  We present and demonstrate a new protocol for practical quantum cryptography,
tailored for an implementation with weak coherent pulses to obtain a high key
generation rate. The key is obtained by a simple time-of-arrival measurement on
the data line; the presence of an eavesdropper is checked by an interferometer
on an additional monitoring line. The setup is experimentally simple; moreover,
it is tolerant to reduced interference visibility and to photon number
splitting attacks, thus featuring a high efficiency in terms of distilled
secret bit per qubit.
