  Spontaneous decay of an excited atomic state is a fundamental process that
originates from the interaction between matter and vacuum modes of the
electromagnetic field. The rate of decay can thus be engineered by modifying
the density of final states of the joint atom-photon system. Imposing suitable
boundary conditions on the electromagnetic field has been shown to alter the
density of vacuum modes near the atomic transition, resulting in modified
atomic decay rates. Here we report the first experimental demonstration of
suppression of atomic radiative decay by reducing the density of available
energy-momentum modes of the atomic motion when it is embedded inside a Fermi
sea.
