Observation of false vacuum decay via bubble formation in ferromagnetic superfluids

Abstract Metastability is ubiquitous in nature and is observed through the crossing of an energy barrier toward a configuration of lower energy as, for example, in chemical processes [1] or electron field ionization [2]. In classical many-body systems, metastability naturally emerges in the presence of a first-order phase transition and finds a prototypical example in supercooled vapour. In the last decades, the extension to quantum field theory and quantum many-body systems has attracted significant interest in the context of statistical physics [3, 4], protein folding [5, 6], and cosmology [7–9], where thermal and quantum fluctuations are expected to trigger the transition from the metastable state (false vacuum) to the ground state (real vacuum) via the probabilistic nucleation of spatially localized bubbles [10, 11]. However, the long-standing theoretical progress in estimating the relaxation rate of the metastable field via bubble nucleation has not yet found a counterpart in terms of experimental observations. Here we experimentally observe and characterize bubble nucleation in isolated and coherently-coupled atomic superfluids, and support our observations with numerical simulations. The agreement between our results and a novel analytic formula based on instanton theory confirms the quantum-field character of the observed decay, and promotes coherently-coupled atomic superfluids as emulators of out-of-equilibrium quantum field phenomena.