Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A28: Superconducting Qubit Gates, Measurement and CharacterizationLive

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Sponsoring Units: DQI Chair: Kevin Satzinger, Google Inc  Santa Barbara 
Monday, March 15, 2021 8:00AM  8:12AM Live 
A28.00001: Fast Initialization Experiment of Superconducting Qubit Using SINIS Teruaki Yoshioka, Shuji Nakamura, NobuHisa Kaneko, Jaw Shen Tsai We report an experiment of fast initialization of superconducting qubit using SINIS. 
Monday, March 15, 2021 8:12AM  8:24AM Live 
A28.00002: Twoqubit tomography in the presence of stray couplings Tanay Roy, Ziqian Li, Eliot Kapit, David I Schuster Tomography is an indispensable part of quantum computation as one needs to verify or determine the state of a quantum system after certain evolution. Existing tomographic protocols are based on determining various correlators assuming noninteracting qubits. However, in realistic systems, qubits often develop some form of unavoidable stray coupling. These couplings lead to incorrect correlator measurements resulting in unfaithful reproduction of the true quantum state. We have developed a protocol that can correct for these errors by computing the evolution of the system in software and is able to correctly determine the quantum state. We demonstrate the performance of our scheme on a system of two transmon qubits with alwayson ZZ coupling. This technique is general enough to allow an extension to larger systems with different types of stray coupling and even allows the use of nonπ/2 pulses for prerotations during tomography. 
Monday, March 15, 2021 8:24AM  8:36AM Live 
A28.00003: Experimental implementation of nonClifford interleaved randomized benchmarking with a controlledS gate Shelly Garion, Naoki Kanazawa, Haggai Landa, David C McKay, Sarah Sheldon, Andrew Cross, Christopher J Wood Hardware efficient transpilation of quantum circuits is essential for the execution of quantum algorithms on noisy quantum computers. However, typical devices limit their gate set to a single Clifford twoqubit gate, e.g. the CNOT gate, which is easy to characterize via randomized benchmarking. Yet, for some applications, access to a nonClifford twoqubit gate can result in more optimal circuit decompositions and also allows more flexibility in optimizing over noise. Furthermore, recent methods have been proposed for benchmarking gates in the CNOTDihedral group, which include gates such as the control pi/2 gate (CS). To that goal, here we demonstrate calibration of a high fidelity CS gate on an IBM cloud device. To measure the gate error of the calibrated CS gate we perform the first experimental demonstration of nonClifford CNOTDihedral interleaved randomized benchmarking. We are able to obtain a low gate error close to the coherence limit of the associated qubits, and lower error than the backends standard calibrated CNOT gate. The experiments were implemented entirely using open source software tools available in the Qiskit software library, and run on a cloud accessible quantum computer. Hence the presented techniques can be readily employed by other researchers. 
Monday, March 15, 2021 8:36AM  8:48AM Not Participating 
A28.00004: Mitigating backaction in parametric quantum amplifiers Anja Metelmann, Archana Kamal Parametric quantum amplifiers are of paramount importance for quantum information processing with superconducting circuits. A promising route to design quantum amplifiers is based on parametric modulation of coupled modes, where the required modemixing processes are realized by utilizing Josephson junctionbased tunable couplers. All designs face the challenge of higherorder nonlinearities, resulting in a limitation of the dynamical range of the amplifier. However, even without any higherorder nonlinearities, the amplification process is itself nonlinear, e.g., it involves the mixing of three waves: the pump, the idler and the signal. Only for weak enough signal intensity the pump can be considered stiff and the amplification process becomes linear. Once the signal strength grows this approximation does not hold true anymore. The nonlinear nature of the mixing process leads to backaction, limiting the dynamical range of the amplifier.Here we present possible ways to face these challenges, and how to avoid unwanted backaction effects in engineered quantum systems. Furthermore, we discuss routes for optimizing the design of quantumlimited parametric amplifiers that one can avoid pumpdepletion effects completely. 
Monday, March 15, 2021 8:48AM  9:00AM Live 
A28.00005: Multipexed Photon Number Measurement Antoine Essig, Quentin Ficheux, Alain Sarlette, Pierre Rouchon, Audrey Bienfait, Benjamin Huard A single superconducting qubit can reveal the number of photons in a dispersively coupled resonator through a continuous measurement. The trick consists in using many frequency modes of a transmission line to encode the occupancy of each Fock state [1,2]. The qubit thus provides a multiplexed quantum measurement of the photon number. In this talk, I will report on our progress on performing this measurement in a single shot manner using stateoftheart microwave engineering techniques. 
Monday, March 15, 2021 9:00AM  9:12AM Live 
A28.00006: Efficient tomography of microwave photonic cluster states Yoshiki Sunada, Shingo Kono, Jesper Ilves, Takanori Sugiyama, Yasunari Suzuki, Tsuyoshi Okubo, Yasunobu Nakamura Sequential generation of entangled photonic qubits can provide us with large resource states for quantum computation and communication. However, the generated manybody entanglement is exponentially difficult to characterize using conventional quantum state tomography methods. Here, we propose and experimentally demonstrate an efficient procedure for estimating the density matrix of a sequentially generated string of photons. Because the photons do not interact with each other after they are emitted, the density matrix is constrained to be a matrix product operator with a fixed bond dimension. This means that the number of parameters to be estimated grows only linearly with the number of photonic qubits. Furthermore, since a matrix product operator is fully determined by its local reductions [1], we can choose the measurement bases such that the measurement time is also linear. To demonstrate our tomography scheme, we generate microwave photons in 1D cluster states and reconstruct their density matrices from measurements using a Josephson parametric amplifier. 
Monday, March 15, 2021 9:12AM  9:24AM Live 
A28.00007: Stabilization of squeezing beyond 3 dB in a microwave resonator by reservoir engineering. Rémy Dassonneville, Réouven Assouly, Théau Peronnin, Aashish Clerk, Audrey Bienfait, Benjamin Huard Squeezed states, whose fluctuations on one quadrature are below the zero point fluctuations (ZPF) at the expense of the other, are an instrumental resource for quantum sensing and information processing. Squeezing is usually generated by parametrically pumping a resonator. While any amount of squeezing can theoretically be obtained for the outgoing field, the intraresonator squeezing is limited to 3 dB below the ZPF. Indeed, inputoutput relations impose that the intraresonator fluctuations result from the average of the ingoing ZPF and outgoing squeezed fluctuations. Using reservoir engineering techniques [Kronwald PRA 88 (2013)], the 3 dB limit has recently been overcome in a mechanical resonator [Lei PRL 117 (2016)]. However, a proof of principle is still missing for electromagnetic modes. 
Monday, March 15, 2021 9:24AM  9:36AM Live 
A28.00008: Influence of a strong pump field on controls of superconducting quantum parametrons Shumpei Masuda, Toyofumi Ishikawa, Yuichiro Matsuzaki, Shiro Kawabata Universal quantum computation and adiabatic quantum computation using parametrons as qubits were proposed and have been studied theoretically and experimentally. A parametron can work as a qubit when it is pumped at approximately twice the natural frequency. A strong pump field can increase T1 decreasing the overlap of two coherent states with opposite phases. However, the pump field of the parametron can be an intrinsic origin of the imperfection of controls because the pump field can disturb the state of parametrons due to the violation of the rotating wave approximation. We report the influence of unwanted rapidly oscillating terms in the Hamiltonian called counter rotating terms (CRTs) on the accuracy of controls of a parametron: a catstate creation and a singlequbit gate. We also show a method to suppress both of the nonadiabatic transitions and the disturbance of the state of the parametron due to the CRTs. 
Monday, March 15, 2021 9:36AM  9:48AM Live 
A28.00009: Vacuuminduced multipartite entanglement in superconducting microwave cavity under multiple pump tones Michael Perelshtein, Ilari Lilja, Kirill Petrovnin, Terro Korkalainen, Gheorghe Sorin Paraoanu, Pertti Juhani Hakonen Quantum correlations are an essential resource in advanced information processing based on quantum phenomena. Remarkably, the vacuum state of a quantum field may act as a key element for the generation of strong quantum correlations. Besides, superconducting microwave cavities offer an excellent platform for experimental studies of such quantum effects. In this work, we experimentally investigate vacuum correlations in a fluxtunable superconducting cavity under multiple pump tones. We consider double and triple pumping cases and explore multipartite entanglement between frequency bands. Utilizing the developed scheme that provides us comprehensive control of the entanglement structure, we demonstrate genuine tripartite entanglement of the states. We envision quantum resources facilitated by the multiple pump configuration offers enhanced prospects for quantum data processing using parametric microwave cavities. 
Monday, March 15, 2021 9:48AM  10:00AM Live 
A28.00010: Fluxdriven impedancematched Josephson parametric amplifier with improved pump efficiency Yoshiro Urade, Kun Zuo, Syotaro Baba, C. W. Sandbo Chang, Kohichi Nittoh, Kunihiro Inomata, Zhirong Lin, Tsuyoshi Yamamoto, Yasunobu Nakamura Impedancematched Josephson parametric amplifiers (IMPAs) with the instantaneous bandwidth of several hundred MHz are crucial for frequencymultiplexed dispersive readout of superconducting qubits. However, for further integration of qubits, required pump power for such amplifiers can be a concern, since a high pump power may result in the increase of refrigerator temperature as well as the degradation of qubit performance due to pump leakage. In this presentation, we experimentally demonstrate highly improved pump efficiency of fluxdriven IMPAs by adding two features to their pump structures: (i) kineticinductance coupling between the SQUID in the amplifiers and pump waveguide [1] and (ii) a lowQ resonator to store pump photons and enhance the parametric process. Owing to these improvements, we can operate a fluxdriven IMPA at about 20dB gain in a nearly 1GHz bandwidth with a pump power less than 60 dBm at base temperature, which is hundred times smaller compared with our previous device [2]. 
Monday, March 15, 2021 10:00AM  10:12AM Live 
A28.00011: Theory of deterministic threephoton downconversion in ultrastrong cavity QED Kazuki Koshino, Kouichi Semba In ultra and deepstrong cavity quantum electrodynamics (QED) systems, in which the atomcavity coupling strength is comparable or even larger than the bare transition frequencies of the atom and cavity, many intriguing phenomena that do not conserve the number of excitations are expected to occur. In this study, we theoretically analyze the optical response of an ultrastrong cavity QED system in which an atom is coupled to the fundamental and third harmonic modes of a cavity, and report the possibility of deterministic threephoton downconversion, in which a single parent photon is converted into triplet photons by reflection from the cavity. In principle, such downconversion may occur in the usual strongcoupling systems that does not reach the ultrastrong coupling regime. However, considering the required intrinsic loss rates, this phenomenon would be characteristic to the ultrastrong cavity QED. 
Monday, March 15, 2021 10:12AM  10:24AM Live 
A28.00012: Implementation of continuous multiqubit gate families on superconducting quantum processors Alexander Hill, Nicolas Didier Quantum processors in the noisy intermediatescale (NISQ) era will rely on the decomposition of complex, errorprone circuits into the fewest possible native pulse operations. This is especially true of computations involving entangling operations parameterized by one or more continuous variables, such as the UXY (universal XY) and UZZ (universal CPHASE) gates, which are useful in nearterm quantum chemistry and combinatorial optimization problems and can dramatically reduce overall circuit length relative to circuits relying only on simple Clifford rotations, such as CZ. Here we extend our technique for generating the UXY family of gates [1] to produce the UZZ gates gates via the calibration of a single pulse, enabling high fidelity for arbitrary rotation angles in the twoqubit space. We present our latest results for each of these gate families on our current class of superconducting quantum processors, and additionally construct arbitrary fermionic simulation (fSIM) gates and the three qubit controlledcontrolled phase (CCPHASE) gates. 
Monday, March 15, 2021 10:24AM  10:36AM Live 
A28.00013: Parametricresonance entangling gates for superconducting qubits Eyob Sete, Nicolas Didier, Angela Chen, Shobhan Kulshreshtha, Riccardo Manenti, Stefano Poletto We demonstrate experimentally parametric resonance iSWAP and controlledZ entangling gates via radiofrequency flux modulation of one of the qubits. In contrast to the standard parametric gates where sideband frequencies are used to activate the gates, parametricresonance gates are activated when the average frequency of the modulated qubit is in resonance with the unmodulated qubit. This allows us to approximately retain the bare qubitqubit coupling, as opposed to a reduction by 30% at best in the case of standard parametric gates. We show that fast high fidelity iSWAP and CZ gates can be realized by leveraging the flexibility of parametric modulation while maintaining the bare coupling rates. Moreover, because the modulation frequency is not used to activate the interaction, parametricresonance gates provide flexibility in avoiding collisions with unwanted resonances. Finally, these entangling gates are protected from slow flux noise by operating the modulated qubit at a dynamical sweet spot 
Monday, March 15, 2021 10:36AM  10:48AM Live 
A28.00014: Realization of Highfidelity CZ and ZZfree iSWAP Gates with a Tunable Coupler Youngkyu Sung, Leon Ding, Jochen Braumüller, Antti Vepsäläinen, Bharath Kannan, Morten Kjaergaard, Amy Greene, Gabriel O Samach, Chris McNally, David K Kim, Alexander Melville, Bethany Niedzielski, Mollie Schwartz, Jonilyn Yoder, Terry Philip Orlando, Simon Gustavsson, William Oliver Highfidelity twoqubit gates at scale are a key requirement to realize the full promise of quantum computation and simulation. The advent and use of coupler elements to tunably control twoqubit interactions has improved operational fidelity in manyqubit systems by reducing parasitic coupling and frequency crowding issues. Nonetheless, twoqubit gate errors still limit the capability of nearterm quantum applications. The reason, in part, is the existing framework for tunable couplers based on the dispersive approximation does not fully incorporate threebody multilevel dynamics, which is essential for addressing coherent leakage to the coupler and parasitic longitudinal interactions during twoqubit gates. Here, we present a systematic approach that goes beyond the dispersive approximation to exploit the engineered level structure of the coupler and optimize its control. Using this approach, we experimentally demonstrate CZ and ZZfree iSWAP gates with twoqubit interaction fidelities of 99.76 +/ 0.10 % and 99.87 +/ 0.32 %, respectively, which are close to their T1 limits. 
Monday, March 15, 2021 10:48AM  11:00AM Live 
A28.00015: Ultimate quantum limit for amplification: a single atom in front of a mirror Emely Wiegand, Anton Frisk Kockum, Ping Yi Wen, IoChun Hoi We investigate three types of amplification processes for light fields coupling to an atom near the end of a onedimensional semiinfinite waveguide. We consider two setups where a drive creates population inversion in the bare or dressed basis of a threelevel atom and one setup where the amplification is due to higherorder processes in a driven twolevel atom. In all cases, the end of the waveguide acts as a mirror for the light. We find that this enhances the amplification in two ways compared to the same setups in an open waveguide. Firstly, the mirror forces all output from the atom to travel in one direction instead of being split up into two output channels. Secondly, interference due to the mirror enables tuning of the ratio of relaxation rates for different transitions in the atom to increase population inversion. We quantify the enhancement in amplification due to these factors and show that it can be demonstrated for standard parameters in experiments with superconducting quantum circuits. 
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