Practical free space quantum cryptography

A successful free space quantum cryptography experiment over a distance of 500 m using the BB84 protocol is reported. The obtained bit error rates of this experiment using faint coherent pulses with an average photon number ranging from 0.1 to 1.0 was measured to be below 3 percent for experiments carried out during night, leading to average raw key rates (before error correction and privacy amplification) of 50 kBits per second.