Critical velocities in superfluids and the nucleation of vortices

The problem of critical velocities in superfluids, that is the comprehension of superfluidity breakdown by flow, has been long standing. One difficulty stems from the existence of several breakdown mechanisms. A major advance has come from the observation of single $2\pi$ phase slips, which arise from the nucleation of quantised vortices, that is, their creation {\it ex nihilo}. The statistical properties of the nucleation process in both the thermal regime and the quantum regime are identified and analysed: vortex nucleation provides a well-documented case of macroscopic quantum tunnelling (MQT). In particular, a close scrutiny of the experimental data obtained on ultra-pure ${}^4$He reveals the influence of damping on tunnelling, a rare occurrence where the effect of the environment on MQT can be studied.