General Cram\'er-Rao bound for parameter estimation using Gaussian multimode quantum resources

TL;DR

This paper derives the ultimate sensitivity limit for parameter estimation using multimode Gaussian quantum light and shows that optimal sensitivity is achieved by concentrating squeezing in a single mode, with homodyne detection reaching this bound.

Contribution

It establishes a general Cramér-Rao bound for Gaussian quantum resources and demonstrates the optimal strategy for maximizing sensitivity in quantum metrology.

Findings

Maximum sensitivity is achieved by using the most squeezed state in a single mode.

Quantum correlations and entanglement between modes do not enhance sensitivity beyond the single-mode squeezing.

Homodyne detection can reach the fundamental sensitivity limit.

Abstract

Multimode Gaussian quantum light, including multimode squeezed and/or multipartite quadrature entangled light, is a very general and powerful quantum resource with promising applications to quantum information processing and metrology involving continuous variables. In this paper, we determine the ultimate sensitivity in the estimation of any parameter when the information about this parameter is encoded in such Gaussian light, irrespective of the exact information extraction protocol used in the estimation. We then show that, for a given set of available quantum resources, the most economical way to maximize the sensitivity is to put the most squeezed state available in a well-defined light mode. This implies that it is not possible to take advantage of the existence of squeezed fluctuations in other modes, nor of quantum correlations and entanglement between different modes. We show that an appropriate homodyne detection scheme allows us to reach this Cramr-Rao bound. We apply finally these considerations to the problem of optimal phase estimation using interferometric techniques.