Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y34: CV Quantum Computation and Simulation IIIFocus Recordings Available

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Sponsoring Units: DQI Chair: Ruoming Peng, University of Chicago Room: McCormick Place W193A 
Friday, March 18, 2022 8:00AM  8:36AM 
Y34.00001: Scalable Neural Decoder for Topological Surface Codes Invited Speaker: Simon Trebst With the advent of noisy intermediatescale quantum (NISQ) devices, practical quantum computing has seemingly come into reach. However, to go beyond proofofprinciple calculations, the current processing architectures will need to scale up to larger quantum circuits which in turn will require fast and scalable algorithms for quantum error correction. In this talk, I will present a neural network based decoder that, for a family of stabilizer codes subject to depolarizing noise and syndrome measurement errors, is scalable to tens of thousands of qubits (in contrast to other recent machine learning inspired decoders) and exhibits faster decoding times than the stateoftheart union find decoder for a wide range of error rates (down to 1%). The key innovation is to autodecode error syndromes on small scales by shifting a preprocessing window over the underlying code, akin to a convolutional neural network in pattern recognition approaches. We show that such a preprocessing step allows to effectively reduce the error rate by up to two orders of magnitude in practical applications and, by detecting correlation effects, shifts the actual error threshold, up to fifteen percent higher than the threshold of conventional error correction algorithms such as union find or minimum weight perfect matching, even in the presence of measurement errors. An insitu implementation of such machine learningassisted quantum error correction will be a decisive step to push the entanglement frontier beyond the NISQ horizon. 
Friday, March 18, 2022 8:36AM  8:48AM 
Y34.00002: Deterministic Generation of Multidimensional Microwave Photonic Cluster States with a Single Quantum Emitter  Part 1 Andreas Butler, Vinicius S Ferreira, Gihwan Kim, Oskar Painter Multidimensional photonic graph states, such as cluster states, are important resource states for a variety of protocols in quantum metrology, quantum communication, and measurementbased quantum computing. While previous works have demonstrated protocols for generating 1D photonic cluster states via quantum emitters, expanding such protocols for generation of multidimensional states requires an increasing number of emitters and control complexity. Here we present an implementation in the microwave regime of a scheme, originally proposed by Pichler et al. (PNAS 2017), for generating higherdimensional cluster states using a single emitter in a resourceefficient manner via a timedelayed feedback operation between previously emitted itinerant photons and the emitter. Our implementation uses a conventional flux tunable transmon as emitter, a superconducting slow light waveguide to introduce large group delay for itinerant photons, and a second auxiliary transmon as a switchable mirror. We discuss how this scheme is realizable by using the first three levels of the emitter as a ladder system with only one transition damped to the finitebandwidth waveguide, and by control of the decay rate into the waveguide via fluxmodulation, which allows for emission of shaped wavepackets. 
Friday, March 18, 2022 8:48AM  9:00AM 
Y34.00003: Deterministic Generation of Multidimensional Microwave Photonic Cluster States with a Single Quantum Emitter  Part 2 Gihwan Kim, Vinicius S Ferreira, Andreas Butler, Oskar Painter Multidimensional photonic graph states, such as cluster states, have prospective applications in quantum metrology, secure quantum communication, and measurementbased quantum computation. We report on the results of an experimental implementation of a resourceefficient scheme for the deterministic generation of 2D microwave photonic cluster states utilizing a slowlight waveguide with roundtrip delay ??_{d} = 240 ns, a fluxtunable transmon qubit as a quantum emitter, and a second auxiliary transmon as a switchable mirror. Strong decay rates of the qubits into the waveguide, in excess of 150??_{d}^{1} when resonant with the passband, allow for rapid emission of photons, while negligible decay rates when detuned from the passband allow for coherent qubit control. Using only single qubit gates and fast flux control of the transmon qubits we generate a state of four photons with entanglement structure consistent with that of a twobytwo cluster state, as verified by heterodyne tomographic techniques. We comment on how future design strategies could allow for generation of even larger cluster states and singleshot measurement of emitted photons, enabling the integration of quantum information processing techniques previously confined to optics into the microwave domain. 
Friday, March 18, 2022 9:00AM  9:12AM 
Y34.00004: Efficient simulation of broadband nonGaussian quantum optics with matrix product states Ryotatsu Yanagimoto, Edwin Ng, Logan G Wright, Tatsuhiro Onodera, Hideo Mabuchi Ultrashort pulses in nanophotonic waveguides can enjoy tight temporal and spatial field confinements to significantly leverage material nonlinearity, offering a unique route towards alloptical quantum engineering and (nonGaussian) gate operations. Numerical simulation of lightwave propagation in this strongly nonlinear regime, however, is a highly nontrivial task since the simultaneous presence of multimode spectraltemporal entanglement and nonGaussian quantum dynamics naïvely requires an exponentially large Hilbert space. Here, we realize efficient simulations of the quantum pulse propagation via the use of the matrix product states (MPS), with which we exploit the specific entanglement structure of the optical fields. We also develop an algorithm to access nonlocal information encoded in an MPS, e.g., reduced density matrices of arbitrary supermodes. As a demonstration, we perform fullquantum simulations of optical solitons, where the emergence of genuine nonGaussian quantum features, e.g., Wigner function negativity, are observed. Our approach can readily incorporate dissipations as well via the quantum trajectory theory. We expect our work to establish MPSbased techniques as a powerful tool for the research at this frontier of broadband nonGaussian quantum optics. 
Friday, March 18, 2022 9:12AM  9:24AM 
Y34.00005: Towards energyconstrained continuousvariable designs Joseph T Iosue, Kunal Sharma, Victor V Albert, Michael J Gullans Exactly characterizing and verifying the behavior of multiqubit devices typically requires sampling from exponentially large sets. Approximate approaches often utilize state and unitary designs  "evenly distributed" subsets of states and unitaries, respectively, that reduce the computational cost of calculating averages of functions over the aforementioned sets. In the continuousvariable (CV) domain, such tools would help characterize novel optical and microwavebased quantum devices that are quickly becoming too large for an exact analysis. However, the noncompact nature of the CV phase space presents obstructions to creating analogous tools using simple resources. For example, positiveweighted CV designs cannot be constructed with Gaussian operations [BlumeKohout, Turner 2014]. In this work, we present various approximate designs formed by relaxing the positiveweight constraint and, in some cases, also using nonGaussian resources. Our constructions are constrained in either the average or the total occupation number, utilizing displaced squeezed states, displaced Fock states, and combinations of finitesimplex and torus designs. 
Friday, March 18, 2022 9:24AM  9:36AM 
Y34.00006: Pairwise exponential level kissing in the Kerrcat eigenspectrum (part 1/2) Rodrigo G Cortiñas, Nicholas E Frattini, Jayameenakshi Venkatraman, Xu Xiao, Chan U Lei, Vidul R Joshi, Benjamin J Chapman, Steven M Girvin, Shruti Puri, Michel H Devoret

Friday, March 18, 2022 9:36AM  9:48AM 
Y34.00007: Pairwise exponential level kissing in the Kerrcat eigenspectrum: part 2/2 Nicholas E Frattini, Rodrigo G Cortiñas, Jayameenakshi Venkatraman, Xu Xiao, Chan U Lei, Vidul R Joshi, Benjamin J Chapman, Steven M Girvin, Shruti Puri, Michel H Devoret Schrödinger cat states, superpositions of coherent states in an oscillator, can be stabilized by a driven effective Hamiltonian thanks to the interplay between Kerr nonlinearity and singlemode squeezing. The pair of resulting degenerate Kerrcat states form a qubit whose coherence along one Bloch sphere axis increases exponentially with the average photon number, while decreasing only linearly along the other axes. The qubit protection arises from the progressive pairwise kissing of consecutive levels as the average photon number is increaseda quantum manifestation of robust period doubling in this system. We experimentally observe the pairwise kissing via spectroscopy, and the associated increase in the perioddoubled coherent state lifetime, which reaches 1 ms, a factor of 480 improvement over the bare lifetime of the constituent SNAILtransmon circuit. In the second part of this twopart presentation, we discuss the spectroscopy and the lifetime scaling for the coherent states and their superpositions in our system. 
Friday, March 18, 2022 9:48AM  10:00AM 
Y34.00008: Progress in the High Coherence 3D SRF Qubit Architecture Alexander Romanenko, Taeyoon Kim, Daniil Frolov, Roman Pilipenko, Matthew J Reagor, Srivatsan Chakram, Sergey A Belomestnykh, Silvia Zorzetti, Shaojiang Zhu, Mustafa Bal, Mattia Checchin, David Van Zanten, Anna Grassellino Superconducting radio frequency (SRF) cavities with seconds of coherence coupled with the superconducting transmon qubits to enable quantum operations present a powerful very high coherence platform for quantum computing and sensing  "3D SRF qubits". Long coherence times of SRF cavities allow access to multiple energy levels within the same mode, enabling e.g. the qudit architecture for quantum state encoding and manipulation  with a big decrease in the required hardware/wiring overhead and the potential alltoall qubit connectivity versus the nearest neighbor in 2D approaches. 3D SRF qubits form a foundation for the devices effort of the Superconducting Quantum Materials and Systems (SQMS) center  one of the five DOE National QIS Research Centers [*]. 
Friday, March 18, 2022 10:00AM  10:12AM 
Y34.00009: A new model for characterizing stable magnetic levitation in SRF cavities of nontrivial geometry Jeff Miller, Nabin K Raut, Demetrius Zulevic, Raymond Y Chiao, Jay E Sharping The coupling of a levitated magnet’s mechanical oscillations to a radio frequency (RF) mode inside of a superconducting cavity may lay the groundwork for coupling to quantum objects whose states can be probed and controlled, such as magnons or transmons. We have previously reported levitation of a strong, mmscale neodymium magnet within a cmscale coaxial microwave resonator. In trying to better understand the behavior of the system, we have developed a finite element model that allows us to calculate the potential energy landscape of the cavitymagnet system. This can be done for a wide array of cavity and magnet specifications as well as any magnet orientation. By identifying the stable points, we can design the cavity such that levitation can be forced in regions of interest, such as regions of maximum electric or magnetic fields. We can also control the system’s sensitivity to the magnet’s motion and approximate the mechanical frequency at which the magnet vibrates about equilibrium. The calculated potential energy landscapes are used, in combination with other geometrybased FEM simulations, in comparison with experimental measurements of the shift in resonance frequency with the motion of the magnet when there is no optical access inside of the cavity at cryogenic temperatures. 
Friday, March 18, 2022 10:12AM  10:24AM 
Y34.00010: Frequency Comb Formation in Driven High Impedance Josephson Junction Arrays Elif Cuce, Kanupriya Sinha, Saeed A Khan, Hakan E Tureci Optical frequency combs (OFCs) have a broad array of applications in metrology, spectroscopy, communications and have been realized across a range of platforms including modelocked lasers, nonlinear microresonators, microcavity excitonpolaritons and superconducting circuits. The latter has enabled a platform for generation of OFCs in a fundamentally quantum setting, mediated by a single nonlinear Josephson Junction (JJ) coupled to a single LC mode. In this work, we study the multimode extension of such a system, analyzing the drivendissipative dynamics of a nonlinear JJ coupled to an open highimpedance JJ array. Starting from a circuitlevel description, we analyze the linearized system exactly using a nonHermitian modal description. By then including the Josephson nonlinearity via a perturbative approach, we explore instability thresholds for the formation of frequency combs and analyze the dependence of comb properties on the circuit and input parameters. Our results are pertinent to recent experiments with artificial atoms coupled to high impedance JJ arrays that exhibit strong lightmatter couplings. 
Friday, March 18, 2022 10:24AM  10:36AM 
Y34.00011: Multimode quantum correlations of soliton microcombs in silicon carbide photonics Melissa A Guidry, Daniil M Lukin, Ki Youl Yang, Rahul Trivedi, Jelena Vuckovic Soliton microcombs may possess multimode entanglement across their spectral modes. In this work, we measure secondorder photon correlations between the belowthreshold modes of a soliton crystal in an integrated silicon carbide microresonator and match the correlation matrix to the theoretical model based on the linearization of soliton optical fields; we infer the entanglement structure of the state. In addition, we study the underlying quantum processes of three stages of soliton formation: (i) the belowthreshold biphoton comb, (ii) the merging of secondary combs, and (iii) merged but notyet phaselocked secondary combs. 
Friday, March 18, 2022 10:36AM  10:48AM 
Y34.00012: Shadow Tomography of ContinuousVariable Quantum Systems Srilekha Gandhari, Victor V Albert, Jacob M Taylor, Michael J Gullans Shadow tomography is a framework for constructing succinct descriptions of quantum states, called classical shadows, with powerful methods to bound the estimators used. Classical shadows are wellstudied in the discretevariable case, which consists of states of several qubits. Here, we extend this framework to continuousvariable quantum systems, such as optical modes and harmonic oscillators. We show how to adapt homodyne and photon number resolving (PNR) experimental methods from optical tomography to efficiently construct finitedimensional classical shadows for an infinitedimensional unknown state. We provide rigorous bounds on the variance of estimating density matrices from both of these experimental methods. We show that, to reach a desired precision on the classical shadow of an Nphoton density matrix with a high probability, homodyne detection requires ~N^{5} measurements in the worst case, whereas PNR detection requires only ~N^{4} measurements in the worst case. 
Friday, March 18, 2022 10:48AM  11:00AM 
Y34.00013: Simple, reliable and noiseresilient continuousvariable quantum state tomography with convex optimization Ingrid Strandberg, Shahnawaz Ahmed, Isaac Quijandria Diaz Precise reconstruction of unknown quantum states from measurement data has been researched for decades. This process, known as quantum state tomography, is of fundamental interest and nowadays also a crucial component in the development of quantum information processing technologies. Many different tomography methods have been proposed over the years. Maximum likelihood estimation is a prominent example, being the most popular method for a long period of time. Recently, more advanced neural network methods have started to emerge. Here, we go back to basics and present a method for continuous variable state reconstruction that is both conceptually and practically simple, based on convex optimization. Convex optimization has been used for process tomography and qubit state tomography, but seems to have been overlooked for continuous variable quantum state tomography. We demonstrate highfidelity reconstruction of an underlying state from data corrupted by thermal noise or imperfect detection, for both homodyne and heterodyne measurements. A major advantage over widely used iterative maximum likelihood methods is that convex optimization algorithms are guaranteed to converge to the optimal solution. 
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