Subradiance in a Large Cloud of Cold Atoms

TL;DR

This paper reports the first experimental observation of subradiance in a large, dilute cold-atom cloud, demonstrating very slow decay times that scale with optical depth and are consistent with coupled-dipole theory.

Contribution

The study provides the first experimental evidence of subradiance in extended cold-atom samples, confirming theoretical predictions and characterizing its dependence on optical depth.

Findings

Observed subradiant decay times up to 100 times the atomic lifetime.

Subradiance scales linearly with the optical depth of the sample.

Decay times are independent of laser detuning, matching coupled-dipole model predictions.

Abstract

Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes.Here we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model.