Quantum probes for the characterization of nonlinear media

TL;DR

This paper explores how quantum probes, especially squeezed states, can enhance the precision of characterizing nonlinear optical media, outperforming classical methods in estimating key parameters.

Contribution

It demonstrates the advantages of quantum probes, including squeezed states, for joint and individual estimation of nonlinear media parameters, surpassing classical approaches.

Findings

Quantum probes enable compatible joint estimation of parameters.

Squeezed probes improve precision at fixed energy.

Optimal squeezing strategies depend on probe energy.

Abstract

Active optical media leading to interaction Hamiltonians of the form $ H = \tilde{\lambda}\, (a + a^{\dagger})^{\zeta}$ represent a crucial resource for quantum optical technology. In this paper, we address the characterization of those nonlinear media using quantum probes, as opposed to semiclassical ones. In particular, we investigate how squeezed probes may improve individual and joint estimation of the nonlinear coupling $\tilde{\lambda}$ and of the nonlinearity order $\zeta$. Upon using tools from quantum estimation, we show that: i) the two parameters are compatible, i.e. the may be jointly estimated without additional quantum noise; ii) the use of squeezed probes improves precision at fixed overall energy of the probe; iii) for low energy probes, squeezed vacuum represent the most convenient choice, whereas for increasing energy an optimal squeezing fraction may be determined; iv) using optimized quantum probes, the scaling of the corresponding precision with energy improves, both for individual and joint estimation of the two parameters, compared to semiclassical coherent probes. We conclude that quantum probes represent a resource to enhance precision in the characterization of nonlinear media, and foresee potential applications with current technology.