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arXiv:2104.02669 · Physical Review Applied

Suppressing Coherent Two-Qubit Errors via Dynamical Decoupling

Jiawei Qiu, Yuxuan Zhou, Chang-Kang Hu, Jiahao Yuan, Libo Zhang, Ji Chu, Wenhui Huang, Weiyang Liu, Kai Luo, Zhongchu Ni, Xianchuang Pan, Zhixuan Yang, Yimeng Zhang, Yuanzhen Chen, Xiu–Hao Deng, Ling Hu, Jian Li, Jingjing Niu, Yuan Xu, Tongxing Yan, Youpeng Zhong, Song Liu, Fei Yan, Dapeng Yu

Published Nov 29, 2021• 21 citations

Abstract

Scalable quantum information processing requires the ability to tune multiqubit interactions. This makes the precise manipulation of quantum states particularly difficult for multiqubit interactions because tunability unavoidably introduces sensitivity to fluctuations in the tuning parameters, leading to erroneous multiqubit gate operations. The performance of quantum algorithms may be severely compromised by coherent multiqubit errors. It is therefore imperative to understand how these fluctuations affect multiqubit interactions and, more importantly, to mitigate their influence. In this study, we demonstrate how to implement dynamical-decoupling techniques to suppress the two-qubit analogue of the dephasing on a superconducting quantum device featuring a compact tunable coupler, a trending technology that enables the fast manipulation of qubit-qubit interactions. The pure-dephasing time shows up to an approximate 14 times enhancement on average when using robust sequences. The results are in good agreement with the noise generated from room-temperature circuits. Our study further reveals the decohering processes associated with tunable couplers and establishes a framework to develop gates and sequences robust against two-qubit errors.

DOI: 10.1103/physrevapplied.16.054047

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Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev. Appl. 10, 054061 (2018)] we experimentally demonstrate a simply designed and flux-controlled tunable coupler with a continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a different type of controlled-phase (cz) gate with ``dynamically decoupled regime,'' which allows the qubit-qubit coupling to be only ``on'' at the usual operating point while dynamically ``off'' during the tuning process of one qubit frequency into and out of the operating point. This scheme not only efficiently suppresses the leakage out of the computational subspace, but also allows for the acquired two-qubit phase being geometric at the operating point. We achieve an average cz gate fidelity of $98.3\ifmmode\pm\else\textpm\fi{}0.6\mathrm{%}$, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.

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Recent advances have significantly enhanced the coherence times of superconducting qubits. However, high-fidelity storage of multiqubit quantum states in large-scale circuits is still limited by factors such as the flux noise and crosstalk. We propose an X-iSwap decoupling sequence combining single-qubit ${X}_{\ensuremath{\pi}}$ gates and two-qubit iSwap gates to effectively mitigate dephasing from the low-frequency flux noise and eliminate crosstalk between qubits. Numerical simulations of noisy random evolution show excellent agreement with the theoretical models. In quantum memory applications, our method substantially extends the decay time of two-qubit fidelity and eliminates fidelity oscillations caused by the ZZ crosstalk. Compared to the traditional decoupling methods based on single-qubit gates, the X-iSwap sequence exhibits superior robustness against small system errors, which holds promise for widespread application in two-qubit systems and large-scale quantum processors for tasks such as long-term quantum memory and entanglement protection.