Quantum interference gives rise to behaviour that cannot be described classically. In the famous experiment by Hong, Ou and Mandel (HOM), two photons incident on the ports of a balanced beam splitter exhibit bunching behaviour at the outputs due to the interference of probability amplitudes for coincident detection. The strength of this interference is governed by the pairwise distinguishability of the photons’ quantum states.

In Oxford we have been investigating new features of interference that arise when more than two photons interfere. It turns out that three photon interference includes a dependence on the phase of the overlaps of the participating quantum states - something not seen in HOM interference. By using probabilistic photon sources, we have demonstrated this new feature with a three-port fibre interferometer.

Things get even more interesting when another photon is added to the mix: a four-photon collective phase appears in counting statistics from a four-port interferometer, and it persists even if pairs of photons are in orthogonal states! We present our current experimental efforts to reveal this quirk of multiphoton interference.