Output Field-Quadrature Measurements and Squeezing in Ultrastrong Cavity-QED

TL;DR

This paper develops a comprehensive theoretical framework for analyzing output quadrature squeezing in cavity-QED systems across all interaction regimes, correcting previous misconceptions about squeezing in the ground state.

Contribution

It introduces a generalized approach valid for weak to ultrastrong coupling, clarifies that no squeezing occurs in the ground state, and applies this to specific quantum systems.

Findings

Standard input-output theory fails in ultrastrong coupling regimes.

No output squeezing in the ground state of the system.

Output squeezing can arise from virtual photons in certain systems.

Abstract

We study the squeezing of output quadratures of an electro-magnetic field escaping from a resonator coupled to a general quantum system with arbitrary interaction strengths. The generalized theoretical analysis of output squeezing proposed here is valid for all the interaction regimes of cavity-quantum electrodynamics: from the weak to the strong, ultrastrong, and deep coupling regimes. For coupling rates comparable or larger then the cavity resonance frequency, the standard input-output theory for optical cavities fails to calculate the correct output field-quadratures and predicts a non-negligible amount of output squeezing, even if the system is in its ground state. Here we show that, for arbitrary interactions and cavity-embedded quantum systems, no squeezing can be found in the output-field quadratures if the system is in its ground state. We also apply the proposed theoretical approach to study the output squeezing produced by: (i) an artificial two-level atom embedded in a coherently-excited cavity; and (ii) a cascade-type three-level system interacting with a cavity field mode. In the latter case the output squeezing arises from the virtual photons of the atom-cavity dressed states. This work extends the possibility of predicting and analyzing continuous-variable optical quantum-state tomography when optical resonators interact very strongly with other quantum systems.