Many-body systems relaxing to equilibrium can exhibit complex dynamics even if their steady state is trivial. In situations where relaxation requires highly constrained local particle rearrangements, such as in glassy systems, this dynamics can be difficult to analyze from first principles. The essential physical ingredients, however, can be captured by idealized lattice models with so-called kinetic constraints. While so far constrained dynamics has been considered mostly as an effective and idealized theoretical description of complex relaxation, 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 kinetic constraints can be tailored through the detuning of the excitation lasers from resonance. The system undergoes a dynamics which is characterized by pronounced spatial correlations or anticorrelations, depending on the detuning. Our results confirm recent theoretical predictions, and highlight the analogy between the dynamics of interacting Rydberg gases and that of certain soft-matter systems. DOI: 10.1103/PhysRevA.93.040701 Complex collective relaxation in many-body systems is often accompanied by a dramatic slowdown of diffusion processes and the emergence of nonergodic and glassy phases [1][2][3][4]. At low temperatures or high densities their evolution is often dominated by steric hindrances affecting particle motion. Local rearrangements are highly constrained, giving rise to collective-and often slow-relaxation. These features can be seen to be the consequence of effective kinetic constraints in the dynamics [5]. A kinetic constraint is a condition on the rate for a local transition dependent on the local environment: The transition and its reverse-irrespective of whether they are energetically favorable or unfavorable-can only occur if the constraint is satisfied. Kinetic constraints [6] can severely restrict relaxation in situations where local particle arrangements make satisfying them unlikely, which is typical of fluid systems with excluded volume interactions such as dense colloids or supercooled liquids [1][2][3][4]. When a constraint is satisfied, however, the transition is allowed and a local rearrangement is "facilitated" [5,6]. Kinetic constraints naturally give rise [7] to collective and spatially heterogeneous relaxation, and are used to describe situations where the correlation properties of the dynamics go beyond those of the static stationary state, a salient feature of glassy systems [4].Steric hindrances and dynamic facilitation are argued to play a central role in the behavior of glass formers [5]. However, it can be difficult [8] to establish unambiguously the relation between microscopic processes and emerging kinetic constraints, or between idealized models with explicit kinetic constraints and actual physical systems. In this Rapid Communication we establish such a direct connection by reporting the experimental observation of correlated...

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 socalled 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.Absorbing state phase transitions are among the simplest non-equilibrium phenomena displaying critical behavior and universality. They can occur for instance in models describing the growth of bacterial colonies or the spreading of an infectious disease among a population (see, e.g., [1][2][3]). Once an absorbing state, e.g., a state in which all the bacteria are dead, is reached, the system cannot escape from it [4]. However, there might be a regime where the proliferation of bacteria overcomes the rate of death and thus a finite stationary population density is maintained for long times. The transition between the absorbing and the active state may be continuous, with observables displaying universal scaling behaviour [5][6][7][8][9]. Although conceptually of great interest, the unambiguous observation of even the simplest non-equilibrium universality class -directed percolation -is challenging and has only been achieved in recent years in a range of soft-matter systems and fluid flows [10][11][12][13][14][15][16] (see also the references in [11,12]). The exploration of such universal non-equilibrium phenomena is currently an active topic across different disciplines with a number of open questions concerning, among others, their classification, the role of disorder, and quantum effects. In particular, cold atomic systems have proven to constitute a versatile platform for probing this and related physics [17][18][19][20][21][22][23][24][25][26].Here we experimentally observe signatures of an absorbing state phase transition in a driven ope...

We report an experimental investigation of the facilitated excitation dynamics in off-resonantly driven Rydberg gases by separating the initial off-resonant excitation phase from the facilitation phase, in which successive facilitation events lead to excitation avalanches. We achieve this by creating a controlled number of initial seed excitations. Greater insight into the avalanche mechanism is obtained from an analysis of the full counting distributions. We also present simple mathematical models and numerical simulations of the excitation avalanches that agree well with our experimental results.