Measuring entanglement of photons produced by a pulsed source

Abstract

A source of entangled photons based on well-separated laser pump pulses (PDs) with a duration in the nanosecond range may be useful in many applications. Yet, such a source has intrinsic problems arising from the simultaneous arrival of signal and noise photons to the detectors, which makes the methods unsuitable for dealing with accidental (or spurious) coincidences applied in the continuous-wave or mode-locked pump regimes. Those problems are analyzed, and practical methods to calculate the number of accidental coincidences are described and experimentally checked. These methods are useful not only to measure entanglement, but also in every situation where extracting the number of valid two-photon coincidences from noisy data generated by such a pulsed process is required. As an original example of the use of these methods, we present the time-resolved measurement of the concurrence of the field produced by spontaneous parametric downconversion with PDs of duration in the ns range at a repetition of kHz.