It's actually highly inefficient to do (general) computation with photons. In contrast to electrons, photons really don't like to interact so you need a lot of

It's actually highly inefficient to do (general) computation with photons. In contrast to electrons, photons really don't like to interact so you need a lot of optical power to implement an optical transistor-like, this was studied by Prof. David Miller some time ago (doi:10.1038/nphoton.2009.240, I think this is the correct citation am on the phone right now). Like others said, the device proposed in the paper here is more like an analog computer (with ML they have really seen a revival, both in electronics and optics), for a specific "calculation" they can be vastly more efficient, but they can't do general computation. The device here relies on interference and the photodetectors provide the nonlinearity, people have been researching these type of devices for neural networks for quite a while.