Experimental signatures of an absorbing-state phase transition in an open driven many-body quantum system

TL;DR

This study experimentally observes signatures of a non-equilibrium absorbing-state phase transition in an open driven quantum system of cold Rydberg atoms, revealing critical scaling and the role of atomic motion in long-range correlations.

Contribution

It provides the first experimental evidence of an absorbing-state phase transition in an open quantum system with Rydberg atoms, highlighting the role of atomic motion and disorder.

Findings

Power-law scaling of Rydberg excitation density near transition

Increased fluctuations close to the critical point

Atomic motion enables long-range correlations

Abstract

Understanding and probing phase transitions in non-equilibrium systems is an ongoing challenge in physics. A particular instance are phase transitions that occur between a non-fluctuating absorbing phase, e.g., an extinct population, and one in which the relevant order parameter, such as the population density, assumes a finite value. Here we report the observation of signatures of such a non-equilibrium phase transition in an open driven quantum system. In our experiment rubidium atoms in a quasi one-dimensional cold disordered gas are laser-excited to Rydberg states under so-called facilitation conditions. This conditional excitation process competes with spontaneous decay and leads to a crossover between a stationary state with no excitations and one with a finite number of excitations. We relate the underlying physics to that of an absorbing state phase transition in the presence of a field (i.e. off-resonant excitation processes) which slightly offsets the system from criticality. We observe a characteristic power-law scaling of the Rydberg excitation density as well as increased fluctuations close to the transition point. Furthermore, we argue that the observed transition relies on the presence of atomic motion which introduces annealed disorder into the system and enables the formation of long-ranged correlations. Our study paves the road for future investigations into the largely unexplored physics of non-equilibrium phase transitions in open many-body quantum systems.