Quantum sensing of phase-covariant optical channels

TL;DR

This paper establishes universal bounds on quantum sensing performance for phase-covariant optical channels, demonstrating near-optimal probe states and providing a framework for evaluating limits in Gaussian channel sensing.

Contribution

It introduces a universal, probe-independent bound for quantum sensing of Gaussian channels and identifies near-optimal entangled probes under energy constraints.

Findings

Two-mode squeezed vacuum probes are near-optimal at low photon numbers.

Universal upper bounds on quantum Fisher information are derived for Gaussian channels.

The framework applies broadly to various quantum sensing scenarios involving Gaussian channels.

Abstract

We obtain universal (i.e., probe and measurement-independent) performance bounds on ancilla-assisted quantum sensing of multiple parameters of phase-covariant optical channels under energy and mode-number constraints. We first show that for any such constrained problem, an optimal ancilla-entangled probe can always be found whose reduced state on the modes probing the channel is diagonal in the photon-number basis. For parameters that are encoded in single-mode Gaussian channels, we derive a universal upper bound on the quantum Fisher information matrix that delineates the roles played by the energy and mode constraints. We illustrate our results for sensing of the transmittance of a thermal loss channel under both the no-passive-signature and passive-signature paradigms, and in the problem of sensing the noise variance of an additive-noise channel. In both cases, we show that two-mode squeezed vacuum probes are near-optimal under the constraints in the regime of low signal brightness, i.e., per-mode average photon number. More generally, our work sets down a uniform framework for readily evaluating universal limits for any sensing problem involving Gaussian channels.