Reconfigurable processing device for quantum communications

A configuration method for a processing device (1), the processing device (1) comprising i) an input interface (2) configured to receive photons having first quantum states, ii) an encoder (3) configured to generate a photon having a first fixed quantum state from a received electrical signal, iii) a transformer (4) configured to transform said first quantum state of a photon into a second quantum state, iv) a decoder (5) configured to transform said second quantum state of a received photon into an electrical signal, and v) an output interface (6) configured to deliver photons having second quantum states and to be transmitted, said processing device (1) also comprising a switch (7) comprising at least three inputs connected respectively to outputs of said input interface (2), encoder (3) and transformer (4), and at least three outputs connected respectively to inputs of said output interface (6), decoder (5) and transformer (4), and selectively coupling at least one of its inputs to one of its outputs according to a received command,

• the method comprising a step in which the processing device (1) is configured so that:
• either the output of said encoder (3) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said decoder (5), for it to perform a procedure of prepare and measure type,
• or the output of said encoder (3) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said output interface (6), for it to perform a procedure of prepare and send type,
• or the output of said input interface (2) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said decoder (5), for it to perform a procedure of receive and measure type,
• or the output of said input interface (2) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said output interface (6), for it to perform a procedure of receive and send type.

A method for distributing a quantum key using a processing device (1),

• the processing device comprising i) an input interface (2) configured to receive photons having first quantum states, ii) an encoder (3) configured to generate a photon having a first fixed quantum state from a received electrical signal, iii) a transformer (4) configured to transform said first quantum state of a photon into a second quantum state, iv) a decoder (5) configured to transform said second quantum state of a received photon into an electrical signal, and v) an output interface (6) configured to deliver photons having second quantum states and to be transmitted, said processing device (1) also comprising a switch (7) comprising at least three inputs connected respectively to outputs of said input interface (2), encoder (3) and transformer (4), and at least three outputs connected respectively to inputs of said output interface (6), decoder (5) and transformer (4), and selectively coupling at least one of its inputs to one of its outputs according to a received command,
• the method comprising:
• a first step in which i) the processing device (1) is configured so that the output of said encoder (3) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said decoder (5), ii) said encoder (3) is configured so that it delivers 2N photons having first states corresponding respectively to 2N quantum information units, iii) said transformer (4) is configured so that it transforms the first state of each photon, received from said encoder (3) via said switch (7), into a second quantum state resulting from a uniform superimposition of first |0 and second |1 quantum information units corresponding to a phase difference of π/2, and iv) said decoder (5) is used, with said 2N photons received from said transformer (4) via said switch (7) to determine a sequence of 2N electrical signals each corresponding to a value chosen from 0 and 1, this sequence being subdivided into two subsequences of N electrical signals respectively denoted xi and yi (with i=1 to N), and
• a second step in which i) said processing device (1) is configured so that the output of said encoder (3) is coupled to the input of said transformer (4) and the output thereof (4) is coupled to the input of said output interface (6), ii) said encoder (3) is supplied with N electrical signals in order for it to generate, from these N electrical signals, a sequence of N photons having a first fixed quantum state, and said transformer (4) is supplied with said two subsequences xi and yi so that it transforms the first quantum state of each photon, received from said encoder (3) via said switch (7), into a second quantum state defined by a superimposition of said first |0 and second |1 quantum information units corresponding to a phase difference which is a function of said values of these xi and yi, with a view to the transmission of this photon by said output interface (6).