  We investigate the cavity excitation spectrum and the photon number
distribution in a cavity QED system driven by a broadband squeezed vacuum. In
an empty cavity, we show that only states with even number of photons can be
measured under resonant condition since the squeezed vacuum consists of states
with even number of photons only. When a single atom is trapped in the cavity,
the strong coupling between the atom and cavity results in energy splittings of
the system, and there exist two peaks in the cavity excitation spectrum at
two-photon transition frequencies. At the central frequency, however, all
photon states can be detected because of the interaction between the atom and
cavity. Therefore, it can be used to detect whether a single atom is trapped in
the cavity. We also show that the squeezed vacuum can promote multiphoton
excitations in the cavity. Using a coherent probe field, it is possible to
explore higher Jaynes-Cummings doublet even if the probe field intensity is
very weak.
