The following thought experiment appears to violate the uncertainty principle.(See diagram.)

Photons emitted from a light source pass through a narrow band colour filter before impinging on a photomultiplier. The light source is reduced in intensity so as to detect photons one at a time with the photomultiplier. The pulse from the photomultiplier can be made very narrow as can the bandwidth of the colour filter; so violating the uncertainty principle, if the photons are considered as particles. However, if a photon is considered purely as a wave packet then there is uncertainty as to which position in the wave packet coincides with emission of an electron in the photomultiplier and the uncertainty principle holds.


John Fleming, 2004