Experimental implementation of time-coding quantum key distribution

We have implemented an experimental setup 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 one to evaluate the quantum bit error rate (QBER). The minimum obtained QBER is 1.62%. The possible loss of coherence in the setup can be exploited by Eve to eavesdrop the line. Therefore, the second arm of Bob setup is a Mach-Zehnder interferometer with a 10 ns propagation delay between the two paths. Contrast measurement of the output beams allows one to measure the autocorrelation function of the received pulses that characterizes their average coherence. In the case of an ideal setup, 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.