Quantum key distribution: Closing the gap to perfect sources

Quantum key distribution (QKD) allows two parties to communicate securely even in the presence of an arbitrarily powerful eavesdropper. A multitude of protocols have been suggested in the last decades. They were shown to be secure in the presence of the bit rates over which secure communications can be guaranteed are strongly constricted by experimental imperfections. Decoy-state QKD improves the situation, but achieves only about (70%) of the maximal secure distance imposed by fundamental physics with conventional implementations. In this talk, we show how we can close the gap between practical QKD implemented with state-of-the-art devices and idealized QKD assuming perfect single-photon signals. A parametric downconversion (PDC) source in conjunction with a photon number resolving detector is utilized to implement a decoy-like QKD scheme. It allows to improve the effectively sent signal statistics. Strict photon-number correlations between the two PDC outputs allow to infer the complete statistical information about one of them by measuring the photon number distribution of the other. For all practical purposes, our protocol accomplishes up to few percent the power of a single photon source in terms of distance, while the key generation rate is on par with the best available schemes