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 spontaneous parametric down-conversion to generate a pair of photons in the state with the least distinguishing information consistent with the set of modes available. The technique also allows the preparation of the two photons in a highly entangled space-time state provided certain criteria are met.

We demonstrate a general method of engineering the joint quantum state of photon pairs produced in spontaneous parametric down-conversion. The method makes use of a superlattice structure of nonlinear and linear materials, in conjunction with a broadband pump, to manipulate the group delays of the signal and idler photons relative to the pump pulse, and realizes photon pairs described by a joint spectral amplitude with arbitrary degree of entanglement. This method of group-delay engineering has the potential of synthesizing a broad range of states including factorizable states crucial for quantum networking and states optimized for Hong-Ou-Mandel interferometry. Experimental results for the latter case are presented, illustrating the principles of this approach.

We report a fourth-order interference experiment in which pairs of photons produced in parametric downconversion pumped by short optical pulses interfere in a Hong-Ou-Mandel interferometer. The visibility of the interference pattern is reduced for larger pump bandwidths. This effect can be understood in terms of the spectral distinguishability of the photon pairs. The interference can be restored by blocking the distinguishing information with a spectral filter. ͓S1050-2947͑98͒50704-4͔