Demonstration of strong coupling of a subradiant atom array to a cavity vacuum

TL;DR

This paper experimentally demonstrates that a subradiant atom array can strongly couple to a cavity vacuum, leading to observable effects like vacuum Rabi splitting and polarization rotation, revealing new quantum light-matter interaction regimes.

Contribution

It shows for the first time that a subradiant atomic array can achieve strong coupling with a cavity vacuum, modifying excitation spectra and inducing polarization effects.

Findings

Observation of vacuum Rabi splitting in a subradiant atom array

Suppression of Bragg scattering due to destructive interference

Significant polarization rotation induced by the strongly coupled vacuum

Abstract

By considering linear scattering of laser-driven cold atoms inside an undriven high-finesse optical resonator, we experimentally demonstrate effects unique to a strongly coupled vacuum field. Arranging the atoms in an incommensurate lattice with respect to the radiation wavelength, the Bragg scattering into the cavity can be suppressed by destructive interference: the atomic array is subradiant to the cavity mode under transverse illumination. We show however, that strong collective coupling leads to a drastic modification of the excitation spectrum, as evidenced by well-resolved vacuum Rabi splitting in the intensity of the fluctuations. Furthermore, we demonstrate a significant polarization rotation in the linear scattering off the subradiant array via Raman scattering induced by the strongly coupled vacuum field.