Mikolaj Lasota

Numerous imperfections of realistic setup elements required for the implementation of quantum key distribution (QKD) protocols impose strong limitation on their performance, particularly in terms of the obtainable key generation rate. A possible way to increase this quantity in the case of utilizing spontaneous parametric down-conversion (SPDC) source, producing strongly correlated pairs of photons, is to use wavelength-division-multiplexing (WDM) modules in order to direct photons of different wavelengths into separate detection systems utilized by the trusted parties. In this way it can be possible to generate multiple cryptographic keys in paralell, significantly increasing the overall key rate. In this work we theoretically investigate the potential improvement offered by such possibility in the case of entanglement-based version of the BB84 protocol realized with pulsed-pump SPDC source. We optimize the properties of the produced photon pairs and the intensity of the utilized pumping laser in order to maximize the potential improvement over the traditional, non-WDM, setup configuration. We consider both noiseless case, with ideal single-photon detectors utilized by the trusted parties and more practical situation with imperfect binary detectors, in which case we develop formulas for optimizing the detection window when the temporal filtering method is used to reduce the quantum bit error rate.

In this work we perform an analysis of a general setup for subtracting photons from a given state of light and use the obtained results to derive criteria characterizing usefulness of realistic single photon sources in two recently proposed quantum repeater schemes. We also describe possible tests of Bell’s inequalities using these two schemes and find requirements on the statistics of photon sources which would enable us to show their violation in realistic situation.