Experimental Observation of BCS Dynamical Phases in a Strontium Cavity-QED System

Out-of-equilibrium dynamics in many-body systems can exhibit rich behavior not found in equilibrium systems. In particular, certain non-dissipative systems quenched out of equilibrium can experience distinct “dynamical phases” and corresponding phase transitions as either Hamiltonian parameters or initial conditions are tuned. The Bardeen-Cooper-Schrieffer (BCS) model of superconductivity provides a notable example: studies of this model predict the existence of three dynamical phases when a superconductor in its ground state experiences a sudden quench in interaction strength [1]. Despite this, experimental observation of all three phases in condensed matter systems has yet to be realized. Following a previous theory proposal [2], we present results simulating the BCS model using an ensemble of ⁸⁸Sr atoms subject to a cavity-mediated spin-exchange interaction. By tuning both the interaction strength and the shape of the single-particle energy distribution, we observe three dynamical phases and identify them with the predicted dynamics in a quenched BCS superconductor.

[1] R. A. Barankov and L. S. Levitov, PRL 96, 230403 (2006)

[2] R. J. Lewis-Swan et al., PRL 126, 173601 (2021)