Time-dependent electric field fluctuations at the subphoton level

We report studies of a third-order correlation function that records the conditional time evolution of the field of a fraction of a photon. We present results in the strong-coupling regime of cavity QED, where the detection of a photon from the cavity prepares a state with a well-defined phase that evolves back to equilibrium via a damped vacuum Rabi oscillation. We perform a homodyne measurement conditioned on the photon detection to observe the regression of the field amplitude. The measured field-photon correlation is nonclassical and can be used to determine the spectrum of squeezing in an efficiency independent way.