Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D36: Quantum Control: Quantum GatesRecordings Available

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Sponsoring Units: DQI Chair: Sara Sussman, Princeton Room: McCormick Place W194A 
Monday, March 14, 2022 3:00PM  3:12PM 
D36.00001: Qutrit Entanglement With Differential AC Stark Shift Noah Goss, Alexis Morvan, Brad Mitchell, Brian Marinelli, Ravi K Naik, David I Santiago, Irfan Siddiqi Ternary quantum information processing in circuit quantum electrodynamics devices poses a promising alternative to its more popular binary counterpart through larger computational spaces and proposed more efficient error correction schemes. Recent advancements in ternary quantum computing, such as qutrit randomized benchmarking and quantum information scrambling on a qutrit device, have been key in enabling qutrit development and in demonstrating its value in quantum simulation. However, effectively engineering two qutrit entanglement remains a central challenge towards realizing ternary quantum information processing. In this work, we present a generalized Joint Amplification of ZZ (JAZZ) method for measuring the entanglement between two nearest neighbor transmon qutrits. Leveraging this method, we apply the differential AC Stark shift to enable larger driven ZZ interactions and implement a scheme for an efficient two qutrit CPhase gate. 
Monday, March 14, 2022 3:12PM  3:24PM 
D36.00002: A quantum module with alltoall gates via parametric control Pinlei Lu, Chao Zhou, Mingkang Xia, Ryan Kaufman, Israa Yusuf, Param J Patel, Boris Mesits, Maria M Mucci, Michael J Hatridge For quantum computing in the NISQ era, most platforms for superconducting systems, including surface code, employ a network of twobody interactions between nearestneighbor qubits. Alternatively, modular quantum computers seek to create networks with dense local couplings among small 'quantum modules' which are in turn connected via a quantum bus. In this talk, we present such a quantum module comprised of a central Superconducting Nonlinear Asymmetric Inductive eLement (SNAIL) coupled with four transmon qubits. Twoqubit interactions are created via threewave coupling driving the SNAIL at the difference frequency of a pair of qubits. The module's architecture allows us to realize alltoall twoqubit couplings with experimental SWAP times of ~100 ns in our prototype. Moreover, we can also drive single qubit gates in the module as fast as ~20 ns by driving the central SNAIL at one half of each qubit's resonant frequency, allowing the entire module's gates to be implemented via a single drive line. The module is also directly compatible with our previously realized quantum state router (C. Zhou, et al. arXiv (2021)). We will present data characterizing the device's performance and discuss the prospects for its integration into largerscale modular quantum machines. 
Monday, March 14, 2022 3:24PM  3:36PM 
D36.00003: Modular coupling approach using ancilla transmons with fluxtunable hybridization Daniel L Campbell, Archana Kamal, Leonardo M Ranzani, Michael Senatore, Matthew LaHaye Recent demonstrations of small quantum processors (tens of qubits) based upon superconducting circuits have incorporated coupling elements to facilitate fast multiqubit operation without sacrificing data storage fidelity. A recently proposed [Yan2018] and now stateoftheart coupling approach leverages the interference of a static interaction between two data transmons and a separate virtual interaction through a noncomputational flux tunable ancilla transmon to produce an effective tunable coupling. The resultant coupling is a nontrivial function of all three transmons’ frequencies and the static interactions between them. Consequently, adapting the coupler to new architectures and use cases can require considerable remodeling. To overcome this challenge, we introduce a new ancillabased coupler that utilizes the hybridization of two ancillas to mediate the interaction of external circuitry across the coupler. Our proposed coupler is modular in design and simulation, has first order insensitivity to datatransmon parameters, and can mediate both degenerate and parametric interactions to implement a diverse set of entangling operations. 
Monday, March 14, 2022 3:36PM  3:48PM 
D36.00004: Mitigating offresonant error in the crossresonance gate Moein Malekakhlagh, Easwar Magesan Offresonant error for a driven quantum system refers to interactions due to the input drives having nonzero spectral overlap with unwanted system transitions. Here, we quantify offresonant error for the crossresonance interaction with application to a direct CNOT gate implementation [1, 2]. We show that pulse parameters should be optimized so that offresonant transition frequencies coincide with the local minima due to the pulse spectrum sidebands. Additionally, we show that a YDRAG [3, 4] pulse on the control qubit can significantly help to mitigate the effects of offresonant error. Depending on system parameters, the proposed methods can improve the average offresonant error by up to an order of magnitude for a direct CNOT calibration. 
Monday, March 14, 2022 3:48PM  4:00PM 
D36.00005: Towards highfidelity twoqubit gates on fluxonium qubits Haonan Xiong, Quentin Ficheux, Konstantin Nesterov, Aaron Somoroff, Ray A Mencia, Roman Kuzmin, Maxim G Vavilov, Vladimir Manucharyan Recently we demonstrated microwave twoqubit gate schemes [1, 2] using high levels of fluxoniums, where the gate fidelity is limited by the decoherence outside the computational space. To solve this problem, here we demonstrate the implementation of microwave twoqubit gates using only computational states. By applying a strong microwave drive between the two qubit frequencies, we can flip 00 to 11 in ~80 ns through the twophoton transition. On the other hand, we can induce a ZZ interaction (~5 MHz) by offresonantly driving near the qubit frequencies. With these techniques, we can construct a bSWAP gate and a CZ gate and over 99.9% fidelities can be expected. These highfidelity twoqubit gates on fluxoniums can benefit the development of Noisy Intermediate Scale Quantum (NISQ) processors and universal quantum computing. 
Monday, March 14, 2022 4:00PM  4:12PM 
D36.00006: Microwave activated twoqubit gate for fluxonium qubits via a tunabletransmon coupler Leon Ding, Youngkyu Sung, Bharath Kannan, Agustin Di Paolo, Junyoung An, Max Hays, Roni Winik, Kyle Serniak, Thomas M Hazard, David K Kim, Bethany M Niedzielski, Alexander Melville, Jonilyn L Yoder, Mollie E Schwartz, Devin L Underwood, Terry P Orlando, Simon Gustavsson, William D Oliver Qubit lifetimes in superconducting transmon based quantum computers are a leading cause of gate infidelity. Furthermore, the transmon’s anharmonicity gives rise to frequency crowding on multiqubit devices and limits the gate speed. The fluxonium qubit is a promising alternative to transmons, with coherence times reaching the order of milliseconds and anharmonicities on the order of gigahertz. In this work, we present a device containing two fluxonium qubits connected by a tunabletransmon coupler. By utilizing the higher levels of the fluxonium qubits and the transmon excited state, we explore the potential of a microwave activated CPHASE gate. We present results on a device designed to operate in a parameter space that has large qubittoqubit couplings and a reduced alwayson ZZ interaction. This architecture is expected to facilitate faster, higher fidelity twoqubit gates. 
Monday, March 14, 2022 4:12PM  4:24PM 
D36.00007: The siZZle Gate – Using AC Stark tones to modulate ZZ in Superconducting Transmon Qubits David C McKay, Xuan Wei, Easwar Magesan, Isaac Lauer, Srikanth Srinivasan, Daniela F Bogorin, Santino Carnevale, George Keefe, Youngseok Kim, David Klaus, William Landers, Neereja Sundaresan, Cindy Wang, Eric J Zhang, Matthias Steffen, Oliver E Dial, Abhinav Kandala Fixed frequency superconducting transmon qubits are an attractive technology for scaling due to their high coherence and stability. However, there are a number of challenges associated with always on coupling. In particular, the higher levels cause shifts in the computational levels that leads to unwanted ZZ quantum crosstalk. Here, we will discuss a novel technique to manipulate the energy levels and mitigate this crosstalk via a simultaneous AC Stark effect on coupled qubits. This breaks a fundamental deadlock between qubitqubit coupling and crosstalk, leading to a 90ns CNOT with a gate error of (0.19 ± 0.02)% and the demonstration of a novel CZ gate with fixedcoupling singlejunction transmon qubits. Furthermore, we show a definitive improvement in circuit performance with crosstalk cancellation over seven qubits, demonstrating the scalability of the technique. This talk is based on work published in arXiv:2106.00675 (2021). 
Monday, March 14, 2022 4:24PM  4:36PM 
D36.00008: Understanding the speed limits of parametrically pumped quantum gates Chao Zhou, Pinlei Lu, Daniel K Weiss, Mingkang Xia, Ryan Kaufman, Param J Patel, Boris Mesits, Israa Yusuf, Maria M Mucci, David Pekker, Jens Koch, Michael J Hatridge Controllable couplings between qubits are vital for realizing largescale quantum machines. In superconducting systems, highfidelity twoqubit gates can be performed by offresonant parametric pumping of a nonlinear element dispersively coupled to two qubits. In this scheme, the performance of fast highfidelity gates requires strong pumping. However, high drive strengths may activate unwanted transitions which can ruin gate fidelity and coherence properties. Moreover, strong coupling between the pump port and the nonlinear mode may limit the lifetime of the quantum modes being controlled. In this work we will use our previously built quantum state router [Zhou and Lu, arxiv: (2021)] and a new 4qubit quantum module as platforms (which both operated via 3wavebased parametric gates) to study how to characterize and control the factors that limit our gate speed. We show how to identify and mitigate the effects of parasitic parametric processes while maintaining qubit lifetimes, and engineer the drive port's impedance to allow stronger parametric drives while maintaining mode lifetimes. In total, our results open a pathway to realizing a modular qubit architecture featuring highfidelity parametric gates. 
Monday, March 14, 2022 4:36PM  4:48PM 
D36.00009: Robust entangling shaped pulses for cavitycoupled silicon quantum dot spin qubits with alwayson coupling Utkan Güngördü, Jason P Kestner, Charles Tahan The fidelity of cavitymediated longrange entanglement between singlyloaded double quantum dot spin qubits in silicon is ultimately limited by charge noise. Leading order effects of quasistatic charge noise can be suppressed by using a composite pulse sequence provided that the coupling can be switched on and off rapidly by pulsing on the tunnel barrier voltage [1]. However, a different approach is needed when bandwidth limitations [2] preclude rapid pulsing of the tunnel barrier. Here, we report a robust iSWAP gate for such a system, with both qubits driven slightly offresonantly using a shaped pulse obtained using a physics informed neural network [3], in the presence of an alwayson exchange coupling. The resulting gate is robust against quasistatic charge noise as well as driving amplitude errors, and the control requirements are within experimental limitations. 
Monday, March 14, 2022 4:48PM  5:00PM 
D36.00010: Crosstalk resistant CZ gate robust against charge noise in silicon two qubit devices David W Kanaar, Jason P Kestner, Utkan Güngördü

Monday, March 14, 2022 5:00PM  5:12PM 
D36.00011: Demonstration of an entangling gate between noninteracting qubits using the quantum Zeno effect Eliya Blumenthal, Birgitta Whaley, asaf A diringer, Leigh S Martin, Daniel Burgrath, Shay HacohenGourgy, Chen Mor The quantum Zeno effect occurs in systems when frequent measurements are applied to effectively freeze the system dynamics, holding it at an eigenstate of the measurement observable. The measurements divide the Hilbert space into subspaces with distinct eigenvalues of the measured observable, and give rise to 'Zeno dynamics' within each. Transitions between subspaces are suppressed by measurement, but the evolution inside each subspace is completely coherent. We show that Zeno dynamics can deterministically create entanglement between two 
Monday, March 14, 2022 5:12PM  5:24PM 
D36.00012: Crosstalk Cancellation in The TunableCoupling Architecture with FixedFrequency Qubits Orkesh Nurbolat, Ji Chu, Zhikun Han, Yang Yu, Fei Yan Scalability of superconducting quantum processors is undermined by both quantum and classical crosstalk between qubits. Here, we implement a set of scalable and robust techniques for characterizing and cancelling crosstalk in a tunablecoupling architecture with fixedfrequency transmon qubits, and show improvement in simultaneous benchmarking results. 
Monday, March 14, 2022 5:24PM  5:36PM 
D36.00013: Implementing dispersive readout of superconducting qubits with simultaneous resonator reset Markus Jerger, Felix Motzoi, Christian Dickel, Daniel J Weigand, Jonas Bylander, David P DiVincenzo, Rami Barends, Pavel Bushev A key challenge in quantum computing is speeding up measurement and initialization. Here, we experimentally demonstrate a dispersive measurement method for superconducting qubits that simultaneously returns the readout resonator to its initial state. The approach is based on analytical pulses [1] and requires knowledge of the qubit and resonator parameters, but needs no direct optimization of the pulse shape. Moreover, the method generalizes to an arbitrary number of modes. In the case of qubit readout, we are able to drive the resonator to 10^{2} and back to 10^{2} photons in less than 4/κ, achieving a T_{1}limited assignment fidelity of 98.5%. We also present results for qutrit readout. 
Monday, March 14, 2022 5:36PM  5:48PM 
D36.00014: Halving iontrap twoqubit gate time while enhancing frequencydrift robustness Seyed Shakib Vedaie, Eduardo Paez, Barry C Sanders Twoqubit gate performance is vital for scaling up iontrap quantum computing, and reducing gate time τ and gate error rate is achieved by quantumcontrol methods. We develop a full model for twoqubit gates effected in a Paul trap with multiple ions, described by a master equation incorporating the singleion quadrupolar effective Rabi frequency, 
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