Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L28: Fluxonium QubitsFocus Live
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Sponsoring Units: DQI Chair: David Zajac, IBM TJ Watson Research Center |
Wednesday, March 17, 2021 8:00AM - 8:36AM Live |
L28.00001: High-fidelity gates on fluxonium qubits Invited Speaker: Quentin Ficheux A promising path to reduce gate errors in transmon-based quantum processors consists in developing highly anharmonic circuits with some degree of protection from prevailing decoherence sources. At present, properly designed single fluxonium qubits can have over 1 ms coherence time via the trick of slowing down the qubit transition about tenfold or more compared to transmons. We describe recent progress in the implementation of high-fidelity single and two-qubit gates in fluxonium circuits. This includes a fast microwave-activated controlled-Z gate completed in less than 9 qubit Larmor cycles (about 60 ns) with a fidelity of 99.2%, which is on-par with the best microwave-activated gates reported on transmon qubits. Finally, we discuss the prospects of extending our two-qubit gates to large scale quantum processors. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L28.00002: High-fidelity entangling gates for fluxonium qubits Feng Wu, Dawei Ding, Tenghui Wang, Xiaotong Ni, Hsiang-Sheng Ku, Gengyan Zhang, Additional Contributing AQL Members, Yaoyun Shi, Jianxin Chen, Chunqing Deng, Hui-Hai Zhao Fluxonium is the one of the promising candidates for the next generation of superconducting quantum processors due to its long coherence time and large anharmonicity. These qualities make it feasible to achieve ultra-high fidelity entangling gates. In this work, we theoretically analyze and compare various two-qubit gate schemes with all-microwave drives, as well as those with qubit frequency tuning, and numerically demonstrate how to obtain high fidelities. We will show further improvement of the gate fidelity by suppressing ZZ crosstalk and dephasing noise via dynamical decoupling with continuous drives. In addition, we will also discuss the many-body effect on two-qubit gates in multi-qubit devices. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L28.00003: Two Qubit Cross Resonance Gate in Fluxonium Jacob Elvin Bryon, Martin A Ritter, Maya M Amouzegar, Alicia Kollar, Andrew Houck A potential alternative to the widely used transmon qubit is the fluxonium circuit, which has desirable coherence properties when operated at half flux [1]. In contrast to the transmon there are few proposed two qubit gates for the fluxonium, a critical component for quantum computing. One popular two qubit gate for the transmon is the cross resonance (CR) gate, achieved by irradiating the control qubit at the target qubit frequency. This scheme has been shown to achieve high fidelities above 0.99 for transmons [2]. The CR gate is limited by low anharmonicities, forcing longer gate times to avoid leakage to noncomputational states. Fluxonium, having a much larger anharmonicity, has an advantage over the transmon in this respect. We propose a scheme to drive a CR gate in two fluxonium qubits coupled via an on-chip cavity and present the experimental progress in this effort. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L28.00004: Cross resonance gate for a capacitively coupled two fluxonium device Ebru Dogan, Dario Rosenstock, Quentin Ficheux, Haonan Xiong, Aaron Somoroff, Ray Mencia, Konstantin Nesterov, Maxim G Vavilov, Vladimir Manucharyan, Chen Wang Cross resonance gate is an entangling gate widely used with transmon qubits for its 'all microwave' structure and no tunability requirements. Yet the weak anharmonicity of the transmon qubit puts some limitations on the CR gate such as the long gate duration. Fluxonium qubits on the other hand are promising candidates for better logic gate performances due to their large anharmonicity and longer coherence times: Application of the cross resonance gate on fluxoniums with fixed frequencies at the half flux quantum suggests shorter gate durations and less leakage to higher states. This work outlines an experimental scheme for calibration and realization of cross resonance gates on fluxonium devices. We will report our progress about the tune up procedures and the benchmarking for a cross resonance gate on a capacitively coupled two fluxonium device in a 3D cavity resonator. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L28.00005: Entanglement of fluxonium qubits without leaving the computational space Konstantin Nesterov, Quentin Ficheux, Chen Wang, Vladimir Manucharyan, Maxim G Vavilov The superconducting fluxonium circuit in its flux sweet spot possesses unique spectral properties. Its main qubit transition has low frequency and exceptionally long lifetime reaching 500 us [1], while the transition between its first and second excited states has an order of magnitude higher frequency and stronger coupling to a microwave field, which has been utilized in a fast controlled-Z gate [2, 3]. On the other hand, strong anharmonicity of the fluxonium also simplifies qubit control by driving transitions in the computational subspace to perform two-qubit gates. The first example discussed here is a controlled-X gate reminiscent of the cross-resonance gate in transmons [4]. It is activated by driving at the frequency of the target qubit and requires two independent controls. The second example is a swapping gate operation in the 00-11 subspace activated by a high-power drive at half the frequency of the 00-11 two-photon transition, which can be compared to bSWAP gate with transmons [5]. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L28.00006: Cavity-photon induced state transitions in a coupled Fluxonium qubit system Jeremy Stevens, Alexis Jouan, Nathanael Cottet, Long B Nguyen, Aaron Somoroff, Quentin Ficheux, Audrey Bienfait, Vladimir Manucharyan, Benjamin Huard Superconducting qubits are a subject of intense research as a platform for scalable quantum computing. While transmon qubits have received a lot of attention, the less ubiquitous Fluxonium qubit has been shown to have long life-times and gates unlimited by level anharmonicity [1]. Despite this, little research has been put into studying multi-Fluxonium devices. Here, one of the difficulties is understanding how their rich level structure can make them prone to measurement photons inducing transitions out of the qubit subspace [2]. We present a systematic study of a system comprising two capacitively coupled Fluxonium qubits sharing the same read-out cavity. By tracking the state dependent transmission of the read-out pulse, we determine the transition rates from state i to state j of the coupled system using a forward-backward analysis [3] to characterize these cavity induced transitions. By varying the flux bias of the system and the population of the cavity, we characterize the fidelity of this read-out. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L28.00007: Excitation Dynamics in an Inductively Coupled Fluxonium Chain A. Baris Ozguler, Vladimir Manucharyan, Maxim G Vavilov We propose a highly coherent near-term quantum simulator based on the fluxonium qubits with inductive coupling. This system provides long coherence time, large anharmonicity, and strong coupling, making it suitable to study strongly interacting clean and disordered transverse field Ising models (TFIM). A weak detuning from the sweet spot is equivalent to the additional random longitudinal field in the TFIM. We evaluate the propagation of qubit excitations through the system with quenched flux disorder. In the clean limit, we find that the excitation propagation is in agreement with the TFIM. As the flux disorder increases, the excitations become localized. We demonstrate that the localization and ergodicity in the system can be identified by measuring excitations at the edges. Such measurements do not require tunable couplers and are easily accessible via circuit QED methods. Thus, inductively coupled fluxoniums provide unique opportunities to study localization and many-body effects in highly coherent quantum systems. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L28.00008: Understanding and mitigating decoherence in fluxonium qubits – Energy relaxation Hantao Sun, Feng Wu, Hsiang-Sheng Ku, Hao Deng, Dawei Ding, Ran Gao, Xun Gao, Xun Jiang, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Chengchun Tang, Tenghui Wang, Wenlong Yu, Tian Xia, Gengyan Zhang, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Additional Contributing AQL Members, Yaoyun Shi, Jianxin Chen, Hui-Hai Zhao, Chunqing Deng Fluxonium qubits with long coherence times and large anharmonicities are one of the promising alternatives to transmons for achieving higher-fidelity gates. While the coherence time of fluxonium qubits has improved greatly in the past few years to the hundreds of microseconds level, theoretical and numerical evidence indicates that even longer coherence times can be achieved. Here, we experimentally study the energy relaxation of 2D fluxonium qubits to understand and then mitigate decoherence sources. Using a high-bandwidth flux control, we measure the relaxation time over a wide range of flux biases, mapping out the noise spectrum over a wide frequency range. We find that spurious tunneling two-level systems and excess quasiparticles are the dominant sources of energy relaxation. By improving the device fabrication process and the shielding against stray high-energy radiation, we achieve relaxation times on the order of hundreds of microseconds for 2D qubits in the light-fluxonium regime. We will also discuss other decoherence mitigation techniques for fluxonium qubits. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L28.00009: Novel two-qubit gates for the light fluxonium qubit Joachim Cohen, Agustin Di Paolo, Larry Chen, Trevor Chistolini, John Mark Kreikebaum, Long B Nguyen, Ravi K. Naik, David Ivan Santiago, Irfan Siddiqi, Alexandre Blais The fluxonium qubit, taken in the light regime where phase-slip rate/energy is of the order of the inductive energy, presents a low-energy spectrum with reduced flux dispersion. With flux noise amplitude being weaker than that of charge noise, the light fluxonium qubit should benefit from a high coherence time without having to pay the price of a lower anharmonicity as it is the case of the transmon. Here, we introduce a two-qubit gate for the light fluxonium in a parameter regime where the coherence times are predicted to be long and that is within the reach of current circuit-QED technology. Our proposal exploits an analogy between flux- and charge-noise insensitive circuit modes [Pechenezhskiy et al., Nature 585, 368–371 (2020)] alongside lessons learned from the transmon qubit. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L28.00010: Toward an ultra-high fidelity and scalable fluxonium quantum processor Hsiang-Sheng Ku, Hao Deng, Dawei Ding, Ran Gao, Xun Gao, Xun Jiang, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Hantao Sun, Chengchun Tang, Tenghui Wang, Feng Wu, Wenlong Yu, Tian Xia, Gengyan Zhang, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Additional Contributing AQL Members, Yaoyun Shi, Jianxin Chen, Hui-Hai Zhao, Chunqing Deng Fluxonium qubits are promising candidates for next-generation superconducting quantum processors due to their long coherence time and large anharmonicity. Moreover, fluxonium can be directly integrated into the existing circuit-QED schemes for quantum computing. In this talk, we present our design and experiments of a 2D multi-qubit fluxonium circuit in a scalable, high-fidelity quantum processor. The processor features tunable qubits with individual high-bandwidth fast control, which facilitates the demonstration of the circuit’s high-fidelity reset, gates, and readout. We believe that such processors will enable ultra-high fidelity quantum information processing through a scalable multi-qubit system. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L28.00011: Understanding and mitigating decoherence in fluxonium qubits – Dephasing Gengyan Zhang, Hantao Sun, Hao Deng, Dawei Ding, Ran Gao, Xun Gao, Hsiang-Sheng Ku, Xun Jiang, Zhisheng Li, Xiaotong Ni, Jin Qin, Zhijun Song, Chengchun Tang, Tenghui Wang, Feng Wu, Wenlong Yu, Tian Xia, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Additional Contributing AQL Members, Yaoyun Shi, Jianxin Chen, Hui-Hai Zhao, Chunqing Deng The fluxonium qubit is a promising candidate for the physical realization of quantum computation on the superconducting circuits platform. State-of-the-art devices have achieved relaxation time (T1) and coherence time (T2) of several hundred microseconds, but the reported T2 in the literature for various parameter regimes and flux biases has not reached the limit of 2T1. We present theoretical models and experimental data to study dephasing mechanisms in fluxonium qubits. Statistics and temporal fluctuations in measured data are analyzed to reveal the impact of environmental noise on device performance at different time scales. We will discuss the choice of device parameters, the characterization and suppression of noise levels, and the strategies for mitigating dephasing mechanisms. The improvement of coherence time paves the way for high-fidelity gate operations in fluxonium-based quantum processors. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L28.00012: Machine Learning Approach to Characterization of Multiple Fluxonium Qubits Yinqi Chen, Maxim G Vavilov Three energy parameters characterize fluxonium qubits: the charging energy EC, inductance energy EL, and Josephson energy EJ . Fitting of the fluxonium spectral lines as functions of the external flux provides estimates for these parameters. The fitting method based on the exact |
Wednesday, March 17, 2021 10:48AM - 11:00AM Live |
L28.00013: Experimental realization of ultra-high fidelity qubit operations with tunable fluxonium qubits Tenghui Wang, Jin Qin, Hao Deng, Dawei Ding, Ran Gao, Xun Gao, Hsiang-Sheng Ku, Xun Jiang, Zhisheng Li, Xiaotong Ni, Zhijun Song, Hantao Sun, Chengchun Tang, Feng Wu, Wenlong Yu, Tian Xia, Gengyan Zhang, Xiaohang Zhang, Jingwei Zhou, Xing Zhu, Additional Contributing AQL Members, Yaoyun Shi, Jianxin Chen, Hui-Hai Zhao, Chunqing Deng High-fidelity gates are critical for reducing the number of physical qubits required by quantum error correction. A main challenge for realizing high-fidelity gates is balancing controllability with coherence times. Fluxonium has reduced sensitivity to multiple noise sources, thereby exhibiting high coherence while maintaining large anharmonicity and strong coupling between circuits. These properties make fluxonium a promising qubit design for the next generation of superconducting quantum processors. However, due to its lower transition frequency, a fluxonium always stabilizes at a thermal state. Therefore, an active qubit reset needs to be implemented, in addition to a universal gate set. In this talk, we will demonstrate an efficient qubit reset and ultra-high fidelity single and two-qubit gates in a system with two coupled fluxonium qubits. The technologies demonstrated here are compatible with current methods for scaling up a multi-qubit system. |
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