Optical microcavities as platforms for entangled photon spectroscopy

TL;DR

Optical microcavities significantly influence the quantum entanglement of photon pairs in spectroscopy, especially under strong coupling, revealing their active role beyond simple filtering in quantum-light applications.

Contribution

This study demonstrates that optical microcavities can enhance entanglement entropy in photon pairs, highlighting their active role in quantum spectroscopy beyond classical filtering effects.

Findings

Strong coupling microcavities increase entanglement entropy

Propagation alters joint spectrum of entangled photons

Microcavities impact quantum-light spectroscopy measurements

Abstract

Optical microcavities are often proposed as platforms for spectroscopy in the single- and few-photon regime due to strong light-matter coupling. For classical-light spectroscopies, an empty microcavity simply acts as an optical filter. However, we find that in the single- or few-photon regime treating the empty microcavity as an optical filter does not capture the full effect on the quantum state of the transmitted photons. Focusing on the case of entangled photon-pair spectroscopy, we consider how the propagation of one photon through an optical microcavity changes the joint spectrum of a frequency-entangled photon pair. Using the input-output treatment of a Dicke model, we find that propagation through a strongly coupled microcavity above a certain coupling threshold enhances the entanglement entropy between the signal and idler photons. These results show that optical microcavities are not neutral platforms for quantum-light spectroscopies and their effects must be carefully considered when using change in entanglement entropy as an observable.