35 research outputs found
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 with
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 phaselocked 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 to observe new
types of time crystals.Comment: 9 pages, 8 figure
Optical control of collective states in 1D ordered atomic chains beyond the linear regime
Driven by the need to develop efficient atom-photon interfaces, recent
efforts have proposed replacing cavities by large arrays of cold atoms that can
support subradiant or superradiant collective states. In practice, subradiant
states are decoupled from radiation, which constitutes a hurdle to most
applications. In this work, we study theoretically a protocol that bypasses
this limit using a one dimensional (1D) chain composed of N three-level atoms
in a V-shaped configuration. Throughout the protocol, the chain behaves as a
time-varying metamaterial: enabling absorption, storage and on-demand emission
in a spectrally and spatially controlled mode. Taking into account the quantum
nature of atoms, we establish the boundary between the linear regime and the
nonlinear regime where singly and doubly excited subradiant states compete in
the instantaneous decay rate during the storageComment: 7 pages, 4 figure