We report measurements in cavity QED of a wave-particle correlation function which records the conditional time evolution of the field of a fraction of a photon. Detection of a photon prepares a state of well-defined phase that evolves back to equilibrium via a damped vacuum Rabi oscillation. We record the regression of the field amplitude. The recorded correlation function is nonclassical and provides an efficiency independent path to the spectrum of squeezing. Nonclassicality is observed even when the intensity fluctuations are classical.
In a driven cavity QED system with two nonidentical qubits of different resonant frequencies, a multiphoton nonclassical state can be generated by single-photon and two-photon cavity-induced transparency (CIT). Due to the existence of the quasi-subradiant state, the parameter of radiance witness
R can be greater than unity, which demonstrates that the cavity QED system is in the hyperradiance regime. In this hyperradiance regime, although the second-order correlation function
g(2)(0) is greater than unity, the photon statistics still exhibits the higher-order sub-Poissonian distribution. Because the single-photon and two-photon processes are suppressed by CIT-like effects, the three-photon process is enhanced, which leads to Klyshko's criterion
Kn<1 for
n>2, indicating the appearance of multiphoton nonclassicality. Based on this principle, we propose a scheme for preparing multiphoton nonclassical light with a three-photon probability of ${10}^{\ensuremath{-}4}$, which is feasible for experiment.