Layered optical quantum circuit

An optical quantum circuit comprising:

• a substrate;
• a first plurality of waveguides formed on a first layer of the substrate wherein the first plurality of waveguides connect a first plurality of input ports to a first plurality of output ports;
• and
• a second plurality of waveguides formed on a second layer of the substrate wherein the second plurality of waveguides connect a second plurality of input ports to a second plurality of output ports;
• and wherein:
• at least some of the waveguides in the first plurality of waveguides are formed to interface, in a pairwise fashion, with another waveguide from the first or second plurality of waveguides in each of a plurality of interaction stages arranged between the first and second plurality of input ports and the first and second plurality of output ports, such that:
• in a at least first some interaction stages of the plurality of interaction stages each of a at least first some waveguides of the first plurality of waveguides interface with a respective neighbouring waveguide of the first plurality of waveguides, wherein each of the at least first some waveguides of the first plurality of waveguides is within the layer of the respective neighbouring waveguide of the first plurality of waveguides;
• and
• in a at least second some interaction stages of the plurality of interaction stages each of a at least second some waveguides of the first plurality of waveguides interface with a respective adjacent waveguide of the second plurality of waveguides, wherein the respective adjacent waveguide of the second plurality of waveguides is formed on the second layer to be a neighbouring waveguide to the corresponding waveguide from the at least second some waveguides of the first plurality of waveguides on the first layer.