In quantum communication systems, information is transmitted between a sender and a receiver by encoded single quanta, such as single photons.  Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time.  The photon may even carry more than one bit of information, for example, by using properties such as angular momentum. 
Quantum key distribution which is a technique for forming a shared cryptographic key between two parties; a transmitter, often referred to as “Alice”, and a receiver often referred to as “Bob”.  The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve).  The laws of quantum mechanics dictate that measurement of the photons by Eve causes an unavoidable change to the state of some of the photons.  These changes to the states of the photons will cause errors in the bit values formed between Alice and Bob.  By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information. 
Previous quantum key distribution systems have used a point-to-point link.