quoted from physicsworld.com:
To Sascha Agne and Thomas Jennewein of the University of Waterloo and colleagues and Stefanie Barz, Steve Kolthammer and Ian Walmsley of the University of Oxford and colleagues for independently measuring quantum interference involving three photons. Seeing the effect is very difficult because it requires the ability to deliver three indistinguishable photons to the same place at the same time and also to ensure that single-photon and two-photon interference effects are eliminated from the measurements. As well as providing deep insights into the fundamentals of quantum mechanics, three-photon interference could also be used in quantum cryptography and quantum simulators.