Achieving the Heisenberg limit using fault-tolerant quantum error correction

          @misc{ pirsa_PIRSA:25100180,
            doi = {10.48660/25100180},
            url = {https://pirsa.org/25100180},
            author = {Sahu, Himanshu},
            keywords = {},
            language = {en},
            title = {Achieving the Heisenberg limit using fault-tolerant quantum error correction},
            publisher = {Perimeter Institute},
            year = {2025},
            month = {oct},
            note = {PIRSA:25100180 see, \url{https://pirsa.org}}
          }
          

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 Pauli-Z signal with single-qubit bit-flip noise and measurement noise. We propose a fault-tolerant metrological protocol where a repetition code is prepared via repeated syndrome measurements and logically measured after being merged into a thin surface code. We demonstrate the existence of an error threshold, below which errors are effectively suppressed and the HL is attained.