The security of quantum cryptography relies on the foundations of quantum mechanics, in contrast to traditional public key cryptography which relies on the computational difficulty of certain mathematical functions. Also traditional public key cryptography cannot provide any indication of eavesdropping or guarantee of key security. Quantum key distribution has an important and unique properly; it is the ability of the two communicating users (traditionally referred to as Alice and Bob) to detect the presence of any third party (referred to as Eve) trying to gain knowledge of the key. A third party trying to eavesdrop on the key must in some way measure it, thus introducing detectable anomalies. By using quantum superpositions or quantum entanglement and transmitting information in quantum states over a quantum channel (such as an optical fiber or free air), a communication system can be implemented which detects eavesdropping.