Kibble-Zurek universality in a strongly interacting Fermi superfluid

The Kibble-Zurek mechanism describes the spontaneous formation of topological defects in a system crossing a continuous phase transition(1,2). Its central premise is the notion of universality, which states that the characteristic scaling exponent describing the dependence of the defect density on the quench rate is determined by the underlying symmetries of the system. Whether this universality can be extended to strongly interacting systems, such as a unitary Fermi gas, is an open question that has recently drawn attention in the context of holographic theories(3,4). Here, we report the observation of the Kibble-Zurek universality in a strongly interacting Fermi superfluid. As the microscopic nature of superfluidity is tuned from Bose-Einstein condensation of tightly bound molecules to Bardeen-Cooper-Schrieffer superfluidity of long-range fermion pairs, the thermal quench formation of vortices reveals a constant scaling exponent arising from the U(1) gauge symmetry of the system. In rapid quenches, destructive vortex collisions lead to the saturation of vortex densities, the values of which can be universally scaled by the interaction-dependent area of the vortex cores. This work paves the way for precision studies of non-equilibrium dynamics in a highly tunable, strongly correlated many-fermion systems(5,6).