  A detailled analysis of quantum key distribution employing entangled states
is presented. We tested a system based on photon pairs entangled in energy-time
optimized for long distance transmission. It is based on a Franson type set-up
for monitoring quantum correlations, and uses a protocol analogous to BB84.
Passive state preparation is implemented by polarization multiplexing in the
interferometers. We distributed a sifted key of 0.4 Mbits at a raw rate of 134
Hz and with an error rate of 8.6% over a distance of 8.5 kilometers. We discuss
thoroughly the noise sources and practical difficulties associated with
entangled states systems. Finally the level of security offered by this system
is assessed and compared with that of faint laser pulses systems.
