Quantum state reduction and conditional time evolution of wave-particle correlations in cavity QED

We report measurements in cavity QED of a wave-particle correlation functio n which records the conditional time evolution of the field of a fraction o f a photon. Detection of a photon prepares a state of well-defined phase th at evolves back to equilibrium via a damped vacuum Rabi oscillation We reco rd the regression of the field amplitude. The recorded correlation function is nonclassical and provides an efficiency independent path to the spectru m of squeezing. Nonclassicality is observed even when the intensity fluctua tions are classical.