Achieving the Heisenberg limit using fault-tolerant quantum error correction

TL;DR

This paper develops a fault-tolerant quantum metrology protocol that maintains the Heisenberg limit even when all quantum operations, including error correction steps, are affected by noise, demonstrating an error threshold for effective noise suppression.

Contribution

It introduces a fault-tolerant quantum metrology scheme that accounts for noise in all operations, extending the applicability of the Heisenberg limit to more realistic noisy quantum systems.

Findings

Existence of an error threshold for noise suppression.

Fault-tolerant protocol achieves the Heisenberg limit under realistic noise.

Repetition code with syndrome measurements enables noise resilience.

Abstract

Quantum effect enables enhanced estimation precision in metrology, with the Heisenberg limit (HL) representing the ultimate limit allowed by quantum mechanics. Although the HL is generally unattainable in the presence of noise, quantum error correction (QEC) can recover the HL in various scenarios. A notable example is estimating a Pauli-$Z$ signal under bit-flip noise using the repetition code, which is both optimal for metrology and robust against noise. However, previous protocols often assume noise affects only the signal accumulation step, while the QEC operations -- including state preparation and measurement -- are noiseless. To overcome this limitation, we study fault-tolerant quantum metrology where all qubit operations are subject to noise. We focus on estimating a Pauli-$Z$ signal under bit-flip noise, together with state preparation and measurement errors in all QEC operations. We propose a fault-tolerant metrological protocol where a repetition code is prepared via repeated syndrome measurements, followed by a fault-tolerant logical measurement. We demonstrate the existence of an error threshold, below which errors are effectively suppressed and the HL is attained.