Spontaneous Emission in a Matter-Wave Open Quantum System

One of the paradigms of a small quantum system in a dissipative environment is the decay of an excited atom undergoing spontaneous photon emission into the fluctuating quantum electrodynamic vacuum. Recent experiments have demonstrated that the gapped photon dispersion in periodic structures can give rise to novel spontaneous-decay behavior including the formation of dissipative bound states. So far, these effects have been restricted to the optical domain. Here, we experimentally demonstrate similar behavior in a system of artificial atoms in an optical lattice that decay by emitting matter-wave, rather than optical, radiation into free space. By controlling the vacuum coupling and excitation energy, we directly observe exponential and partly reversible, non-Markovian dynamics and detect a tunable bound state containing evanescent matter waves for emission at negative excitation energies. Our system provides a flexible platform for the emulation of open-system quantum electrodynamics and studies of dissipative many-body physics with ultracold atoms.