An optical system including: a photon source; first directing elements configured to direct photons to follow a first path through the optical system; second directing elements configured to direct photons to follow a second path through the optical system, wherein the second path is the reverse of the first path, photons travelling through the first path having a different polarization to those travelling through the second path; and a mechanism varying the relative phase shift between photons following the first path and photons following the second path.