Multi-pair signal states in entanglement-based quantum cryptography

Pairs of entangled photons can be employed for quantum key distribution. For each bit exactly one pair of photons is needed. Unfortunately, the quantum states produced by real sources, like a parametric down conversion, contain also terms with more than only one photon pair. We discuss several aspects of the use of such states. for quantum key distribution. It is shown that the presence of multi-pair signals (together with low detection efficiencies) causes errors in transmission even in the absence of an eavesdropper. However, the most important result is that all individual eavesdropping on multi-pair signals increases the error rate. This fact represents the important advantage of the entanglement-based quantum key distribution.