Repeated Quantum Error Detection in a Surface Code

TL;DR

This paper demonstrates repeated quantum error detection using a 7-qubit surface code, significantly extending logical qubit coherence times and marking progress toward fault-tolerant quantum computing.

Contribution

It experimentally implements the smallest surface code capable of detecting single errors repeatedly, showing improved logical qubit coherence times.

Findings

Logical fidelity of 96.1% for encoded states

Logical qubit coherence time exceeds constituent qubits when no errors are detected

Successful repeated error detection in a 7-qubit surface code

Abstract

The realization of quantum error correction is an essential ingredient for reaching the full potential of fault-tolerant universal quantum computation. Using a range of different schemes, logical qubits can be redundantly encoded in a set of physical qubits. One such scalable approach is based on the surface code. Here we experimentally implement its smallest viable instance, capable of repeatedly detecting any single error using seven superconducting qubits, four data qubits and three ancilla qubits. Using high-fidelity ancilla-based stabilizer measurements we initialize the cardinal states of the encoded logical qubit with an average logical fidelity of 96.1%. We then repeatedly check for errors using the stabilizer readout and observe that the logical quantum state is preserved with a lifetime and coherence time longer than those of any of the constituent qubits when no errors are detected. Our demonstration of error detection with its resulting enhancement of the conditioned logical qubit coherence times in a 7-qubit surface code is an important step indicating a promising route towards the realization of quantum error correction in the surface code.