Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G68: Superconducting Qubit Quantum Simulation and AlgorithmsFocus

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Sponsoring Units: DQI Chair: David McKay, IBM TJ Watson Research Center Room: Four Seasons 4 
Tuesday, March 3, 2020 11:15AM  11:51AM 
G68.00001: Quantum Supremacy: Benchmarking the Sycamore Processor Invited Speaker: Kevin Satzinger The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. A fundamental challenge is to build a highfidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with 53 programmable superconducting qubits. In our Sycamore processor, each qubit interacts with four neighbors in a rectangular lattice using tunable couplers. A key systems engineering advance of this device is achieving highfidelity single and twoqubit operations, not just in isolation but also while performing a realistic computation with simultaneous gate operations across the entire processor. We benchmark the Sycamore processor using crossentropy benchmarking, a scalable method to evaluate system performance. Our largest system benchmarks feature circuits that are intractable for classical hardware, culminating in the demonstration of quantum supremacy. Furthermore, the fidelities from fullsystem benchmarks agree with what we predict from individual gate and measurement fidelities, verifying the digital error model and presenting a path forward to quantum error correction. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G68.00002: Quantum supremacy using the Sycamore processor Charles Neill The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. A fundamental challenge is to build a highfidelity processor capable of running quantum algorithms in an exponentially large computational space. Recently, our team has demonstrated a dramatic speedup relative to all known classical algorithms providing an experimental realization of quantum supremacy on a computational task. In this talk, I will focus on the hardware details of the Sycamore processor and how this device enabled supremacyquality twoqubit gates. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G68.00003: Quantum simulation in circuit QED: Observation of quantum manybody effects due to zero point fluctuations in
superconducting circuits  I: Theory Nicolas Roch, Sebastien Leger, Javier Puertas, Karthik Srikanth Bharadwaj, Remy Dassonneville, Jovian Delaforce, Farshad Foroughi, Vladimir Milchakov, Luca Planat, Olivier Buisson, Cécile Naud, Wiebke HaschGuichard, Serge Florens, Izak Snyman Electromagnetic fields possess zero point fluctuations (ZPF) which lead to observable effects such as the Lamb shift and the Casimir effect. In the traditional quantum optics domain, these corrections remain perturbative due to the smallness of the fine structure constant. To provide a direct observation of nonperturbative effects driven by ZPF in an open quantum system we wire a highly nonlinear Josephson junction to a high impedance transmission line, allowing large phase fluctuations across the junction. Consequently, the resonance of the former acquires a relative frequency shift that is orders of magnitude larger than for natural atoms. Detailed modelling confirms that this renormalization is nonlinear and quantum. Remarkably, the junction transfers its nonlinearity to about 30 environmental modes, a striking backaction effect that transcends the standard CaldeiraLeggett paradigm. This work opens many exciting prospects for longstanding quests such as the tailoring of manybody Hamiltonians in the strongly nonlinear regime, the observation of Bloch oscillations, or the development of highimpedance qubits. 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G68.00004: Quantum simulation in circuit QED: Observation of quantum manybody effects due to zero point fluctuations  II: Experiment Sebastien Leger, Javier Puertas, Karthik Srikanth Bharadwaj, Remy Dassonneville, Jovian Delaforce, Farshad Foroughi, Vladimir Milchakov, Luca Planat, Olivier Buisson, Cécile Naud, Wiebke Guichard, Serge Florens, Izak Snyman, Nicolas Roch Electromagnetic fields possess zero point fluctuations (ZPF) which lead to observable 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G68.00005: Analog quantum simulation of a Kondo impurity with superconducting circuits Nicholas Grabon, Roman Kuzmin, Nitish Jitendrakumar Mehta, Moshe Goldstein, Vladimir Manucharyan

Tuesday, March 3, 2020 12:39PM  12:51PM 
G68.00006: Simulating a Dirac particle with coupled transmon circuits Elisha Svetitsky, Nadav Y Katz The core concept of quantum simulation is the mapping of an inaccessible quantum system onto a controllable one by identifying analogous dynamics. We map the Dirac equation of relativistic quantum mechanics in 3+1 dimensions onto a multilevel superconducting Josephson circuit. Resonant drives determine the particle mass and momentum and the quantum state represents the internal spinor dynamics, which are cast in the language of multilevel quantum optics. The degeneracy of the Dirac spectrum corresponds to a degeneracy of bright/dark states within the system and particle spin and helicity are employed to interpret the multilevel dynamics. We simulate the Schwinger mechanism of electronpositron pair production by introducing an analogous electric field as a doubly degenerate LandauZener problem. All proposed measurements can be performed well within typical decoherence times. This work opens a new avenue for experimental study of the Dirac equation and provides a tool for control of complex dynamics in multilevel systems. 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G68.00007: Quantum Simulation of Hyperbolic Space with Circuit Quantum Electrodynamics: From Graphs to Geometry Igor Boettcher, Przemyslaw Bienias, Ron Belyansky, Alicia Kollar, Alexey V Gorshkov We give a quantum field theoretic perspective on recent breakthrough experiments in circuit quantum electrodynamics, where hyperbolic lattices are realized with superconducting resonators and photons are tricked into believing that space is hyperbolic. We show how these finite lattice geometries can be mapped onto quantum field theories in continuous negatively curved space. We use this as a computational tool to quantitatively reproduce ground state energy, spectral gap, and correlation functions of the noninteracting lattice system by means of analytic formulas on the Poincare disk, and show how conformal symmetry emerges for large lattices. I will discuss how interaction effects can be induced by coupling qubits to the hyperbolic lattice. This sets the stage for studying interactions and disorder on hyperbolic graphs, and to resolve fundamental open problems at the interface of interacting manybody systems, quantum field theory in curved space, and quantum gravity using tabletop experiments. 
Tuesday, March 3, 2020 1:03PM  1:15PM 
G68.00008: Demonstration of programmable quantum simulations of lattice models using a superconducting parametric cavity Jamal Busnaina, Jimmy ShihChun Hung, M.V. Moghaddam, Chung Wai Sandbo Chang, A.M. Vadiraj, Hadiseh Alaeian, Enrique Rico, C.M. Wilson There has been a growing interest in realizing quantum simulators for important physical systems where perturbative methods are ineffective. The scalability and flexibility of circuit quantum electrodynamics (cQED) make it a promising platform for implementing various types of simulators, including lattice models of stronglycoupled field theories. With this in mind, we use a multimode superconducting parametric cavity to create programmable lattices of bosonic modes by parametrically pumping at modedifference frequencies. The choice of pump frequencies allows changing the graph of the lattice in situ. Further, the resulting hopping terms induced between the modes can be made complex by controlling the relative phases of the parametric drives. This enables us to study a wide variety of interesting lattice models. For instance, controlling the total loop phase in closed plaquettes allows us to simulate the motion of particles in a static gauge field, including producing nonreciprocal transport. The system can also realize models with topological features such as the bosonic Creutz ladder. In this talk, we present experimental results on a variety of different small lattice models. 
Tuesday, March 3, 2020 1:15PM  1:27PM 
G68.00009: Quantum simulation of a spin chain with superconducting circuits Quentin Ficheux, Aaron Somoroff, Nitish Jitendrakumar Mehta, Roman Kuzmin, Ivar Martin, Maxim G Vavilov, Vladimir Manucharyan An Ising chain is one of the simplest manybody systems to exhibit a quantum phase transition. In the presence of a transverse field and disorder, this model produces a rich variety of non integrable ground states giving rise to exotic phase transitions including many body localization and Anderson localization. 
Tuesday, March 3, 2020 1:27PM  1:39PM 
G68.00010: Growth and preservation of entanglement in a manybody localized system Ben Chiaro, Brooks Foxen, Matthew McEwen, John M Martinis

Tuesday, March 3, 2020 1:39PM  1:51PM 
G68.00011: Approximating finitetemperature dynamic correlation functions on quantum computers Jeffrey Cohn, Khadijeh 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 subclusters 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. 
Tuesday, March 3, 2020 1:51PM  2:03PM 
G68.00012: Band engineering for quantum simulation with superconducting circuits Christie Chiu, Andrew Houck Quantum simulation has been implemented on a variety of experimental platforms such as neutral atoms, ions, quantum dots, and superconducting circuits, each offering unique features. Superconducting circuits can and have been used to realize artificial photonic materials in a wide range of lattice geometries and graph connectivities, due to the flexibility of onchip fabrication. In addition, photonphoton interactions are highly tunable using nonlinearities such as superconducting qubits, leveraging the vast toolkit developed for quantum computation. Here I report on recent progress towards engineering flat bands for studies of strongly correlated manybody physics. 

G68.00013: Propagation and Localization of Collective Excitations on a 24Qubit Superconducting Processor Yangsen Ye Superconducting circuits have emerged as a powerful platform of quantum simulation, especially for emulating the dynamics of quantum manybody systems. Here we construct a BoseHubbard ladder with a ladder array of 20 qubits. We then use pairs of controllable qubits to study the dynamics of the ladder model. We investigate theoretically and demonstrate experimentally the dynamics of single and doubleexcitation states with distinct behaviors. We observe linear propagation of photons in the singleexcitation case. The doubleexcitation state, initially placed at the edge, localizes; while placed in the bulk, it splits into two singleexcitation modes. Our work paves the way to simulation of exotic logic particles by subtly encoding physical qubits and exploration of rich physics by superconducting circuits. 
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