Quantum communication system

1. A system for transmitting quantum information comprising a sending unit and a receiving unit,

said sending unit comprising an encoder including hardware, and configured to encode photon pulses of a sequence of photon pulses with quantum information by setting for each photon pulse of the sequence of photon pulses a single quantum state of a single parameter of that photon pulse, the single quantum state of the single parameter determined according to the combination of a first component and a second component, said encoder further comprising a random number generator configured to select said first component randomly, the second component being determined from information which is pre-shared with the receiving unit, said information being pre-shared before said sequence of photon pulses is sent, said information indicating a first value for the second component for a first pulse of the sequence of photon pulses and a second, different value for the second component for a second pulse of the sequence of photon pulses,

said receiving unit comprising a decoder including hardware, and configured to perform a measurement on said photon pulses of a sequence of photon pulses using a measurement basis selected for each photon pulse of the sequence of photon pulses, using said pre-shared information, to cancel said second component and decode said quantum information from the photon pulses.

2. A system according to claim 1 , wherein the encoder is configured to change the second component between each of successive photon pulses.

3. A system according to claim 1 , wherein the encoder is configured to select the second component for each photon pulse from a fixed set of n values, where n is an integer greater than 1.

4. A system according to claim 1 , wherein the encoder and decoder are provided with means to share or derive information concerning the second component.

5. A system according to claim 4 , wherein both the encoder and decoder are configured to determine the second component from expansion of a shared secret seed key.

6. A system according to claim 1 , wherein the encoder and decoder are a phase encoder and phase decoder respectively and wherein the second component is a phase shift.

7. A system according to claim 1 , wherein the encoder and decoder are a polarisation encoder and polarisation decoder respectively and the second component is a rotation of the polarisation.

8. A system according to claim 1 , wherein the encoder is configured to select the first component from a fixed set of N states, where N (≧2) is an integer.

9. A system according to claim 8 , wherein some of the N states are non-orthogonal.

10. A system according to claim 8 , wherein the N states can form encoding sets in which two states can either be orthogonal or non-orthogonal.

11. A system according to claim 1 , wherein the system is fibre optics based.

12. A system according to claim 1 , wherein the system is free-space based.

13. A decoder, for a quantum communication system, configured to decode quantum information from photon pulses of a sequence of photon pulses when the quantum information is held in a single quantum state of a single parameter of each of the photon pulses and the quantum state is set using the combination a first component and a second component of the single parameter, the decoder comprising an authentication controller configured to determine pre-shared information about the second component received from a source other than the source of the photon pulses and an interferometer configured to perform a measurement on a measurement basis selected to cancel the second component before determining said quantum information from the photon pulses, said decoder further comprising a random number generator configured to randomly select a measurement basis for determining said quantum information,

wherein said source other than the source of photon pulses indicates a first value for the second component for a first pulse of the sequence of photon pulses and a second different value for the second component for a second pulse of the sequence of photon pulses.

14. A quantum communication method comprising:

in a sending unit, encoding a sequence of photon pulses with quantum information by setting a single quantum state of a single parameter of each photon pulse of the sequence of photon pulses, said quantum state being set using a combination of a first component and a second component, said first component comprising a randomly selected measurement basis, the second component being determined from information which is pre-shared with the receiving unit, said information being pre-shared before the sequence of photon pulses is sent, said information indicating a first value for the second component for a first pulse of the sequence of photon pulses and a second, different value for the second component for a second pulse of the sequence of photon pulses,

sending the sequence of encoded photon pulses to a receiving unit comprising a decoder, and

decoding the said first component of said quantum information by performing a measurement on each photon pulse of the sequence of photon pulses using a measurement basis selected, for each photon pulse of the sequence of photon pulses, using said pre-shared information, to cancel the second component.

15. A method according to claim 14 , further comprising sharing secret information between the sending unit and receiving unit concerning the second component.

16. A method according to claim 15 , wherein the secret information is a seed key and both the sending unit and the receiving unit expand the seed key.

17. A method according to claim 14 , further comprising the receiving unit communicating with the sending unit to disclose what types of measurements were performed on the photons by the receiving unit.

18. A method according to claim 14 , wherein the first component is chosen randomly by the sending unit.

19. A method of receiving information from a sending unit, the method comprising:

in a receiving unit, receiving a sequence of photon pulses which has been encoded with quantum information, said quantum information being encoded by setting a quantum state of a single parameter of each photon pulse, said quantum state being set using a combination of a first component and a second component of the single parameter;

receiving pre-shared information from which the second component can be determined from a source using a communication route other than a communication route by which the sequence of photon pulses is received; and

decoding said photon pulse by first determining said second component from said pre-shared information and then performing a measurement on said photon pulse using a measurement basis selected to cancel said second component from said photon pulse, wherein the measurement basis is randomly chosen from n measurement basis where n is an integer of at least 2,

wherein said pre-shared information indicates a first value for the second component for a first pulse of the sequence of photon pulses and a second, different value for the second component for a second pulse of the sequence of photon pulses.