A non-equilibrium superradiant phase transition in free space

A class of systems exists in which dissipation, external drive and interactions compete and give rise to non-equilibrium phases that would not exist without the drive. There, phase transitions could occur without the breaking of any symmetry, yet with a local order parameter-in contrast to the Landau theory of phase transitions at equilibrium. One of the simplest driven-dissipative quantum systems consists of two-level atoms enclosed in a volume smaller than the wavelength of the atomic transition cubed, driven by a light field. The competition between collective coupling of the atoms to the driving field and their cooperative decay should lead to a transition between a phase where all the atomic dipoles are phase-locked and a phase governed by superradiant spontaneous emission. Here, we realize this model using a pencil-shaped cloud of laser-cooled atoms in free space, optically excited along its main axis, and observe the predicted phases. Our demonstration is promising in view of obtaining free-space superradiant lasers or observing new types of time crystal. Some driven systems sustain non-equilibrium phases in which phase transitions occur without symmetry breaking. The use of a laser-cooled atomic cloud confined in a pencil beam now allows the demonstration of such a system.