One of the fundamental laws of quantum mechanics, the Heisenberg uncertainty relation, tells us that every quantum measurement significantly influences the observed system. Quantum Cryptography utilizes this feature to guarantee secure communication between Alice (transmitter) and Bob (receiver). In wide contrast to the case of classical communication, where an eavesdropper (Eve) would be able to measure the transmitted signals without arresting Alice's or Bob's attention (Fig. 1, left), in Quantum Cryptography eavesdropping can immediately be detected by Alice or Bob (Fig. 1, right).