Cryptography that is based on quantum mechanics. Since any attempt to measure the quantum state of a system alters that state, the basic idea of quantum cryptography is that if a message exists as a quantum state then any eavesdropper would change the state. At present, much work is being undertaken to make quantum cryptography a practical possibility. An important aspect of quantum cryptography is quantum key distribution. In ordinary cryptography it is necessary to use a key, i.e. a mathematical method for encrypting and decrypting the message. The key distribution problem in cryptography is that the key has to be established very securely between the sender and the receiver of messages. Quantum key distribution uses the quantum entanglement of states such as the polarization of photons.