There is often a need to communicate a message in secret over a channel which can potentially be intercepted by an eavesdropper. Traditionally, such a problem has been addressed by encrypting or enciphering the message using a secret key. Quantum communication provides a highly secure method for sending such a key. By encoding the key on a series of single photons, where each photon carries one bit of information encoded as a quantum state of the photon e.g. polarisation, phase or energy/time of a photon, an eavesdropper cannot intercept the key without at least partially changing the key. It is not possible to completely prevent an eavesdropper from obtaining the key, but providing that there is a single photon in each light pulse sent from the sender to the receiver, the eavesdropper will be detected.
However, many current quantum communication systems use an attenuated laser as a source of weak light pulses. Such pulses often contain more than one photon which introduces security issues since it is possible for an eavesdropper to split off one photon from a pulse without affecting the other photons in the pulse, a so-called photon number splitting attack PNS. To address the PNS attack, security can be increased by reducing the distance and/or the bit rate at which information can be transferred securely by quantum key distribution systems.
One such method is a decoy pulse protocol which has been proposed to improve security and transmission distance. This method sends signal pulses which carry the encoded information and decoy pulses of a second intensity. By determining the transmission efficiency of the signal and decoy pulses, it is possible to statistically spot the presence of a photon number splitting attack.
Considerable work has been performed on methods for determining a secure bit rate for such systems. The secure bit rate is the number of bits per unit time which may be transferred securely between an emitter(Alice) and a receiver (Bob). It will be lower than the actual rate of pulses sent from Alice.
However, in a real quantum communication system, parameters such as the intensity of the pulses which are sent will drift over time, thus reducing the security of the transmission.