Two Non-Orthogonal States Quantum Cryptography Method and Apparatus with Inter-and Inter-Qubit Interference for Eavesdropper Detection

A receiver station for cooperating with an emitter station for distributing a sequence of symbols between the emitter station and the receiver station when interconnected by a quantum channel and a conventional channel, and assessing the amount of information an eavesdropper having access to both of the channels can have obtained on the sequence, the receiver station comprising:

• control means for controlling operation of the receiver station and coordinating operations and communications with the emitter station, so as to enable the receiver station to command and control its own components in cooperation with associated components of the emitter station having a quantum source for generating a stream of quantum systems, the quantum systems having a coherent phase relationship between systems located in nearby positions of the stream, and, when carrying a symbol of the sequence, being prepared in a state selected in a first set, this set comprising at least two non-orthogonal states;
• a switching device adapted for directing the quantum systems to one of at least two measurement subsystems;
• a first measurement subsystem connected by an appropriate optical path to the switching device, and adapted for performing a first measurement on some of the quantum systems, this measurement allowing determination in some cases in which of the quantum states the quantum systems were prepared by the emitter station;
• a measurement subsystem connected by an appropriate optical path to the switching device, and adapted for performing a second measurement on groups of at least two quantum systems, this measurement capable of obtaining information on the degree of coherence of the phase relationship existing between two quantum systems received by the receiver station;
• a measurement subsystem connected by an appropriate optical path to the switching device, and adapted for performing a third measurement on quantum systems, this measurement allowing, when applied to quantum systems prepared in a state belonging to a second set of quantum states, this second set comprising at least one state, the states of this second set being non-orthogonal to some of the states of the first set, the states of this second set not being a superposition of some of the states of the first set, the states of this second set being selected in such a way that they are perturbed by a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determine at least in some cases in which state this quantum system was prepared;
• at least in some cases determination of whether they have been subjected, between the emitter and receiver stations, to a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determining at least in some cases in which state this quantum system was prepared;
• and
• communication means enabling communication of data with the emitter station adapted for announcing to the emitter station
• the position in the stream of at least some of the quantum systems on which this first measurement yielded conclusive results, thereby allowing determination of which of the quantum states of the first set a particular quantum system had been prepared in and of which symbol had been sent by the emitter station,
• at least some of the measurement results of the second measurement and of the third measurement, the communications means further allowing the emitter station and the receiver station to collaborate to estimate the intensity of eavesdropping on the quantum channel.

A method for distributing a sequence of symbols and estimating information that an eavesdropper may know about a key generated therefrom, the method comprising the steps of:

• a) sending, by the emitter station, of a stream of quantum systems, generated by a quantum system source, the quantum systems having a coherent phase relationship between systems located in nearby positions of the stream, and wherein the quantum systems are prepared in a quantum state belonging to a first set of quantum states, this set comprising at least two non-orthogonal quantum states, and the quantum states of this first set being associated with symbol values,
• b) inserting quantum system belonging to a second state between some of the quantum systems associated with symbols, this second set comprising at least one state, the states of this second set being non-orthogonal to some of the states of the first set, the states of this second set not being a superposition of some of the states of the first set, the states of this second set being selected in such a way that they are perturbed by a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determine at least in some cases in which state this quantum system was prepared
• c) performing, by the receiver, a first measurement on some of the quantum systems to try to determine in which of the quantum states they were prepared by the emitter station,
• d) performing, by the receiver, a second measurement on groups of at least two quantum systems, this measurement capable of obtaining information on the degree of coherence of the phase relationship existing between two quantum systems received by the receiver station;
• e) performing, by the receiver, a third measurement on quantum systems, this measurement allowing, when applied to quantum systems prepared in a state belonging to the second set of quantum states, at least in some cases determination of whether they have been subjected, between the emitter and receiver stations, to a measurement adapted, when applied to a quantum system prepared in a state belonging to the first set, to determining at least in some cases in which state this quantum system was prepared;
• and
• f) announcing to the emitter station the position in the stream of at least some of the quantum systems on which this first measurement yielded conclusive results, thereby allowing determination of which of the quantum states of the first set a particular quantum system had been prepared in and of which symbol had been sent by the emitter station, and, at least some of the measurement results of the second measurement and of the third measurement, and
• g) collaborating, by the emitter and the receiver stations, to assess the intensity of eavesdropping on the quantum channel.

A method for distributing a sequence of symbols between an emitter station and a receiver station connected by a quantum channel wherein the symbols are coded on quantum systems belonging to a first set of at least two non-orthogonal quantum states and wherein the apparatus assesses the amount of information an eavesdropper having access to both of the channels can have obtained on the sequence, the method including the steps of:

• (a) generating by the emitter station of a stream of quantum systems having a coherent phase relationship between neighboring quantum systems,
• (b) detecting by the receiver station of the reduction caused by an eavesdropper of the coherence between some of the quantum systems,
• (c) inserting by the emitter station between some of the quantum systems of the stream at least one witness state such that it is modified by a measurement performed by an eavesdropper on this witness state and another quantum system, this measurement being selected by the eavesdropper to leave the coherent phase relationship between two quantum systems unmodified, and
• (d) determining by the receiver station of any modification caused by an eavesdropper of at least some of the witness states.