Conversion of position correlation into polarization entanglement*

Photons entangled in polarization have been a prime tool for experiments in fundamental quantum physics. To generate polarization entanglement from spontaneous parametric down conversion (SPDC), it is necessary to impose coherent superposition of two pump-decay paths. There is a variety of techniques by which this coherent superposition can be achieved. Examples of previous entangled photon-pair sources have utilized momentum correlation, indistinguishable pump decay in two separate crystals, or two distinguishable pump beams. We present a method to convert position correlation of photon-pairs into polarization entanglement. This is achieved by individually manipulating the polarization state of photons generated in different parts of a non-linear medium and putting them in coherent superposition. This concept is experimentally demonstrated using photon-pairs produced by SPDC. The method was used to implement a compact source producing an observed photon-pair rate of 120,000/s/mW with an entanglement fidelity of 0.99. This method can be extended to any photon-pair generation process with initial position correlation.