Noise Intensity-Intensity Correlations and the Fourth Cumulant of Current Fluctuations

We report measurements of the correlation between intensities of noise at different frequencies on a tunnel junction under ac excitation. We show that such correlations exist only for certain relations between the excitation frequency and the two detection frequencies, which are similar to three-wave and four-wave mixing in optics, depending on the dc bias of the sample. We demonstrate that the correlation we measure is proportional to the fourth cumulant of current fluctuations.