  Interferometric photon-correlation measurements, which correspond to the
second-order intensity cross-correlations between the two output ports of an
unbalanced Michelson interferometer, are sensitive to both amplitude and phase
fluctuations of an incoming beam of light. Here, we present the theoretical
framework behind these measurements and show that they can be used to
unambiguously differentiate a coherent wave undergoing dynamical amplitude and
phase fluctuations from a chaotic state of light. This technique may thus be
used to characterize the output of nanolasers and monitor the onset of coherent
emission.
