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

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 time(1). In most QEC codes(2-8), 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 scenarios(9-17). 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 architecture(18), where the logical qubit is binomially encoded in photon-number states of a microwave cavity(8), 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 computation(19).

Automated summary

Quantum error correction (QEC) protects logical qubits by using a large Hilbert space with redundancy to detect and correct errors in real time. In this study, a QEC procedure was demonstrated in a circuit quantum electrodynamics architecture, where a logical qubit was encoded in photon-number states of a microwave cavity and coupled to an auxiliary superconducting qubit. By applying a tailored frequency comb pulse, error syndrome was extracted and error correction was performed, exceeding the break-even point by about 16% lifetime enhancement. This work illustrates the potential of hardware-efficient discrete-variable encodings for fault-tolerant quantum computation.