Does Malus Law apply to counts?
Under classical theory, Malus’ Law applies to EM amplitudes and hence gives a rule relating EM intensities to relative polarisation angle. Experiments with “single-photon” light operate with detectors set in “Geiger Mode”. The detectors have various parameters (temperature, voltage of photomultiplier, threshold voltage of discriminator) chosen so that they approximate as closely as possible to a linear response of counts to intensity. Output counts therefore show the same pattern as input intensities, and it does not matter whether we think of counts or intensities (hence the success of the probabilistic interpretation of QT!). But in reality this linearity can only be approximate. We can organise things so that when we pass light through two polarisers at various angles we get counts following very closely to Malus’ Law. If we leave the setting untouched, though, we are likely to get deviations if we alter the intensity of our source. The curve we see is always, in any case, the weigh
