EFFECT OF CHANNEL IMPERFECTION ON THE SECRECY CAPACITY OF A QUANTUM CRYPTOGRAPHIC SYSTEM

A realistic quantum cryptographic system must function in the presence of the noise and channel loss inevitable in any practical transmissio n. We examine the effects of these channel limitations on the security and throughput of a class of quantum cryptographic protocols known as four-state or BB84 protocols. Provable unconditional security against eavesdropping, which is the principal feature of quantum cryptography , can be achieved despite minor channel defects, albeit at a reduced t ransmission throughput. We present a semiempirical relation between th e fully secure throughput and the loss and noise levels in the channel . According to this relation, an implementation of BB84 utilizing comm ercially available detectors can reach throughputs as high as 10(4)-10 (5) secure bits per second over a practical channel of reasonable qual ity.