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 dataline; 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.

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Our dataline is that of a classical communication channel, but with a photon counter. The same line is used in a different QKD protocol:
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A similar argument applies to a different protocol:
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Alternatively, the source could be a pulsed mode-locked laser followed by a pulse picker.

10.

If dark counts can be neglected (Qdet=0), the optimization can be done analytically: for the present protocol, μoptV[2(2Vt)]; for BB84, μopt=f(V)[2(1t)] with, and μopt=tf(V) without decoy states, where f(V)={Vh[(1V)2]}. For simplicity, in the optimization we have taken tB1 for both protocols, although this may be technically harder to achieve in BB84 because there are more optical components.

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