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...
Circularly polarized (CP) lasers derived from low-cost and renewable raw sources are attracting increasing attention in photonics and material science. Here, we present a facile and effective approach to fabricate CP lasers by the evaporation-induced assembly of cellulose nanocrystals (CNCs) and a laser dye. The obtained laser exhibits a controlled chiral nematic structure, which acts as a chiral optical cavity, and varied chiral coupling interactions. It is shown that the CNC-based laser can modify the polarization state of the laser into left-handed polarization, leading to strong CP laser emission (CPLE) with a dissymmetry factor up to 0.35. The chiral nematic CNC structure proves to be a versatile yet straightforward strategy to generate strong and tailored CPLE.
We report on the direct measurement in real space of the effect of the van der Waals forces between individual Rydberg atoms on their external degrees of freedom. Clusters of Rydberg atoms with inter-particle distances of around 5 µm are created by first generating a small number of seed excitations in a magneto-optical trap, followed by off-resonant excitation that leads to a chain of facilitated excitation events. After a variable expansion time the Rydberg atoms are field ionized, and from the arrival time distributions the size of the Rydberg cluster after expansion is calculated. Our experimental results agree well with a numerical simulation of the van der Waals explosion.
We demonstrate a hybrid method based on field ionization and state-selective de-excitation capable of measuring the lifetimes of high-lying Rydberg states. For nS Rydberg states of Rb atoms with principal quantum number 60 ≤ n ≤ 88, we measure both the individual target state lifetimes and those of the ensemble of Rydberg states populated via black-body radiation-induced transitions. We find good overall agreement with numerical calculations of the expected lifetimes in both cases. However, for the target state lifetimes, we find a local deviation towards shorter lifetimes for states around n = 72, which we interpret as a signature of a modified black-body spectrum in the finite volume in which our experiments take place.
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