Could such a device alter the relative phase of two separate beams of light? This optical device would take as input two beams of light with some arbitrary phase difference, and output the same two beams but with their phases changed to some fixed offset relative to each other.
With such a device an actual switch becomes easy: Make the device change the two inputs to be antiphase and combine them. If one of them is missing, the other passes through unphased (pun intended), but if both are present they cancel each other out; aka you have an xor switch!
With such a device an actual switch becomes easy: Make the device change the two inputs to be antiphase and combine them. If one of them is missing, the other passes through unphased (pun intended), but if both are present they cancel each other out; aka you have an xor switch!