  We report measurements of the correlation between intensities of noise at
different frequencies on a tunnel junction under ac excitation, which
corresponds to two-mode intensity correlations in optics. We observe positive
correlation, \emph{i.e.} photon bunching, which exist only for certain
relations between the excitation frequency and the two detection frequencies,
depending on the dc bias of the sample. We demonstrate that these correlations
are proportional to the fourth cumulant of current fluctuations.
