Encoding, decoding and communication method and apparatus

A communication method for communicating information between a first site and a second site using a stream of single photons, the method comprising the steps of:

• a) encoding information at the first site on the photons by selectively setting a parameter of each photon defined as a quantum parameter such that each photon occupies one basis state with respect to that parameter, wherein the basis state is chosen from n non-orthogonal bases and n is an integer of at least three;
• b) measuring the photons received at the second site wherein the quantum parameter is measured using a randomly chosen measurement basis from n non-orthogonal measurement bases, wherein each basis corresponds to a basis of step (a);
• and
• c) communicating between the first and second sites to establish for which photons the correct type of measurement was used.

A communication method for communicating information between a first site and a second site using a stream of single photons, the method comprising the steps of:

• a) encoding information at the first site on the photons by selectively setting a parameter of each photon defined as a quantum parameter such that each photon occupies one basis state with respect to that parameter, wherein the basis state is chosen from n non-orthogonal bases and n is an integer of at least two;
• b) measuring the photons received at the second site wherein the quantum parameter is measured using a randomly chosen measurement basis from n non-orthogonal measurement bases, wherein each basis corresponds to a basis of step (a);
• c) grouping the measured photons into groups of m photons and m is an integer of at least two;
• d) communicating between the first and second sites to establish for which groups of photons the correct type of measurement was used for all photons in the group;
• e) discarding the results from an entire group of photons if at least one photon from the group of photons was not correctly measured in step (b) such that only the results from groups of photons where all photons in a group were correctly measured are retained;
• and
• f) comparing both the type of measurement and the results of the measurement for all photons in at least one of the retained groups of photons.

A communication apparatus comprising:

• an encoding apparatus. located at a first site, for encoding at least two bits of information on a photon, the apparatus comprising encoding means for variably setting at least two parameters of the photon to respectively correspond to the at least two bits of information such that the at least two bits of information can be encoded onto the photon using the at least two parameters;
• a decoding apparatus located at a second site, for decoding the at least two bits of information encoded on the photon, said decoding apparatus comprising measuring means for measuring the at least two parameters of the photon that respectively correspond to the at least two bits of information such that the at least two bits of information are decoded from the photon using the at least two parameters;
• a link configured to carry single photons to and from the first and second sites;
• and
• a classical unjammable link between the first and second sites configured to carry information between the two sites.

A communication apparatus comprising:

• an encoding apparatus located at a first site for encoding information on a photon comprising:
• encoding means configured to set a quantum parameter chosen from the polarisation, phase or energy/time of the photon, such that the photon occupies one basis state chosen from at least three non-orthogonal bases with respect to the chosen parameter;
• a decoding apparatus located at a second site, for decoding information encoded on a single photon, wherein information is encoded onto the single photon by setting one parameter defined as a quantum parameter chosen from the phase, polarisation or energy/time of the photon, such that the photon occupies a basis state chosen from one of n non-orthogonal bases with respect to his parameter where n is an integer of at least three, the decoding apparatus comprising decoding means configured to measure the photons using a randomly chosen measuring basis from n bases which correspond to the bases used to encode the photon;
• means configured to carry single photons;
• and
• a classical link between the first and second sites configured to carry information between the two sites.