Nat. Photon. (2019)

Linear quantum networks consist of an array of beam splitters and waveguides, either on an optical bench or on a chip. Of course, table-top networks are too unwieldy for applications but integrated networks are just as fixed in their configuration, often only able to perform one task. Saroch Leedumrongwatthanakun and colleagues have now exploited the spatial and polarization mode mixing in a multimode fibre to realize an optical network that is programmable on the fly.

First, the team measured the transmission matrix, which connects the output of the 400-mode fibre they used to the optical input. With this in hand, they turned the fibre into a specific network by injecting the wavefront to match the desired operation. In this way, the team was able to first certify the indistinguishability of photons, then demonstrate controlled coherent absorption. The experiments were limited to two-output × two-input networks by the detection architecture but this can, in principle, be scaled up.