Quantum metrology with imperfect measurements

TL;DR

This paper systematically investigates how measurement imperfections affect quantum metrology, proposing generalized quantum Fisher information and control strategies to mitigate noise, thereby enabling near-ideal sensitivity scaling in large systems.

Contribution

It introduces a generalized quantum Fisher information for noisy detection and demonstrates how control operations can recover optimal sensitivity despite measurement imperfections.

Findings

Global control operations restore Heisenberg scaling asymptotically.

Local control limits quantum enhancement to a constant factor.

Optimal input states and controls can attain ultimate precision under noise.

Abstract

The impact of measurement imperfections on quantum metrology protocols has not been approached in a systematic manner so far. In this work, we tackle this issue by generalising firstly the notion of quantum Fisher information to account for noisy detection, and propose tractable methods allowing for its approximate evaluation. We then show that in canonical scenarios involving $N$ probes with local measurements undergoing readout noise, the optimal sensitivity depends crucially on the control operations allowed to counterbalance the measurement imperfections -- with global control operations, the ideal sensitivity (e.g.~the Heisenberg scaling) can always be recovered in the asymptotic $N$ limit, while with local control operations the quantum-enhancement of sensitivity is constrained to a constant factor. We illustrate our findings with an example of NV-centre magnetometry, as well as schemes involving spin-$1/2$ probes with bit-flip errors affecting their two-outcome measurements, for which we find the input states and control unitary operations sufficient to attain the ultimate asymptotic precision.