The spectroscopic detection of the excited-state resonances of a stron
gly coupled atom and optical cavity mode is complicated by inhomogeneo
us broadening, due to the variation of the dipole coupling constant wi
th location in the cavity. Photon correlation spectroscopy circumvents
the difficulty for slowly moving atoms using frequency-selective two-
photon absorption and photon coincidence detection of the two-photon d
ecay.