Photons can propagate concurrently in two different directions along optical paths in a generalized Mach Zehnder interferometer (GMZI). A counterpropagating GMZI can include a first set of input ports and a second set of input ports, a first set of output ports and a second set of output ports, and optical components interconnected to form a GMZI that can selectably establish a first optical path between one of the first set of input ports and one of the first set of output ports and a second optical path between one of the second set of input ports and one of the second set of output ports. The first optical path and the second optical path can include an overlapping portion though which photons on the first and second optical paths propagate in opposing directions.