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Register a new userNon-equilibrium universality in the dynamics of dissipative cold atomic gases
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The theory of continuous phase transitions predicts the universal collective properties of a physical system near a critical point, which for instance manifest in characteristic power-law behaviours of physical observables. The well-established concept at or near equilibrium, universality, can also characterize the physics of systems out of equilibrium. The most fundamental instance of a genuine non-equilibrium phase transition is the directed percolation universality class, where a system switches from an absorbing inactive to a fluctuating active phase. Despite being known for several decades it has been challenging to find experimental systems that manifest this transition. Here we show theoretically that signatures of the directed percolation universality class can be observed in an atomic system with long range interactions. Moreover, we demonstrate that even mesoscopic ensembles --- which are currently studied experimentally --- are sufficient to observe traces of this non-equilibrium phase transition in one, two and three dimensions.
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Many-body systems relaxing to equilibrium can exhibit complex dynamics even if their steady state is trivial. At low temperatures or high densities their evolution is often dominated by steric hindrances affecting particle motion [1,2,3]. Local rearrangements are highly constrained, giving rise to collective - and often slow - relaxation.This dynamics can be difficult to analyse from first principles, but the essential physical ingredients are captured by idealized lattice models with so- called kinetic constraints [4]. Here we experimentally realize a many-body system exhibiting manifest kinetic constraints and measure its dynamical properties. In the cold Rydberg gas used in our experiments, the nature of the constraints can be tailored through the detuning of the excitation lasers from resonance [5,6,7,8], which controls whether the system undergoes correlated or anti- correlated dynamics. Our results confirm recent theoretical predictions [5,6], and highlight the analogy between the dynamics of interacting Rydberg gases and that of soft-matter systems.
The physics of highly excited Rydberg atoms is governed by blockade or exclusion interactions that hinder the excitation of atoms in the proximity of a previously excited one. This leads to cooperative effects and a relaxation dynamics displaying space-time heterogeneity similar to what is observed in the relaxation of glass-forming systems. Here we establish theoretically the existence of a glassy dynamical regime in an open Rydberg gas, associated with phase coexistence at a first-order transition in dynamical large deviation functions. This transition occurs between an active phase of low density in which dynamical processes take place on short timescales, and an inactive phase in which excited atoms are dense and the dynamics is highly arrested. We perform a numerically exact study and develop a mean-field approach that allows to understand the mechanics of this phase transition. We show that radiative decay --- which becomes experimentally relevant for long times --- moves the system away from dynamical phase coexistence. Nevertheless, the dynamical phase transition persists and causes strong fluctuations in the observed dynamics.
We unveil the universal (model-independent) symmetry satisfied by Schwinger-Keldysh quantum field theories whenever they describe equilibrium dynamics. This is made possible by a generalization of the Schwinger-Keldysh path-integral formalism in which the physical time can be re-parametrized to arbitrary contours in the complex plane. Strong relations between correlation functions, such as the fluctuation-dissipation theorems, are derived as immediate consequences of this symmetry of equilibrium. In this view, quantum non-equilibrium dynamics -- e.g. when driving with a time-dependent potential -- are seen as symmetry-breaking processes. The symmetry-breaking terms of the action are identified as a measure of irreversibility, or entropy creation, defined at the level of a single quantum trajectory. Moreover, they are shown to obey quantum fluctuation theorems. These results extend stochastic thermodynamics to the quantum realm.
In this Colloquium we discuss the anomalous kinetics of atoms in dissipative optical lattices, focusing on the ``Sisyphus laser cooling mechanism. The cooling scheme induces a friction force that decreases to zero for high atomic momentum, which in turn leads to unusual statistical features. We study, using a Fokker-Planck equation describing the semi-classical limit of the system, the shallow optical lattice regime where the momentum distribution of the particles is heavy-tailed and the spatial diffusion is anomalous. As the depth of the optical lattice is tuned, transitions in the dynamical properties of the system occur, for example a transition from Gaussian diffusion to a Levy walk and the breakdown of the Green-Kubo formula for the diffusion constant. Rare events, in both the momentum and spatial distributions, are described by non-normalized states, with tools adapted from infinite ergodic theory. We present experimental observations and elementary explanations for the physical mechanisms of cooling that lead to these anomalous behaviors, comparing theory with available experimental and numerical data.
We introduce a model of negotiation dynamics whose aim is that of mimicking the mechanisms leading to opinion and convention formation in a population of individuals. The negotiation process, as opposed to ``herding-like or ``bounded confidence driven processes, is based on a microscopic dynamics where memory and feedback play a central role. Our model displays a non-equilibrium phase transition from an absorbing state in which all agents reach a consensus to an active stationary state characterized either by polarization or fragmentation in clusters of agents with different opinions. We show the exystence of at least two different universality classes, one for the case with two possible opinions and one for the case with an unlimited number of opinions. The phase transition is studied analytically and numerically for various topologies of the agents interaction network. In both cases the universality classes do not seem to depend on the specific interaction topology, the only relevant feature being the total number of different opinions ever present in the system.
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