Third-order correlations in cavity quantum electrodynamics

The correlation of a photon detection with the record of a homodyne photocurrent has allowed the observation of time-resolved quantum fluctuations of the amplitude of a squeezed electromagnetic wave in a cavity quantum electrodynamic system. The photon detection, through the wavefunction collapse, prepares a conditional state whose evolution back to equilibrium is observed. The perturbation of the wave amplitude is measured at the sub-photon level. The third-order correlation is manifestly non-classical, giving a minimum amplitude at zero delay where there would classically be a maximum; its Fourier transform gives the squeezing spectrum.