Abstract

It is well-known that multi-photon signals in quantum key distribution (QKD) with weak laser pulses represent a security threat resulting in a reduced rate for secret-key production. Similarly, multi-pair contributions^1,2 from spontaneous-parametric-down-conversion (SPDC) sources may cause information leak. In particular, in the case of polarization coding (i.e. when the polarization of photons is used to encode the logical qubits) a security risk occurs when the polarizations show some degree of correlation. In such a case an eavesdropper can perform a Photon Number Splitting (PNS) attack by getting hold of, and analyzing, one photon out of the multi-photon state without causing any disturbance to the polarization of the other ones. It is therefore important to estimate the relative amount of polarization-correlated multi-pairs in SPDC signals. We present and discuss the results of a recent investigation on this subject.

© 2007 IEEE