  The use of atomic decay rates as a probe of sub-vacuum phenomena will be
studied. Because electromagnetic vacuum fluctuations are essential for
radiative decay of excited atomic states, decay rates can serve as a measure of
the suppression of vacuum fluctuation in non-classical states, such as squeezed
vacuum states. In such states the renormalized expectation value of the square
of the electric field or the energy density can be periodically negative,
representing suppression of vacuum fluctuations. We explore the extent to which
atomic decays can be used to measure the mean squared electric field or energy
density. We consider a scheme in which atoms in an excited state transit a
closed cavity whose lowest mode contains photons in a non-classical state. The
change in the decay probability of the atom in the cavity due to the
non-classical state can, under certain circumstances, serve as a measure of the
mean squared electric field or energy density in the cavity. We derive a
quantum inequality bound on the decrease in this probability. We also show that
the decrease in decay rate can sometimes be a measure of negative energy
density or negative squared electric field. We make some estimates of the
magnitude of this effect, which indicate that an experimental test might be
possible.
