Dynamical phase transition in an open quantum system (A4)

Ling-Na Wu, Jens Nettersheim, Julian Feß, Alexander Schnell, Sabrina Burgardt, Silvia Hiebel, Daniel Adam, André Eckardt, Artur Widera 

🔓 arXiv:2208.05164 (2022)

Phase transitions correspond to the singular behavior of physical systems in response to continuous control parameters like temperature or external fields. Near continuous phase transitions, associated with the divergence of a correlation length, universal power-law scaling behavior with critical exponents independent of microscopic system details is found. Recently, dynamical quantum phase transitions and universal scaling have been predicted and also observed in the non-equilibrium dynamics of isolated quantum systems after a quench, with time playing the role of the control parameter. However, signatures of such dynamical phase transition in open systems, whose dynamics is driven by the dissipative contact to an environment, were so far elusive. Here, we demonstrate that dynamical phase transitions with respect to time can also occur in open quantum systems described by mixed states. We experimentally measure the relaxation dynamics of the large atomic spin of individual Caesium atoms induced by the dissipative coupling via spin-exchange processes to an ultracold Bose gas of Rubidium atoms. For initial states far from equilibrium, the entropy of the spin state is found to peak in time, transiently approaching its maximum possible value, before eventually relaxing to its lower equilibrium value. This effect is associated with a prethermal loss of the system's memory of the details of the initial state. Moreover, a finite-size scaling analysis based on numerical simulations shows that it corresponds to a dynamical phase transition of the dissipative system in the limit of large system sizes. Our results show that dynamical phase transitions are not restricted to occur in isolated systems, but, surprisingly, are possible also during the dissipative evolution of open quantum systems.