Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session JJ04: V: Superconducting Quantum Information: Gates and Bosonic Qubits 
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Sponsoring Units: DQI Chair: Sara Sussman, Princeton Room: Virtual Room 4 
Monday, March 20, 2023 3:00PM  3:12PM 
JJ04.00001: DoubleTransmon Coupler I: Elimination of Residual ZZ Coupling for Highly Detuned Transmon Qubits Hayato Goto, Kentaro Kubo Tunable couplers have attracted much attention as a key device to achieve high performance in superconducting quantum computers. Recently, we have proposed a new kind of tunable coupler composed of two fixedfrequency transmons coupled through a loop with an additional Josephson junction, which we call a doubletransmon coupler (DTC) [1]. By tuning the flux in the loop, we can completely eliminate residual ZZ coupling between highly detuned qubits. This is a remarkable feature of the DTC, because conventional tunable couplers with a single frequencytunable transmon (singletransmon couplers) can eliminate the ZZ coupling only if the detuning is smaller than the qubit anharmonicity [2] and therefore they have finite residual ZZ coupling for highly detuned qubits [3,4]. The DTC also allows us to perform fast, highfidelity twoqubit gates [1], as explained in the next talk. 
Monday, March 20, 2023 3:12PM  3:24PM 
JJ04.00002: DoubleTransmon Coupler II: HighFidelity CPHASE and Parametric Gates Kentaro Kubo, Hayato Goto As explained in the previous talk (DoubleTransmon Coupler I), we proposed a new coupler, the doubletransmon coupler (DTC), that can eliminate the ZZ coupling of the highly detuned qubits. As numerically shown in Ref. [1], we can achieve a fast and highfidelity CPHASE gate by controlling the DC magnetic flux in the loop of the DTC. In this talk, we first show another important application of the DTC: a parametric gate by applying the AC magnetic flux to the loop of the DTC. The CPHASE gate in Ref. [1] and the parametric gate in this talk are implemented by utilizing ZZ and transverse coupling, respectively. To use both gates interchangeably with only flux control of the DTC, we tune various parameters for not only ZZ but also transverse coupling. As a result of tuning the circuit design parameters and the flux waveform, we successfully implemented CPHASE and parametric gates only by applying the DC and AC magnetic fluxes, respectively. We show numerical results that both gates can be implemented with highfidelity for highly detuned, fixedfrequency qubits. We expect that a highperformance quantum computer with the DTC can be realized by using these two kinds of gates for different purposes. 
Monday, March 20, 2023 3:24PM  3:36PM 
JJ04.00003: Threequbit interaction in superconducting circuits Xuexin Xu, Mohammad H Ansari Current circuit QED theory for superconducting qubits evaluates the Hamiltonian of quantum subsystem up to qubitqubit interactions. In multiqubit circuits we consider further precision in modeling analysis by considering manybody interaction terms, whose presence can either help reduce the algorithmic depth, or manifest the error rate of spectators. In this talk we propose extended circuit QED modeling and the major differences it can make. 
Monday, March 20, 2023 3:36PM  3:48PM 
JJ04.00004: A universal set of quantum gates on dynamically protected fluxonium qubits Alesya Sokolova, Lucky Kapoor, Farid Hassani Bijarbooneh, Johannes M Fink Fluxonium qubits are a promising platform for quantum computation since they have one of the highest coherence times among superconducting qubits. But, when choosing their parameters, we face the same problem as we do with all the other types of qubits: we need to make a compromise between coherence and control. If the matrix element of a transition between the ground and excited state is low (that is in a heavy fluxonium), the qubit can become almost insensitive to dielectric losses, but it would be impossible to control it. In the opposite regime, when the matrix element is high (light fluxonium), the qubit can be wellcontrolled, but the relaxation is high. For example, the fluxon states of the ultraheavy IST qubit have relaxation times on the order of hours [1], but coherent control is not feasible with a conventional approach. We propose to solve this problem by introducing a dynamically tunable fluxonium architecture that is able to switch between these two regimes insitu. Base band flux pulses can be used to turn the initially heavy fluxonium into the light regime with suitable interactions to form a universal set of single and twoqubit quantum gates. 
Monday, March 20, 2023 3:48PM  4:00PM 
JJ04.00005: Fast and highfidelity qubit initialization based on auxiliary energy levels in fluxonium Tenghui Wang, Feng Wu, Fei Wang, Lu Ma, Xizheng Ma, Gengyan Zhang, HuiHai Zhao, Chunqing Deng Fast and highfidelity qubit initialization is a necessary operation for the implementation of quantum error correction. In circuit quantum electrodynamics, the initialization is realized by implementing a state transfer between the qubit and its readout cavity. However, to ensure the qubit does not predominantly decohere through the cavity, the coupling between them is often kept in the dispersive regime, rendering the microwaveactivated state transfer a twophoton process which ultimately limits the initialization speed. Leveraging the fluxtunability and the large anharmonicity of our fluxonium qubit, we circumvent this limitation using a resonant interaction between the cavity and a noncomputational qubit transition at no cost to the nominal qubit coherence. Specifically, we utilize the secondexcited energy level of the fluxonium as an auxiliary level to perform the initialization protocol with singlefrequency microwave driving. Within hundreds of nanoseconds, we achieve a highfidelity initialization of the first and secondexcited states of a fluxonium qubit. We also demonstrate that the protocol is suitable for initializing multiplequbits. 
Monday, March 20, 2023 4:00PM  4:12PM 
JJ04.00006: Nonperturbative analysis of parametric twoqubit and threequbit gates Zhongyi Jiang, Mohammad H Ansari Realizing high fidelity entanglement gates is a major task for nearterm quantum hardware. With higher fidelity gates achieved in experiments, more accurate theoretical methods are needed. Here, using nonperturbative formalism, we theoretically study an iSWAP gate activated by frequency modulation in a transmontransmon pair. We make a comprehensive analysis to directly solve the timedependency and introduce a continuous set of Fermionic Simulation gates by tuning qubitqubit detuning and pulse phase. We propose a generalized fast and high fidelity threequbitgate that only requires one control pulse, thus eliminating timemisalignment error, which is one of the disadvantages of existing proposals for such gates. Our analysis can serve as an example for studying gates beyond the perturbative regime and RWA. 
Monday, March 20, 2023 4:12PM  4:24PM 
JJ04.00007: Nonlinear response of a transmon using the Schwinger oscillator model of angular momentum Robert P Erickson As an alternative to the JaynesCummings model of a qubit, we consider the Schwinger oscillator model of angular momentum [1] to define the pseudo spin of a transmon. Specifically, we treat the transmon as a strong anharmonic oscillator capacitively coupled to a readout resonator, itself represented by a linear oscillator. We then calculate the nonlinear frequency response of the Schwinger pseudo spin in the presence of a driving sinusoidal voltage and surrounding heat bath. We discuss the coherence of the resulting nonlinear manybody state that underlies the pseudo spin, as it arises in the steady state, and consider its broader application to quantum computation. 1) J. Schwinger, On Angular Momentum, U. S. Atomic Energy Commission Report NYO3071 (1952). 
Monday, March 20, 2023 4:24PM  4:36PM 
JJ04.00008: Detuned twoqubit gate in stabilized cat qubits Rahul Bhowmick, Mohammad H Ansari Stabilized cat qubits, together with a set of biaspreserving (BP) gates, can provide a biased noise channel having a high code capacity threshold for error correction. Compared to qubits without such structured noise channel, cat qubits require less resource overhead for faulttolerant quantum computation as the probability of X and Y errors are exponentially suppressed compared to Z errors. However, fast single and multiqubit gate operations are challenging to execute while ensuring that the noise bias is preserved since they do not necessarily commute with the dominant noise. Recently, a scheme for adiabatically implementing BP ControlledNOT gate (CNOT) has been proposed by exploiting the freedom of moving outside the computational subspace during gate execution. The scheme uses a detuning term to dynamically compensate for the accumulated geometric phase on the target qubit during adiabatic evolution. The detuning however shifts the eigenstates of the target qubit, impacting the overall fidelity of the gate. Here we study the implications of this term on the overall CNOT gate and explore possible improvements in gate fidelity. 
Monday, March 20, 2023 4:36PM  4:48PM 
JJ04.00009: Schrödinger cat in a Kerrfree SNAILterminated resonator Zhirong LIN, Xiaoliang He, Yong Lu, Daqiang Bao, Hang Xue, Wenbing Jiang, Zhen Wang Schrödinger cat states offer variety of application in quantum information processing [1]. Schrödinger cat states were normally observed in 3D cavity with permanent Kerr nonlinearity [2] or concurrent squeezing drive [1]. However, the nonlinearity from the ancillary qubit may induce the collapse of the cat states. Here, we demonstrate a method to generate Schrödinger cat states in a coplanar SNAILterminated resonator biased at Kerrfree point. To manipulate and stabilize the nonclassical superposition of microwave coherent states, we apply a pulse to tune the Kerr coefficient fastly. Through this strategy, multicomponent Schrödinger cat states can be implanted and maintained passively without additional microwave signal. Wigner function measurement on the cat state via an ancillary transmon qubit verifies the fidelity and evolution processes of the nonclassical states of microwave field. Our result shows the potential of a scalable 2D structure for the continuousvariable quantum computing based on Schrödinger cats. 
Monday, March 20, 2023 4:48PM  5:00PM 
JJ04.00010: An ultrahigh gain singlephoton transistor in the microwave regime Yan Li

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