An optical system e.g. for causing entanglement between photons

摘要

An optical system comprises a photon source 101, first directing elements configured to direct photons to follow a first path through said optical system, second directing elements configured to direct photons to follow a second path through said optical system wherein said second path is the reverse of said first path, the system being configured such that photons travelling through the first path have a different polarization, due to polarization rotator 108, to those travelling through the second path and a means of varying the relative phase shift between photons following the first path and photons following the second path.