Engineering the indistinguishability and entanglement of two photons

We present a novel interferometric technique for suppressing distinguishing information in the spacetime component of the state vector of an entangled pair of photons by providing two indistinguishable ways for each photon to occupy any given space-time mode. We demonstrate this method by using spon taneous parametric down-conversion to generate a pair of photons in the sta te with the least distinguishing information consistent with the set of mod es available. The technique also allows the preparation of the two photons in a highly entangled space-time state provided certain criteria are met.