False vacuum decay via bubble formation in ferromagnetic superfluids

Metastability stems from the finite lifetime of a state when a lower-energy configuration is available but only by tunnelling through an energy barrier. It is observed in many natural situations, including in chemical processes and in electron field ionization. In classical many-body systems, metastability naturally emerges in the presence of a first-order phase transition. A prototypical example is a supercooled vapour. The extension to quantum field theory and quantum many-body systems has attracted significant interest in the context of statistical physics, protein folding and cosmology, for which thermal and quantum fluctuations are expected to trigger the transition from the metastable state (false vacuum) to the ground state (true vacuum) through the probabilistic nucleation of spatially localized bubbles. However, the long-standing theoretical progress in estimating the relaxation rate of the metastable field through bubble nucleation has not been validated experimentally. Here we experimentally observe bubble nucleation in isolated and coherently coupled atomic superfluids, and we support our observations with numerical simulations. The agreement between our observations and an analytic formula based on instanton theory confirms our physical understanding of the decay process and promotes coherently coupled atomic superfluids as an ideal platform to investigate out-of-equilibrium quantum field phenomena.