Abstract: A method and apparatus for enhancing the production of polarisation-entangled multiphoton states from a laser pumped parametric down-converting crystal. The pump beam and initial down-converted photons are returned by retro mirrors, in phase to the down-converting crystal where they stimulate the emission of further polarisation-entangled states. The efficiency of the process is increased by including in the path of the returning down-converted photons a crystal of the same substance and thickness as the down-converting crystal to provide spatial and temporal walk-off compensation. Further, the phase of the returning pump beam is adjusted and optimised by control of the retro-mirror while monitoring the rate of production of polarisation-entangled photon pairs. Maximising the rate of production of photon pairs also maximises the rate of production of higher-order states, such as four-photon states.