QUANTUM CRYPTOGRAPHY

After a short introduction to classic cryptography we explain thorough ly how quantum cryptography works. We present then an elegant experime ntal realization based on a self-balanced interferometer with Faraday mirrors. This phase-coding setup needs no alignment of the interferome ter nor polarization control, and therefore considerably facilitates t he experiment. Moreover it features excellent fringe visibility. Next, we estimate the practical limits of quantum cryptography. The importa nce of the detector noise is illustrated and means of reducing it are presented. With present-day technologies maximum distances of about 70 km with bit rates of 100 Hz are achievable.