Experimental characterization of a first order dissipative phase transition in a many-body quantum system.

We experimentally investigate a driven-dissipative Josephson junction array, realized with a weakly interacting Bose-Einstein condensate loaded in a 1-D optical lattice. Tunneling from the neighboring sites makes up the driving force. Engineered losses on one site act as a local dissipative process. The source of these losses is an electron beam, which we also use to image the system (SEM) and monitor the losses. Decreasing the tunnel coupling or increasing the dissipation strength makes the system cross from a superfluid to a resistive state. For intermediate values, the system shows bistable behavior, with coexistence of a superfluid and an incoherent branch. Studying single experimental runs, we see the filling of the lossy site change from the resistive to the superfluid state within a few tunneling times. We explore the hysteresis of the system as a function of sweep time, temperature and initial atom number. Studying the dynamics towards a steady state averaged over many experimental runs, we find a critical slowing down accompanied by large-scale quantum fluctuations. Our results reveal the presence of a first order dissipative phase transition in the system.