above a few terahertz (wave periods less than a picosecond), such a time averaging is unavoidable, since detectors such as photodiodes and photomultiplier tubes cannot produce photocurrents that vary on such short timescales. The correlation function of these time-averaged intensities can then be computed: Most modern schemes actually measure the correlation in intensity fluctuations at the two detectors, but it is not too difficult to see that if the intensities are correlated, then the fluctuations , where is the average intensity, ought to be correlated, since In the particular case that consists mainly of a steady field with a small sinusoidally varying component , the time-averaged intensities are with , and indicates terms proportional to , which are small and may be ignored. The correlation function of these two intensities is then showing a sinusoidal dependence on the delay between the two detectors. Quantum interpretation The above discussion makes it clear that the Hanbury Brown and Twiss (or photon bunching) effect can be entirely described by classical optics. The quantum description of the effect is less intuitive: if one supposes that a thermal or chaotic light source such as a star randomly emits photons, then it is not obvious how the photons "know" that they should arrive at a detector in a correlated (bunched) way. A simple argument suggested by Ugo Fano [Fano, 1961] captures the essence of the quantum explanation. Consider two points and in a source that emit photons detected by two detectors and as in the diagram. A joint detection takes place when the photon emitted by is detected by and the photon emitted by is detected by (red arrows) or when 's photon is detected by and 's by (green arrows). The quantum mechanical probability amplitudes for these two possibilities are denoted by and respectively. If the photons are indistinguishable, the two amplitudes interfere constructively to give a joint detection probability greater than that for two independent events. The sum over all possible pairs in the source washes out the interference unless the distance is sufficiently small. Fano's explanation nicely illustrates the necessity of considering two-particle amplitudes, which are not as intuitive as the more familiar single-particle amplitudes used to interpret most interference effects. This may help to explain why some physicists in the 1950s had difficulty accepting the Hanbury Brown and Twiss result. But the quantum approach is more than just a fancy way to