Quantum Information Group, Single Photon Communications
Quantum cryptography can be implemented by sending encoded single photons ('particles' of light) along standard telecom fibres. These particles of light correspond to extraordinary dim light signals - an ordinary light bulb emits 1 billion billion photons per second. The sender (Alice) encodes one bit, i.e. 0 or 1, onto each photon. This can be done in a number of ways, for example by applying different time delays to the photons to represent 0 and 1 using a device called a 'phase modulator'.
Nearly all the photons launched into the fibre are scattered out enroute, allowing only very few to actually reach the other end. However, quantum cryptography is robust against photon loss, since only photons detected by Bob are used to form the key. Thus, keys can be formed even over fibres in excess of 100km. Although using single photons is a very inefficient way to communicate, its attraction is that it allows a measure of the security of each communication.
Quantum cryptography can also be performed over air based, line-of-sight optical links of the type sometimes used for high bandwidth connections between buildings, or in some local area networks. These free space links may be useful for replenishing the key material of communication satellites in low earth orbits or for secure communication between satellites.
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