  We have implemented an experimental set-up in order to demonstrate the
feasibility of time-coding protocols for quantum key distribution. Alice
produces coherent 20 ns faint pulses of light at 853 nm. They are sent to Bob
with delay 0 ns (encoding bit 0) or 10 ns (encoding bit 1). Bob directs at
random the received pulses to two different arms. In the first one, a 300 ps
resolution Si photon-counter allows Bob to precisely measure the detection
times of each photon in order to establish the key. Comparing them with the
emission times of the pulses sent by Alice allows to evaluate the quantum bit
error rate (QBER). The minimum obtained QBER is 1.62 %. The possible loss of
coherence in the set-up can be exploited by Eve to eavesdrop the line.
Therefore, the second arm of Bob set-up is a Mach-Zender interferometer with a
10 ns propagation delay between the two path. Contrast measurement of the
output beams allows to measure the autocorrelation function of the received
pulses that characterizes their average coherence. In the case of an ideal
set-up, the value expected with the pulses sent by Alice is 0.576. The
experimental value of the pulses autocorrelation function is found to be 0.541.
Knowing the resulting loss of coherence and the measured QBER, one can evaluate
the mutual information between Alice and Eve and the mutual information between
Alice and Bob, in the case of intercept-resend attacks and in the case of
attacks with intrication. With our values, Bob has an advantage on Eve of 0.43
bit per pulse. The maximum possible QBER corresponding to equal informations
for Bob and Eve is 5.8 %. With the usual attenuation of fibres at 850 nm, it
shows that secure key distribution is possible up to a distance of 2.75 km,
which is sufficient for local links.

Quantum Physics