Optical demonstration of quantum fault-tolerant threshold

TL;DR

This paper experimentally demonstrates a quantum fault-tolerance threshold using optical qubits, showing that below a certain error rate, fault-tolerant circuits outperform non-encoded ones, advancing practical quantum computing.

Contribution

First experimental demonstration of a quantum fault-tolerance threshold using optical qubits in a simplified protocol.

Findings

Fault-tolerant circuits outperform non-encoded circuits below the threshold.

No advantage observed when error rate exceeds the threshold.

Optical system provides a platform for studying error propagation.

Abstract

A major challenge in practical quantum computation is the ineludible errors caused by the interaction of quantum systems with their environment. Fault-tolerant schemes, in which logical qubits are encoded by several physical qubits, enable correct output of logical qubits under the presence of errors. However, strict requirements to encode qubits and operators render the implementation of a full fault-tolerant computation challenging even for the achievable noisy intermediate-scale quantum technology. Here, we experimentally demonstrate the existence of the threshold in a special fault-tolerant protocol. Four physical qubits are implemented using 16 optical spatial modes, in which 8 modes are used to encode two logical qubits. The experimental results clearly show that the probability of correct output in the circuit, formed with fault-tolerant gates, is higher than that in the corresponding non-encoded circuit when the error rate is below the threshold. In contrast, when the error rate is above the threshold, no advantage is observed in the fault-tolerant implementation. The developed high-accuracy optical system may provide a reliable platform to investigate error propagation in more complex circuits with fault-tolerant gates.