  We demonstrate single-photon interference over 100 km using a balanced
gated-mode photon detector and a plug & play system for quantum key
distribution. The visibility with 0.1 photon/pulse was more than 80% after 100
km transmission. This corresponds to the fidelity of a quantum cryptography
system of more than 90% and a QBER of less than 10%, satisfying the security
criteria.
