Cooperative non-equilibrium phase transition in a dilute Rydberg ensemble

TL;DR

This paper reports the observation of a non-equilibrium phase transition in a dilute thermal Rydberg gas, characterized by optical bistability and critical slowing down, driven by resonant dipole-dipole interactions.

Contribution

It demonstrates a novel non-equilibrium phase transition in a dilute Rydberg ensemble, including critical phenomena and superradiant behavior, expanding understanding of many-body dynamics in atomic gases.

Findings

Observation of optical bistability above a critical Rydberg density

Measurement of critical slowing down with exponent 0.53

Evidence of superradiant cascade in high Rydberg occupancy phase

Abstract

We demonstrate a non-equilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent $\tau=0.53 \pm 0.10$. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.