  By realizing a quantum cryptography system based on polarization entangled
photon pairs we establish highly secure keys, because a single photon source is
approximated and the inherent randomness of quantum measurements is exploited.
We implement a novel key distribution scheme using Wigner's inequality to test
the security of the quantum channel, and, alternatively, realize a variant of
the BB84 protocol. Our system has two completely independent users separated by
360 m, and generates raw keys at rates of 400 - 800 bits/second with bit error
rates arround 3 percent.
