Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R08: Advances in Qubit Measurement IFocus

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Sponsoring Units: DQI Chair: Luke Govia, BBN Technology  Massachusetts Room: 104 
Thursday, March 5, 2020 8:00AM  8:12AM 
R08.00001: Simultaneous Measurement of 53 Superconducting Transmons on the Sycamore Processor Zijun Chen, Ofer Naaman, Daniel T Sank, Paul Klimov, Chris Quintana, Julian Kelly, Anthony E Megrant, Hartmut Neven, John M Martinis

Thursday, March 5, 2020 8:12AM  8:24AM 
R08.00002: Singleshot readout and state preparation of a fluxonium qubit without the use of a parametric amplifier Daria Gusenkova, Martin Spiecker, Richard Gebauer, Lukas Gruenhaupt, Patrick Winkel, Francesco Valenti, Ivan Takmakov, Dennis Rieger, Alexey V. Ustinov, Wolfgang Wernsdorfer, Oliver Sander, IoanMihai Pop Highfidelity qubit readout is an essential requirement for the implementation of quantum algorithms. A commonly used readout technique involves the dispersive interaction between a qubit and a resonator, which encodes the qubit state into the phase and amplitude of the microwave readout tone. In theory, the state discrimination can be substantially improved if the resonator is populated with high photon numbers [1]. In practice however, the optimal photon number, at which the best readout fidelity is obtained, is usually in the range of a few photons [2, 3], and parametric amplifiers with near quantumlimited noise are needed to overcome the noise of even the best commercial high electron mobility transistor amplifiers. 
Thursday, March 5, 2020 8:24AM  8:36AM 
R08.00003: Measurement of quantum jumps of a fluxonium qubit using a Dimer Josephson Junction Array Amplifier operated at high power Ivan Takmakov, Patrick Winkel, Farshad Foroughi, Martin Spiecker, Lukas Gruenhaupt, Daria Gusenkova, Luca Planat, Dennis Rieger, Alexey V. Ustinov, Wolfgang Wernsdorfer, IoanMihai Pop, Nicolas Roch Josephson parametric amplifiers have become an essential element in cQED dispersive readout measurement schemes, enabling singleshot qubit readout. Over the last decade there was significant progress in the increase of their saturation power [1,2,3,4], which now reaches several thousand photons per μs. 
Thursday, March 5, 2020 8:36AM  8:48AM 
R08.00004: Towards autonomous digital feedback on a superconducting qubit Ziyi Zhao, Eric Rosenthal, Maxime Malnou, Christian M. F. Schneider, Leila Vale, Gene C. Hilton, Gerhard Kirchmair, Jiansong Gao, K. W. Lehnert Quantum error correction requires conditional operations predicated on measurement results. Such feedback schemes in superconducting qubit systems, however, can be hardware intensive. They may require dedicated control hardware at ambient temperature and consume precious highbandwidth connections between temperature stages. Here we present preliminary data on the state preparation and stabilization of a superconducting qubit where we keep the measurement information at the base temperature stage of the cryostat, and then use it to conditionally π pulse a qubit. This scheme avoids the aforementioned problems and can ease the development of stabilized quantum networks within a scalable architecture. 
Thursday, March 5, 2020 8:48AM  9:00AM 
R08.00005: Isolating a qubit from amplifier backaction by coordinated switching Eric Rosenthal, Christian M. F. Schneider, Maxime Malnou, Ziyi Zhao, Felix Leditzky, Benjamin J. Chapman, Xizheng Ma, Daniel A Palken, Leila R. Vale, Gene C. Hilton, Gerhard Kirchmair, Jiansong Gao, Graeme Smith, K. W. Lehnert We demonstrate a cQED qubit measurement scheme in which the qubit is isolated from amplifier backaction without ferrite circulators. We swap a readout signal from the qubit cavity to an ancillary cavity by temporarily coupling them. The state of the ancillary cavity is then amplified by parametrically pumping past bifurcation into one of two large amplitude states that are correlated with the qubit state. Isolation and efficiency are comparable to readout using a ferrite circulator, but are achieved using a chipscale device compatible with the integration of superconducting qubits. Because the measurement result is contained in the digital state of a superconducting bifurcation amplifier, it is possible to use this scheme to create autonomous feedback loops at the cryostat base temperature. The integrated nature of this readout suggests an avenue toward the scalable measurement of larger quantum networks. 
Thursday, March 5, 2020 9:00AM  9:12AM 
R08.00006: Onchip singlepump interferometric Josephson Isolator Baleegh Abdo, Oblesh Jinka, Nicholas T Bronn, Salvatore Olivadese, Markus Brink, Jerry M. Chow Nonreciprocal microwave devices, such as circulators and isolators, play several critical roles in superconducting quantum processors. They route readout signals in a directional manner, protect the quantum system against noise coming from the output chain, and enable reflection measurements by separating input from output. However, the reliance of these devices, on magnetic materials and strong magnets for breaking reciprocity, makes them disadvantageous in scalable superconducting architectures. In this work, we realize and measure an onchip Josephson isolator, which is formed by coupling two nondegenerate Josephson mixers in an interferometric scheme. Isolation is created, in the active device, by operating the two mixers in frequency conversion mode using balanced, samefrequency microwave pumps, whose phase difference is \pi/2. The applied pumps are generated using an onchip quadrature hybrid, which equally splits the single pump feeding the device and imposes the required phase difference between the split drives. 
Thursday, March 5, 2020 9:12AM  9:24AM 
R08.00007: Novel Measurements for Annealing Capable Flux Qubits Wade DeGottardi, David J. Clarke, Sergey Novikov, James I Basham, Jeffrey Grover, Steven M Disseler, David Ferguson

Thursday, March 5, 2020 9:24AM  9:36AM 
R08.00008: Realizing large, tunable dispersive shifts with parametric couplings Zhihao Xiao, Leonardo M Ranzani, Luke Govia, Raymond W Simmonds, Jose Aumentado, Archana Kamal In recent years, parametric couplings are being deployed in a variety of applications, ranging from parametric gates, state preparation to quantum annealing. Unlike their traditional applications such as parametric amplification, these new functionalities require much stronger parametric coupling strengths that can dominate qubit decay and measurement rates. The theoretical framework to capture the effect of such strong timedependent couplings, however, remains rudimentary. In this work, we analyze the primary building block of such a platform  a single qubit parametrically coupled to a singlemode resonator. We establish that even this simple system can support a rich structure in dispersive shifts, that can be rendered large and tunable by choosing suitable parametric coupling drive amplitudes and frequencies. In addition, this platform can enable enhanced cavity squeezing by multitone parametric pumping. Our scheme can be realized with stateoftheart circuitQED architecture, and we discuss our early experimental efforts to support this approach. 
Thursday, March 5, 2020 9:36AM  9:48AM 
R08.00009: Remote entanglement via two mode squeezed light Xi Cao, Gangqiang Liu, TzuChiao Chien, Chao Zhou, Pinlei Lu, Michael Jonathan Hatridge Remotely entangling qubits which do not interact directly is very desirable for quantum information processing. One method for remote entanglement is to read out two qubits in parallel with their outputs entering the signal and idler mode of a phasepreserving amplifier. The amplifier squeezes away whichpath information and can project the qubits into a Bell state. However, this measurement also produces an outcomedependent phase kick on the qubits’ state. Thus, losses and inefficient amplification can prevent the generation of entangled states [1, 2]. However, we have recently demonstrated a new scheme which uses twomode squeezed light (TMSL) in an interferometer formed by two phasepreserving amplifiers to measure a qubit. We find that the phase backaction of TMSL measurement alters the phase backaction relative to coherent readout, depending on the amplifiers’ relative phase. Here we present a two qubit, TMSL interferometric entangling readout. We will show how TMSL affects the backaction of the entangling readout and discuss the prospects for the use of TMSL to make the scheme more tolerant of hardware imperfections. 
Thursday, March 5, 2020 9:48AM  10:24AM 
R08.00010: Fast high fidelity quantum nondemolition superconducting qubit readout Invited Speaker: Olivier Buisson The most common technique of qubit readout in cQED relies on the transverse dispersive coupling between a qubit and a microwave cavity. However, despite important progresses, implementing fast high fidelity and QND readout remains a major challenge. Indeed, inferring the qubit state is limited by the tradeoff between speed and accuracy due to Purcell effect and unwanted transitions induced by readout photons in the cavity. To overcome this, we propose and experimentally demonstrate a new readout scheme based on a transmon molecule inserted inside a 3Dcavity_[1,2]. The full system presents a transmon qubit mode coupled to a readout mode through an original nonpertubative crossKerr coupling. The readout mode, called polariton mode, results from the hybridization between the microwave cavity and the transmon molecule circuit. The direct crossKerr coupling is a key point of our readout scheme since it protects the qubit from Purcell effect. This first implementation, though perfectible, already enables a very efficient singleshot QND readout of the qubit in only 50ns, with a QNDness of 99% and a fidelity of 97.4%. 
Thursday, March 5, 2020 10:24AM  10:36AM 
R08.00011: Frequencydomain pulse engineering for fast qubit readout and resonator reset Riccardo Borgani, Mats Tholen, Shan Williams Jolin, Daniel Forchheimer, David Haviland The state of a superconducting qubit is typically determined by measuring the dispersive shift χ of a superconducting resonator with coupling rate κ to a transmission line. Large χ/κ ratio is beneficial for highfidelity readout. While small κ protects the qubit from the noisy environment, it has the big disadvantage of limiting the speed of measurement. In particular, photons linger in the resonator for a time of several 1/κ, causing decoherence of the qubit and blocking subsequent measurement. Here we present a microwave pulse scheme which, regardless of the state of the qubit, populates the readout resonator, measures the state of the qubit, and empties the resonator, all in a time much faster than 1/κ. The pulse is designed in the frequency domain by shaping its spectral content at the dressedresonance frequencies. The pulse envelope is derived through inverse Fourier transform, rather than relying on bruteforce numerical optimization. We evaluate the performance of the pulse scheme in different regimes of χ/κ through both simulation and experiment, and we comment on the theoretically achievable singleshot fidelity of the method. 
Thursday, March 5, 2020 10:36AM  10:48AM 
R08.00012: High efficiency measurement of a superconducting qubit using a directional, phasesensitive, parametric amplifier Florent Lecocq, Leonardo M Ranzani, Gabriel Peterson, Shlomi Kotler, Katarina Cicak, X. Y. Jin, Raymond W Simmonds, John Teufel, Jose Aumentado The measurement of a superconducting quantum bit is often performed by encoding its state in a single quadrature of a microwave field. Ideal measurement efficiency of this observable could in principle be achieved by noiselessly amplifying the informationcarrying quadrature, but in practice is limited by technical losses due to circulators, cables and connectors used in stateoftheart amplification chains. 
Thursday, March 5, 2020 10:48AM  11:00AM 
R08.00013: High fidelity dispersive qubit readout in circuit QED without using a Josephson Parametric Amplifier Suman Kundu, Kishor V Salunkhe, Anirban Bhattacharjee, Sumeru Hazra, Meghan P. Patankar, R Vijay In the cQED architecture, the Josephson parametric amplifier (JPA) enables high fidelity readout by providing minimal degradation of the signaltonoise ratio (SNR) of the amplified readout signal. While the SNR can also be increased by increasing readout power and integration time, the fidelity typically gets limited due to unwanted transitions both within and outside the computational subspace. Here, we present and demonstrate an alternate design where qubitcavity coupling does not rely on the dispersive approximation of the JaynesCummings Hamiltonian. This multimodal circuit is reminiscent of the quantronium qubit design but with two differences: the qubit is of the transmon type and the cavity is linear. The device is implemented in the 3D cQED architecture and we use a rectangular waveguide to couple the readout cavity to the measurement line. We achieve a readout fidelity of 97.8% for 800ns integration time with a histogram overlap error of only 0.3% without using a JPA. The best fidelity obtained with the JPA was 98.9% for a 300 ns integration time with an overlap error of less than 0.01%. We will conclude by discussing the variation of readout fidelity with measurement power and compare with a conventional transmon readout in cQED. 
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