Spectroscopy and critical quantum thermometry in the ultrastrong coupling regime

TL;DR

This paper provides an exact solution to the anisotropic Hopfield model, revealing spectral asymmetries and thermometric bounds in the ultrastrong coupling regime, with implications for quantum sensing and phase transitions.

Contribution

It offers a novel analytical solution to the anisotropic Hopfield model and explores spectral and thermometric properties in the ultrastrong coupling regime, highlighting quantum phase transition effects.

Findings

Vacuum Rabi splitting shows asymmetries depending on initial states.

Ultimate bounds on temperature estimation are derived for ultrastrong coupling.

Quantum Fisher information diverges at quantum phase transitions.

Abstract

We present an exact analytical solution of the anisotropic Hopfield model, and we use it to investigate in detail the spectral and thermometric response of two ultrastrongly coupled quantum systems. Interestingly, we show that depending on the initial state of the coupled system, the vacuum Rabi splitting manifests significant asymmetries that may be considered spectral signatures of the counterintuitive decoupling effect. Using the coupled system as a thermometer for quantum thermodynamics applications, we obtain the ultimate bounds on the estimation of temperature that remain valid in the ultrastrong coupling regime. Remarkably, if the system performs a quantum phase transition, the quantum Fisher information exhibits periodic divergences, suggesting that one can have several points of arbitrarily high thermometric precision for such a critical quantum sensor.