A quantum cryptosystem based on comparing an input signal from a communication channel with a reference state prepared at the receiving end is proposed.
All existing quantum cryptosystems use nonorthogonal states as the carriers of information. Nonorthogonal states cannot be cloned (duplicated) by an eavesdropper. As a result, any eavesdropping attempt must introduce errors in the transmission, and, therefore, can be detected by the legal users of the communication channel. Orthogonal states are not used in quantum cryptography, since they can be faithfully cloned without altering the transmitted data. We present a cryptographic scheme based on orthogonal states, which also assures the detection of any eavesdropper.
A new relativistic quantum cryptosystem is proposed in which the information is carried by the extended single-photon states with orthogonal polarizations and effective length exceeding the communication channel length. The light “arrest” effect is used as a procedure for the detection and preparation of extended states. The cryptosystem is secure against any eavesdropping attempts, because its states are quantized and the propagation velocity is limited. In this scheme, the preparation and detection procedures are local in space but require a finite time, depending on the extension of the states. The preparation for detecting at the receiver end begins before the state left the source at the input end.
Quantum cryptography lies at the intersection of information theory and quantum mechanics. Cryptosystems come in two major classes depending on whether the key is shared in public or in secret. Public key cryptosystems are special in that they allow two people, who have not yet met, to communicate securely by looking-up encryption methods in a public directory. In quantum cryptography, the sender and receiver have two types of communication channels: a classical public channel and a quantum channel. The best application of cryptography is to allow a secure communication. In the near future, quantum cryptography may be used in top-secret applications in the military, for example, as the basis of communication links. The first protocol for quantum cryptography was proposed in 1984 by C. H. Bennett and G. Brassard. Some of the basic quantum cryptographic systems are: four-states protocol, two-states protocol, 4+2 protocol and multi-photon and multi-stage protocol.