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

TL;DR

This paper demonstrates a quantum error correction method using photon-number states in a microwave cavity, surpassing the break-even point by about 16% lifetime enhancement, advancing practical quantum computing.

Contribution

The authors introduce a hardware-efficient discrete-variable QEC scheme that exceeds the break-even point through high-fidelity error syndrome extraction and feedback correction.

Findings

Achieved about 16% lifetime enhancement of logical qubits

Implemented high-fidelity error syndrome extraction with a tailored pulse

Demonstrated potential for reliable quantum information processing

Abstract

Quantum error correction (QEC) aims to protect logical qubits from noises by utilizing the redundancy of a large Hilbert space, where an error, once it occurs, can be detected and corrected in real time. In most QEC codes, a logical qubit is encoded in some discrete variables, e.g., photon numbers. Such encoding schemes make the codewords orthogonal, so that the encoded quantum information can be unambiguously extracted after processing. Based on such discrete-variable encodings, repetitive QEC demonstrations have been reported on various platforms, but there the lifetime of the encoded logical qubit is still shorter than that of the best available physical qubit in the entire system, which represents a break-even point that needs to be surpassed for any QEC code to be of practical use. Here we demonstrate a QEC procedure with a logical qubit encoded in photon-number states of a microwave cavity, dispersively coupled to an ancilla superconducting qubit. By applying a pulse featuring a tailored frequency comb to the ancilla, 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 the hardware-efficient discrete-variable QEC codes towards a reliable quantum information processor.