Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session V31: Quantum Computing with ContinuousVariable SystemsFocus Live

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Sponsoring Units: DQI Chair: HoiKwan Lau, Simon Fraser Univ 
Thursday, March 18, 2021 3:00PM  3:12PM Live 
V31.00001: Constructing Qudits from Infinite Dimensional Oscillators by Coupling to Qubits Yuan Liu, Matthew Kearney, Isaac Chuang An infinite dimensional system such as a quantum harmonic oscillator offers a potentially unbounded Hilbert space for computation, but accessing and manipulating the entire state space requires a physically unrealistic amount of energy. When such a simple harmonic oscillator is coupled to a qubit, for example via a JaynesCummings interaction, it is well known that the Hilbert space can be separated into independently accessible subspaces of constant energy, but the number of subspaces is still infinite. Nevertheless, a closed fourdimensional Hilbert space can be formed from the lowest energy states of the qubit + oscillator. We extend this idea, and show for arbitrary N how a 2(N+1)dimensional Hilbert space can be constructed, which is closed under a certain set of unitary operations resulting solely from manipulating standard JaynesCummings Hamiltonian terms. Moreover, these unitaries are a universal set for quantum operations on this “qubitoscillator qudit”. This work suggests that the combination of a qubit and a bosonic system may serve as hardwareefficient quantum resources for quantum computation and memory. 
Thursday, March 18, 2021 3:12PM  3:24PM Live 
V31.00002: Modular bosonic subsystem codes Giacomo Pantaleoni, Ben Baragiola, Nicolas C. Menicucci Bosonic codes[1] introduce a notion of a qubit in the context of infinitedimensional Hilbert spaces. Namely, the GottesmanKitaevPreskill (GKP) bosonic code[2] has several desirable properties[1] and has seen a sharp increase in interest over the past few years. 
Thursday, March 18, 2021 3:24PM  3:36PM Live 
V31.00003: Quantum repeaters based on concatenated bosonic and discretevariable quantum codes Filip Rozpedek, Kyungjoo Noh, Qian Xu, Saikat Guha, Liang Jiang We propose a novel architecture of quantumerrorcorrectionbased quantum repeaters that combines the techniques used in discrete and continuous variable quantum information. Specifically, we propose to encode the transmitted qubits in a concatenated code consisting of two levels. On the first level we use a continuous variable GKP code which encodes the qubit in a single bosonic mode. On the second level we use a small discrete variable code. Such an architecture introduces two major novelties which allow us to make efficient use of resources. Firstly, our architecture makes use of two types of quantum repeaters: the simpler GKP repeaters that need to only be able to store and correct errors on a single GKP qubit and more powerful but more costly multiqubit repeaters that additionally can correct errors on the higher level. The combination of using the two types of repeaters enables us to achieve performance needed in practical scenarios with a significantly reduced cost with respect to an architecture based solely on multiqubit repeaters. Secondly, the use of the GKP code on the lower level provides us with the information about the success probability of the specific GKP correction round. This analog information offers significant boost in performance of our scheme. 
Thursday, March 18, 2021 3:36PM  3:48PM Live 
V31.00004: Improved speed limits for cavityqubit operations using conditional displacements (Part 1) Salvatore Elder, Alec Eickbusch, Christa Flühmann, Zhenghao Ding, Shantanu Jha, Volodymyr Sivak, Nicholas Frattini, Michel Devoret, Robert J Schoelkopf

Thursday, March 18, 2021 3:48PM  4:00PM Live 
V31.00005: Improved speed limits for cavityqubit operations using conditional displacements (Part 2) Alec Eickbusch, Salvatore Elder, Zhenghao Ding, Shantanu Jha, Volodymyr Sivak, Nicholas Frattini, Christa Flühmann, Robert J Schoelkopf, Michel Devoret It has been shown that universal control of an oscillator, realized here as a mode of a microwave cavity, can be achieved by coupling, in the strongdispersive regime, the oscillator to an ancilla twolevel system, such as a transmon [Heeres et al., 2017]. However, the rate of control appears to be limited by the interaction strength. In this twopart talk, we explore how displacements of the oscillator which are large on the scale of zero point fluctuations can generate an effective conditional displacement interaction [CampagneIbarcq et al., 2020], leading in turn to universal oscillator control with a speed limited by the oscillator drive strength rather than the strength of the oscillatortransmon coupling. 
Thursday, March 18, 2021 4:00PM  4:36PM Live 
V31.00006: Bosonic codes for continuousvariable quantum computing Invited Speaker: Ben Baragiola

Thursday, March 18, 2021 4:36PM  4:48PM Live 
V31.00007: Universal Gate Set for ContinuousVariable Quantum Computation with Microwave Circuits Timo Hillmann, Isaac Quijandria Diaz, Göran Johansson, Alessandro Ferraro, Simone Gasparinetti, Giulia Ferrini We provide an explicit construction of a universal gate set for continuousvariable quantum computation with microwave circuits. Such a universal set has been first proposed in quantumoptical setups, but its experimental implementation has remained elusive in that domain due to the difficulties in engineering strong nonlinearities. We show that a realistic threewave mixing microwave architecture based on the superconducting nonlinear asymmetric inductive element [1] allows us to overcome this difficulty. 
Thursday, March 18, 2021 4:48PM  5:00PM Live 
V31.00008: Clifford gates for GottesmanKitaevPreskill codes in superconducting circuits Mackenzie Shaw, Arne Grimsmo The GottesmanKitaevPreskill (GKP) code is one of the most promising bosonic error correcting codes, and has recently been realized both in trapped ion and superconducting circuit experiments. An attractive feature of the GKP code is the fact that all Clifford gates can be implemented with quadratic Hamiltonians, while Pauli measurements can be performed using homodyne detection. We here moreover show that all singlequbit Clifford gates can be performed by simply updating the phase of local oscillators, thus reducing both software and hardware demands. To this end, we present a simple superconducting twomode coupler that allows all Clifford gates to be executed using a single piece of hardware. We numerically estimate the gate fidelity for both ideal and realistic gate implementations. 
Thursday, March 18, 2021 5:00PM  5:12PM Live 
V31.00009: Microwave swap gates with a Kerrcat ancilla Iivari Pietikäinen, Ondrej Cernotik, Shruti Puri, Radim Filip, Steven Girvin Quantum computation requires coherent highly controllable gates with low error rates. A longstanding target is an exponentialswap gate which requires an ancilla qubit to entangle two microwave fields. A transmon ancilla has been previously used (Gao et al. Nature 566, 509 (2019)); it introduces a back propagation of ancilla errors into the fields. We propose a new way of doing a controlled beam splitter with a SNAILbased Kerr cat that is transparent to the dominant error channel. We show that one can implement a beam splitter with a phase that depends on the state of the ancilla cat using suitable driving fields. Encoding quantum information into coherent states of the Kerr cat biases its decoherence towards phase flips, allowing significantly improved performance compared to the transmon setup. With this controlled beam splitter, it is possible to implement controlledswap and further exponentialswap gates for microwave fields, which allows further applications such as swap tests or quantum random access memory protocols. 
Thursday, March 18, 2021 5:12PM  5:24PM Live 
V31.00010: Continuousvariable Gate Teleportation and Bosoniccode Error Correction Blayney Walshe, Ben Baragiola, Rafael N Alexander, Nicolas C. Menicucci Large continuousvariable (CV) cluster states containing 1000s of modes have recently been used to experimentally demonstrate measurementbased single and twomode gate operations. A key primitive component of CV cluster states is a chain of twomode squeezed states (TMSSs) called a macronode wire. Nonlocal, twomode homodyne measurements are used to teleport an input state along the wire. Adjusting the measurement bases applies Gaussian gates to the input states via gate teleportation. 
Thursday, March 18, 2021 5:24PM  5:36PM Live 
V31.00011: Simulation of dissipative conical intersection reaction dynamics in a superconducting circuit Christopher Wang, Nicholas Frattini, Benjamin Chapman, Shruti Puri, Steven Girvin, Michel Devoret, Robert J Schoelkopf Conical intersections (CIs) are ubiquitous features in quantum chemistry where two molecular potential energy surfaces intersect. Characterized by a breakdown of the BornOppenheimer approximation, they result in strong hybridization between electronic and nuclear degrees of freedom. CIs enable nonadiabatic transitions between electronic states and, when combined with vibrational damping, may heavily influence outcomes of chemical reactions. Though conventionally investigated through spectroscopic means, it would be desirable to engineer a synthetic CI with tunable control to systematically explore the parameter space. Superconducting circuits have emerged as a powerful platform for simulating quantum systems, possessing a versatile range of controllable interactions and engineered dissipation. In this talk, we report on experimental progress towards simulating a CI Hamiltonian in a circuit with one nonlinear (electronic) and two linear (nuclear) modes. We engineer the system to support two distinct ground states in one of the nuclear coordinates, representing reactant and product configurations. By preparing an excited state and tuning the dissipation, we highlight the competition between coherent evolution and damping in determining the branching ratio of the model reaction. 
Thursday, March 18, 2021 5:36PM  5:48PM Not Participating 
V31.00012: High Impedance multimode circuit QED Sebastien Leger, Javier Puertas, Karthik Bharadwaj, Remy Dassonneville, Jovian Delaforce, Farshad Foroughi, Vladimir Milchakov, Luca Planat, Olivier Buisson, Cécile Naud, Wiebke Guichard, serge florens, Izak Snyman, Denis Basko, Nicolas Roch Electromagnetic fields possess zero point fluctuations (ZPF) which lead to observable 
Thursday, March 18, 2021 5:48PM  6:00PM On Demand 
V31.00013: Two dimensional multiphoton Quantum Walk in transverse momentum of light Chiara Esposito, Gonzalo Carvacho, Mariana Barros, Andres Duran Hernandez, Filippo Cardano, Nicolò Spagnolo, Lorenzo Marrucci, Fabio Sciarrino Quantum walks (QWs) represent a powerful resource in different fields of physics due to its potential for simulating complex topological systems, paving the way to universal quantum computation. Moreover, twodimensional QWs prove to outperform in the realization of quantum research algorithms and in the simulation of topological characteristics with respect to the onedimensional counterpart. We present an innovative platform feasible for the realization of twodimensional multiphoton QW in the transverse momentum of light. The key element of the platform is a new device called Gplate. When the photons propagate through these devices, they acquire a complex internal structure in the transverse momentum space. We experimentally realize the threestep dynamic of two walkers in different initial positions on the lattice within the quantum regime. Our platform allows us to control the degree of indistinguishability of photons, allowing us to investigate the bosonic behavior of two photons in the QW dynamic. As a future prospect, our platform could be exploited for the implementation of more complex schemes using multiphoton interference, such as multiphoton Boson sampling and the hybrid entanglement state engineering. 
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