Output field-quadrature measurements and squeezing in ultrastrong cavity-QED

Abstract

We study the squeezing of output quadratures of an electro-magnetic field\nescaping from a resonator coupled to a general quantum system with arbitrary\ninteraction strengths. The generalized theoretical analysis of output squeezing\nproposed here is valid for all the interaction regimes of cavity-quantum\nelectrodynamics: from the weak to the strong, ultrastrong, and deep coupling\nregimes. For coupling rates comparable or larger then the cavity resonance\nfrequency, the standard input-output theory for optical cavities fails to\ncalculate the correct output field-quadratures and predicts a non-negligible\namount of output squeezing, even if the system is in its ground state. Here we\nshow that, for arbitrary interactions and cavity-embedded quantum systems, no\nsqueezing can be found in the output-field quadratures if the system is in its\nground state. We also apply the proposed theoretical approach to study the\noutput squeezing produced by: (i) an artificial two-level atom embedded in a\ncoherently-excited cavity; and (ii) a cascade-type three-level system\ninteracting with a cavity field mode. In the latter case the output squeezing\narises from the virtual photons of the atom-cavity dressed states. This work\nextends the possibility of predicting and analyzing continuous-variable optical\nquantum-state tomography when optical resonators interact very strongly with\nother quantum systems.\n