Superradiant phase transition in a large interacting driven atomic ensemble in free space

TL;DR

This paper theoretically investigates the conditions for superradiant phase transitions in large, driven atomic ensembles with strong dipole interactions in free space, revealing both continuous and discontinuous transition behaviors.

Contribution

It introduces a detailed analysis of superradiant phase transitions in free-space atomic ensembles, highlighting the effects of different decay mechanisms on the nature of the transition.

Findings

Superradiant phase transition characterized by abrupt change in light reflection and transmission.

Decay mechanisms influence whether the phase transition is continuous or discontinuous.

Quantum fluctuations are significant near the transition point.

Abstract

Atomic ensembles strongly interacting with light constitute rich quantum-optical many-body systems, with the potential for observing cooperative effects and dissipative nonequilibrium phase transitions. We theoretically analyze the conditions under which a driven atomic ensemble in free space, characterized by strong dipole-dipole interactions and large spatial extent, can undergo a superradiant phase transition, also known as cooperative resonance fluorescence. In an atomic array, stationary states that conserve the collective pseudospin exhibit completely cooperative decay and undergo a second-order phase transition in the large atom number limit. In contrast, decay mechanisms on longer timescales that fail to conserve pseudospin can lead to discontinuous first-order phase transition at a critical finite atom number, disrupting cooperation despite sharing many similar observable characteristics. A hallmark of the superradiant phase transition is an abrupt shift from total light reflection off the atoms to rapidly increasing transmission, accompanied by significant quantum fluctuations, as a function of light intensity.