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Quantum cryptography is based on the use of single photon Fock states. Unfortunately, these states are difficult to realize experimentally. Now-a-days, practical implementations rely on faint laser pulses or entangled photon pairs, where both the photon as well as the photon-pair number distribution obeys Poisson statistics. Hence, both possibilities suffer from a small probability of generating more than one photon or photon pair at the same time. For large losses in the quantum channel even small fractions of these multi-photons can have important consequences on the security of the key. In this paper, the authors briefly comment on sources based on faint pulses as well as on entangled photon-pairs, and they compare their advantages and drawbacks.
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