as shown in FIG. \[fig:optics\_circuit\]. In this we are able to change the outputs from each layer of the network. The spatial shift could be induced by varying the strain or temperature on the waveguides at given locations, to change their refractive indices and hence the relative phase; this may have additional difficulties in that silicon waveguides are birefringent  [@strain]. Alternatively we can tune both polarisation and spatial qubits via the electro-optic effect.

This circuit can be made more robust and minitaturised using silicon or silica optical waveguides [@silicon]. They have been extensively used to control spatial modes and recently also polarisation [@polarisationstates]. Several labs can implement the phase shifting via heaters or the electro-optic effect. Conventionally phase shifters built upon the electro-optic effect are known to work in the megahertz region and have extremely low loss [@silicon]. For many applications this would be considered slow, but our tuning only requires (in the region of) a few thousand steps of tuning, meaning learning tasks for neural networks this small could be completed in milliseconds. While it appears that this effect will be the limiting factor in terms of speed, photodetectors are able to reach reset times in the tens of 