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Transverse correlations and entanglement of biphotons


We present measurements of the transverse entanglement of photon pairs on their propagation from the near to the far field of spontaneous parametric down-conversion. The Fedorov ratio, depending on the widths of conditional and unconditional intensity measurements, is shown to be only able to characterise entanglement in the near and far zones of the source. Therefore we also apply a different technique. Based on the interference of the two-photon amplitude, our measurement provides direct experimental access to the Schmidt number of the state. Our  measurements show that the interferometric scheme always gives the full transverse entanglement of the quantum state.

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