  Half a century ago, T. Kibble proposed a scenario for topological defect
formation from symmetry breaking during the expansion of the early Universe. W.
Zurek later crystallized the concept to superfluid helium, predicting a
power-law relation between the number of quantum vortices and the rate at which
the system passes through the lambda transition. Here, we report the
observation of Kibble-Zurek scaling in a homogeneous, strongly interacting
Fermi gas undergoing a superfluid phase transition. We investigate the
superfluid transition using two distinct control parameters: temperature and
interaction strength. The microscopic physics of condensate formation is
markedly different for the two quench parameters, signaled by their two orders
of magnitude difference in the condensate formation timescale. However,
regardless of the thermodynamic direction in which the system passes through a
phase transition, the Kibble-Zurek exponent is identically observed to be about
0.68 and shows good agreement with theoretical predictions that describe
superfluid phase transitions. This work demonstrates the gedanken experiment
Zurek proposed for liquid helium that shares the same universality class with
strongly interacting Fermi gases.
