze on a quantum processor

**Presenting Author**: Ho Lun Tang, Virginia Tech **Contributing Author(s)**: Harper R. Grimsley, Nicholas J. Mayhall, Edwin Barnes, Sophia E. Economou

Quantum simulation, one of the most promising applications of a quantum computer, is currently pursued on noisy intermediate-scale quantum (NISQ) devices. The variational quantum eigensolver is extensively used for finding the ground state energy of molecular Hamiltonians. The feasibility and performance of this algorithm depend critically on the depths of the state preparation circuits that create the ansatz on the quantum processor and on the number of variational parameters in this ansatz. Recently, an algorithm termed ADAPT-VQE was introduced to build system-adapted ansätze with substantially fewer variational parameters compared to other approaches. ADAPT-VQE provides a way to create an ansatz iteratively based on a predetermined operator pool, from which the algorithm selects the most important to apply at each step. However, deep state preparation circuits remain a challenge. Here, we present a hardware-efficient variant of this algorithm called qubit-ADAPT. By numerical simulations on various molecules, we show that qubit-ADAPT can reduce the circuit depth by one order of magnitude while maintaining the same accuracy as the original ADAPT-VQE. Addressing the high measurement cost, which is proportional to the size of the operator pool, we show how to construct a sufficient pool with size linear in the number of qubits. This result highlights the promise of adaptive simulation algorithms on near-term quantum devices.

### A theory for Trotter Error

**Presenting Author**: Minh Tran, University of Washington **Contributing Author(s)**: Nathan Wiebe, Andrew Childs, Yuan Su, Schchen Zhu

The Lie-Trotter formula and its higher-order generalizations provide direct approaches to implement the exponential of a sum of operators on classical and quantum computers. However, the error scaling of these approximations remains poorly understood despite significant effort. We address this by developing a theory of Trotter error that directly exploits the commutativity of operator summands, producing tighter error bounds for both real- and imaginary-time evolutions. Previous work only achieves these goals for systems with geometrically local interactions and Lie-algebraic structures. We give a host of improved algorithms for digital quantum simulation and quantum Monte Carlo simulation, including simulations of second-quantized plane-wave electronic structure, local Hamiltonians, power-law interactions, clustered Hamiltonians, transverse field Ising model, and quantum ferromagnets, nearly matching or outperforming the state-of-the-art results. We further show that the gate count of simulating local observables can be independent of the system size and we prove a Lieb-Robinson-type bound that nearly matches a recent bound. Our bound is provably tight for low-order formulas. For nearest-neighbor interactions and power-law interactions, our higher-order bound overestimates the complexity by only a factor of 5. This suggests that our theory can accurately characterize Trotter error in terms of both asymptotic scaling and constant prefactor.

### Reaction dynamics on quantum computers

**Presenting Author**: Andrew Tranter, Tufts University **Contributing Author(s)**: Peter Love

The study of quantum chemistry is expected to be a principal use of emergent quantum computing devices. The ability of quantum computers to efficiently provide highly accurate electronic structure data could have major consequences where such accuracy is required, such as in the prediction of the kinetics and dynamics of chemical reactions. However, such simulations often require vast numbers of electronic structure calculations, potentially exacerbating resource limitations of small-scale quantum devices. In this talk, we present theoretical results characterising the quantum resources required for the simulation of reactions involving small numbers of light atoms through semi-classical trajectory simulation. We consider how this process can be aided by recent optimisations to variational quantum algorithms. Finally, we report simulation results and discuss experimental progress to this end.

### Operation and intrinsic error budget of a two-qubit cross-resonance gate

**Presenting Author**: Vinay Tripathi, University of Southern California; University of California Riverside **Contributing Author(s)**: Mostafa Khezri, Alexander N. Korotkov

We analyze analytically, semianalytically, and numerically the operation of a cross-resonance (CR) gate for superconducting qubits (transmons). We find that a relatively simple semianalytical method gives accurate results for the controlled-not (cnot) - equivalent gate duration and compensating single-qubit rotations. It also allows us to minimize the cnot gate duration over the amplitude of the applied microwave drive and find the dependence on the detuning between the qubits. However, full numerical simulations are needed to calculate the intrinsic fidelity of the CR gate. We decompose numerical infidelity into contributions from various physical mechanisms, thus finding the intrinsic error budget. In particular, at small drive amplitudes, the CR gate fidelity is limited by imperfections of the target-qubit rotations, while at large amplitudes it is limited by leakage. The gate duration and fidelity are analyzed numerically as functions of the detuning between qubits, their coupling, drive frequency, relative duration of pulse ramps, and microwave crosstalk. The effect of the echo sequence is also analyzed numerically. Our results show that the CR gate can provide intrinsic infidelity of less than $10^-3$ when a simple pulse shape is used.

Read this article online: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.012301

### Quantum state discrimination circuits inspired by Deutschian closed timelike curves

**Presenting Author**: Christopher Vairogs, University of Florida **Contributing Author(s)**: Vishal Katariya, Mark M. Wilde

The Holevo-Helstrom theorem places a bound on the probability of perfectly distinguishing two non-orthogonal states in a single measurement. However, using quantum states traveling along a Closed Timelike Curve (CTC), one can perfectly distinguish multiple non-orthogonal states and violate the Heisenberg uncertainty principle. A quantum computer can simulate a CTC using a quantum memory, multiple copies of the input state, and multiple iterations of a quantum channel. We determine the probability of correctly distinguishing states with the CTC-inspired scheme and show that this probability converges exponentially to one with respect to the number of iterations. We show how one may achieve practical quantum state discrimination by optimizing this scheme. We then explore applications of this scheme, which may lead to table-top experiments.

### Fast quantum control of 87Rb Bose-Einstein condensates

**Presenting Author**: Denuwan Vithanage, Miami University **Contributing Author(s)**: Skyler Wright, E. Carlo Samson

We present numerical simulations on manipulating Bose-Einstein condensates (BECs) at a fast rate while maintaining the coherence properties of its initial quantum state. The main problem in transporting a quantum system like a BEC at a fast rate is that the energy we add to the system for transport will change the BEC’s initial state or completely destroy the BEC. Two-dimensional (2D) simulations of BEC transport are performed by numerically solving the Gross-Pitaevskii equation (GPE) using a split-step Fourier method. In our simulations, we use trapping potentials in the form of painted potentials because it is possible to achieve arbitrary, dynamic traps with this method. In order to achieve high quantum fidelity, we use shortcuts-to-adiabaticity (STA) for high-speed BEC transport.. With these simulations, we compare different time intervals for a particular spatial displacement that a BEC can travel while keeping high quantum fidelity using experimentally feasible parameters.

### Resource theory of asymmetric distinguishability

**Presenting Author**: Mark Wilde, Louisiana State University **Contributing Author(s)**: Xin Wang

We systematically develop the resource-theoretic perspective on distinguishability. The theory is a resource theory of asymmetric distinguishability, given that approximation is allowed for the first quantum state in general transformation tasks. We introduce bits of asymmetric distinguishability as the basic currency in this resource theory, and we prove that it is a reversible resource theory in the asymptotic limit, with the quantum relative entropy being the fundamental rate of resource interconversion. We formally define the distillation and dilution tasks, and we find that the exact one-shot distillable distinguishability is equal to the min-relative entropy, the exact one-shot distinguishability cost is equal to the max-relative entropy, the approximate one-shot distillable distinguishability is equal to the smooth min-relative entropy, and the approximate one-shot distinguishability cost is equal to the smooth max-relative entropy. We also develop the resource theory of asymmetric distinguishability for quantum channels. For this setting, we prove that the exact distinguishability cost is equal to channel max-relative entropy and the distillable distinguishability is equal to the amortized channel relative entropy.

Read this article online: https://arxiv.org/abs/1905.11629, https://arxiv.org/abs/1907.06306

### Comparison of the quantum Cramer-Rao bounds for quantum-enhanced resonant detection based on phase and transmission estimation

**Presenting Author**: Timothy Woodworth, University of Oklahoma **Contributing Author(s)**: Mohammadjavad Dowran, Ashok Kumar, Alberto M. Marino

The goals of quantum metrology include finding optimal states to probe a system and optimal measurements of those states to obtain as much information as possible about the parameter of interest. Here we study ultimate sensitivity bounds for resonant based detectors, such as plasmonic sensors and optical resonators, when probed with entangled twin beams of light. Resonant detectors experience a resonance shift due to local changes in their environment. This causes both a change in the amplitude and phase of the probing light. We have previously shown experimentally that is possible to use twin beams to obtain a quantum-based enhancement of the sensitivity of a plasmonic sensor when using the change in amplitude to estimate a local change in index of refraction. We now show how further enhancements can be obtained when using the change in phase to estimate the local change in refractive index. We compare ultimate bounds by calculating the quantum Cram\'er-Rao bound for both approaches when using bright twin beams and show experimental schemes that saturate these bounds.

### Optomechanical cooling with time dependent parameters

**Presenting Author**: Pablo Yanes-Thomas, Universidad Nacional Autonoma de Mexico **Contributing Author(s)**: Pablo Barberis-Blostein, Marc Bienert

We obtain a master equation for a parametrically driven optomechanical cavity using a dissipa- tion model that accounts for the modification of the quasi-energy spectrum caused by the driving when the natural frequency of the mechanical object oscillates periodically around its mean value. The master equation with the improved dissipation model is expressed using Floquet operators. We apply the master equation to model the laser cooling of the mechanical object. Using an adiabatic approximation, an analytical expression for its temperature can be obtained. We find that the temperature can be lower than in the non-time dependent case and includes both time dependent and non time dependent corrections. Our results raise the possibility of achieving lower temperatures for the mechanical object if its natural frequency can be controlled as a function of time.

### Quantum feedback error correction of a monitored system evolving adiabatically

**Presenting Author**: Ka Wa Yip, University of Southern California **Contributing Author(s)**: Mostafa Khezri, Daniel Lidar

We devise a quantum feedback error correction method to reverse the effect of thermal excitations in quantum annealing. Conditioned on the output signal I(t) from continuous measurement records, feedback is applied to an adiabatically evolving system in the hopes of increasing the ground state population at the end of the anneal. We propose an experimental setting for such continuous measurement and feedback in the case of superconducting flux qubits. We simulate the error correction performance of a system weakly coupled to a thermal bath based on methods like quantum trajectories and quantum bayesian updates. We also derive a feedback master equation for markovian feedback (feedback delay $\tau\rightarrow 0$) and further give the timescale condition for feedback Markovianity. Realistic feedbacks are also subjected to non-negligible feedback delay, detector efficiency, and restrictions on the form of the feedback Hamiltonian due to experimental challenges. We therefore study the effectiveness of feedback correction under such limitations and explore how the optimized feedback delay time depends on the annealing schedule and limitations in other experimental parameters.

### Noise memory kernel reconstruction via the post-Markovian master equation

**Presenting Author**: Haimeng Zhang, University of Southern California **Contributing Author(s)**: Daniel A. Lidar

Understanding and combating decoherence is one of the central topics in realizing quantum computation. Correlated, non-Markovian noise presents a particularly relevant challenge in superconducting qubit systems. This talk will present results on the construction of a bath memory kernel function from the experimentally measured state dynamics of a superconducting qubit. This phenomenological memory kernel arises in the post-Markovian master equation (PMME) [A. Shabani and D. A. Lidar, PRA 71, 020101 (2005)]. The memory kernel as constructed is of practical interest for quantum computation tasks as it provides insight into the noise origin and the characteristic timescales associated with bath memory effect. It also illuminates how the non-Markovian property of the noise can potentially be utilized to extend coherence timescales relative to the Markovian limit.

### Measurement reduction in variational quantum algorithms

**Presenting Author**: Andrew Zhao, University of New Mexico CQuIC **Contributing Author(s)**: Andrew Tranter, William M. Kirby, Shu Fay Ung, Akimasa Miyake, Peter J. Love

Variational quantum algorithms are promising applications of noisy intermediate-scale quantum (NISQ) computers. These algorithms consist of a number of separate prepare-and-measure experiments that estimate terms in the Hamiltonian. The number of separate measurements required can become overwhelmingly large for problems at the scale of NISQ hardware that may soon be available. We approach this problem from the perspective of contextuality, and use unitary partitioning to define VQE procedures in which additional unitary operations are appended to the ansatz preparation circuit to reduce the number of terms one needs to measure. This approach may be tuned to hardware specifications in order to use all coherent resources available after ansatz preparation. We investigate this technique for a variety of Hamiltonian classes, in particular the electronic structure Hamiltonian from quantum chemistry.

Read this article online: arxiv.org/abs/1908.08067

### Universal logical gate sets with constant-depth circuits for topological and hyperbolic quantum codes

**Presenting Author**: Guanyu Zhu, IBM; Joint Quantum Institute (JQI): University of Maryland **Contributing Author(s)**: Ali Lavasani, Maissam Barkeshli

A fundamental question in the theory of quantum computation is to understand the ultimate space-time resource costs for performing a universal set of logical quantum gates to arbitrary precision. To date, common approaches for implementing a universal logical gate set, such as schemes utilizing magic state distillation, require a substantial space-time overhead. In this work, we show that braids and Dehn twists, which generate the mapping class group of a generic high genus surface and correspond to logical gates on encoded qubits in arbitrary topological codes, can be performed through a constant depth circuit acting on the physical qubits. In particular, the circuit depth is independent of code distance d and system size. The constant depth circuit is composed of a local quantum circuit, which implements a local geometry deformation, and a permutation of qubits. When applied to anyon braiding or Dehn twists in the Fibonacci Turaev-Viro code based on the Levin-Wen model, our results demonstrate that a universal logical gate set can be implemented on encoded qubits in O(1) time through a constant depth unitary quantum circuit, and without increasing the asymptotic scaling of the space overhead. Our results for Dehn twists can be extended to the context of hyperbolic Turaev-Viro codes as well, which have constant space overhead (constant rate encoding). This implies the possibility of achieving a space-time overhead of O(d/log d).

Read this article online: https://quantum-journal.org/papers/q-2019-08-26-180/, https://arxiv.org/abs/1806.02358, https://arxiv.org/abs/1806.06078

### Phase space simulation method for quantum computation with magic states on qubits

**Presenting Author**: Michael Zurel, University of British Columbia **Contributing Author(s)**: Robert Raussendorf, Juani Bermejo-Vega, Emily Tyhurst, Cihan Okay

We propose a method for classical simulation of finite-dimensional quantum systems, based on sampling from a quasiprobability distribution, i.e., a generalized Wigner function. Our construction applies to all finite dimensions, with the most interesting case being that of qubits. For multiple qubits, we find that quantum computation by Clifford gates and Pauli measurements on magic states can be efficiently classically simulated if the quasiprobability distribution of the magic states is non-negative. This provides the so far missing qubit counterpart of the corresponding result [V. Veitch et al., New J. Phys.14, 113011 (2012)] applying only to odd dimension. Our approach is more general than previous ones based on mixtures of stabilizer states. Namely, all mixtures of stabilizer states can be efficiently simulated, but for any number of qubits there also exist efficiently simulable states outside the stabilizer polytope. Further, our simulation method extends to negative quasiprobability distributions, where it provides probability estimation. The simulation cost is then proportional to a robustness measure squared. For all quantum states, this robustness is smaller than or equal to robustness of magic.

Read this article online: arxiv.org/pdf/1905.05374.pdf