Multimode quantum interference of photons in multiport integrated devices

Photonics is a leading approach in realizing future quantum technologies and recently, optical waveguide circuits on silicon chips have demonstrated high levels of miniaturization and performance. Multimode interference (MM I) devices promise a straightforward implementation of compact and robust multiport circuits. Here, we show quantum interference in a 2x2 MM I coupler with visibility of V = 95.6 +/- 0.9%. We further demonstrate the operation of a 4x4 port MM I device with photon pairs, which exhibits complex quantum interference behaviour. We have developed a new technique to fully characterize such multiport devices, which removes the need for phase-sensitive measurements and may find applications for a wide range of photonic devices. Our results show that MM I devices can operate in the quantum regime with high fidelity and promise substantial simplification and concatenation of photonic quantum circuits.