Beating the break-even point with a discrete-variable-encoded logical qubit

Zhongchu Ni, Sai Li, Xiaowei Deng, Yanyan Cai, Libo Zhang, Weiting Wang, Zhen-Biao Yang, Haifeng Yu, Fei Yan, Song Liu, Chang-Ling Zou, Luyan Sun (), Shi-Biao Zheng (), Yuan Xu () and Dapeng Yu ()
Additional contact information
Zhongchu Ni: Southern University of Science and Technology
Sai Li: Southern University of Science and Technology
Xiaowei Deng: Southern University of Science and Technology
Yanyan Cai: Southern University of Science and Technology
Libo Zhang: Southern University of Science and Technology
Weiting Wang: Tsinghua University
Zhen-Biao Yang: Fuzhou University
Haifeng Yu: Beijing Academy of Quantum Information Sciences
Fei Yan: Southern University of Science and Technology
Song Liu: Southern University of Science and Technology
Chang-Ling Zou: University of Science and Technology of China
Luyan Sun: Tsinghua University
Shi-Biao Zheng: Fuzhou University
Yuan Xu: Southern University of Science and Technology
Dapeng Yu: Southern University of Science and Technology

Nature, 2023, vol. 616, issue 7955, 56-60

Abstract: Abstract Quantum error correction (QEC) aims to protect logical qubits from noises by using the redundancy of a large Hilbert space, which allows errors to be detected and corrected in real time1. In most QEC codes2–8, a logical qubit is encoded in some discrete variables, for example photon numbers, so that the encoded quantum information can be unambiguously extracted after processing. Over the past decade, repetitive QEC has been demonstrated with various discrete-variable-encoded scenarios9–17. However, extending the lifetimes of thus-encoded logical qubits beyond the best available physical qubit still remains elusive, which represents a break-even point for judging the practical usefulness of QEC. Here we demonstrate a QEC procedure in a circuit quantum electrodynamics architecture18, where the logical qubit is binomially encoded in photon-number states of a microwave cavity8, dispersively coupled to an auxiliary superconducting qubit. By applying a pulse featuring a tailored frequency comb to the auxiliary qubit, we can repetitively extract the error syndrome with high fidelity and perform error correction with feedback control accordingly, thereby exceeding the break-even point by about 16% lifetime enhancement. Our work illustrates the potential of hardware-efficient discrete-variable encodings for fault-tolerant quantum computation19.

Date: 2023
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DOI: 10.1038/s41586-023-05784-4

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