Quantum cryptography is the oldest and best developed application of the field of quantum information science. Although it is frequently perceived as an encryption method, it is really a scheme to securely distribute correlated random numbers between the communicating parties and thus better described as quantum key distribution (QKD). Any attempt at eavesdropping from a third party is guarantied to be detected by the laws of physics (quantum mechanics) and shows up as an increased error rate in the transmission (the QBER).