Quantum key distribution over 10 km of optical fiber

Quantum cryptography is an important new technique for the distribution of cryptographic keys that exploits the quantum properties of single photons to guarantee the secrecy of the key. Here, we report the transmission over 10 km of optical fiber of pseudorandom bit sequences (PRBS) encoded in the phase of faint pulses of light that contain substantially less than one photon on average. This demonstrates many of the properties required for practical quantum-key distribution systems. The prototype consists of a 10-km-long, time- and polarization-division Mach-Zehnder interferometer. The system operates at a wavelength of 1.3 μm and, as previously reported, exhibits interference visibilities as high as 99%. In order to obtain the single-photon-like properties required for a quantum-cryptography system, a pulsed semiconductor laser source is attenuated to a level where, on average, only 1 pulse in 10 contains a photon (for a Poissonian laser source only a much smaller fraction of pulses, approx.1 in 200, contain two or more photons).