Any-point-to-any-point ("AP2AP") quantum key distribution protocol for optical ring network

Abstract

A QKD node in an optical ring network enables distribution of quantum keys between node pairs having neither photon sources nor photon detectors. The QKD node transmits corresponding pulses P 1 and P 2 into the ring network in opposing directions. A first node (Alice) of the pair randomly modulates pulse P 1 and a second node (Allie) of the pair randomly modulates pulse P 2 , each with phases selected from two encoding bases: B 1 (0, π) and B 2 (π/2, 3+/2). Node Allie then publicly signals to node Alice and the QKD node to indicate which bases were used for encoding QKD bits in sequence, for example, B 1, B 2, B 2, B 1, etc. Node Alice compares the encoding types used by node Allie and publicly signals to nodes Allie and Bob to indicate which encoding types match. The QKD node then deletes all mismatched measurements, and nodes Allie and Alice also delete the corresponding bits. The QKD node then publicly signal to nodes Allie and Alice to indicate the XOR bit string. Nodes Allie and Alice negotiate which is going to do the XOR to their key bit string. After the XORing operation, nodes Allie and Alice form a shifted key and they start quantum error correction and privacy amplification procedures to form a final secret key. Further, the QKD node may modulate a secret key Φs into pulse P 1 before transmission, and into pulse P 2 after receipt, to facilitate security and detection of an eavesdropping attack.

1 . Apparatus for distributing a quantum key between a first node and a second node in a communications network, comprising: 
 a photon source operable to generate a base pulse;    a splitter operable to split the base pulse into corresponding pulses P 1  and P 2 ;    a port operable to transmit pulse P 1  and pulse P 2  into the network, pulse P 1  being modulated by the first node with phases selected from two encoding bases and pulse P 2  being modulated by the second node with phases selected from the two encoding bases, the port being further operable to receive modulated pulses P 1  and P 2 ;    a receiver operable to receive an indication, from the first node, of which bases were employed by the first node, and also to receive an indication from the second node of base matches relative to the bases employed by the second node; and    control logic operable to remove mismatches from consideration and communicate with at least one of the first and second nodes to indicate a remaining XOR bit string,    following which one of the first and second nodes performs an XOR on their respective bit string, and the first and second nodes form a shifted key.    
   
   
       2 . The apparatus of  claim 1  wherein the communications network is a ring network, and further including logic operable to direct pulses P 1  and P 2  into the ring in opposite directions.  
   
   
       3 . The apparatus of  claim 1  further including a phase modulator operable to modulate pulse P 1  with a secret phase key before transmitting the pulse into the network.  
   
   
       4 . The apparatus of  claim 3  wherein the phase modulator is further operable, after receiving pulse P 2  from the network, to modulate pulse P 2  with the secret phase key before the comparator is employed to compare the pulses.  
   
   
       5 . The apparatus of  claim 4  further including control logic operable to indicate potential eavesdropping based on quantum bit error rate.  
   
   
       6 . The apparatus of  claim 1  further including an attenuator operable to reduce the number of photons in the pulse.  
   
   
       7 . The apparatus of  claim 1  wherein the first node is operable to indicate to the second node which base types were used by the first node.  
   
   
       8 . The apparatus of  claim 7  wherein the second node is operable to compare base types used by the first node with base types used by the second node, and to indicate base type matches to the control logic and the first node.  
   
   
       9 . The apparatus of  claim 8  wherein the control logic is further operable to remove mismatched bits.  
   
   
       10 . The apparatus of  claim 9  wherein the control logic is further operable to indicate to the first node which bits are neither matched nor mismatched.  
   
   
       11 . A method for distributing a quantum key between a first node and a second node in a communications network, comprising the steps of: 
 generating a base pulse with a photon source;    splitting the base pulse into corresponding pulses P 1  and P 2  with a splitter;    transmitting pulse P 1  and pulse P 2  via a port into the network;    modulating pulse P 1  by the first node with phases selected from at least two encoding bases;    modulating pulse P 2  by the second node with phases selected from the at least two encoding bases;    receiving, via the port, modulated pulses P 1  and P 2 ;    receiving an indication from the first node of which bases were employed by the first node;    receiving an indication from the second node of base matches relative to the bases employed by the second node;    removing mismatches from consideration; and    communicating with at least one of the first and second nodes to indicate a remaining XOR bit string,    following which one of the first and second nodes performs an XOR on their respective bit string, and the first and second nodes form a shifted key.    
   
   
       12 . The method of  claim 11  wherein the communications network is a ring network, and further including the step of directing pulses P 1  and P 2  into the ring in opposite directions.  
   
   
       13 . The method of  claim 11  further including the step of modulating pulse P 1  with a secret phase key before transmitting the pulse into the network.  
   
   
       14 . The method of  claim 13  further including the step of, after receiving pulse P 2  from the network, modulating pulse P 2  with the secret phase key before the comparator is employed to compare the pulses.  
   
   
       15 . The method of  claim 14  further including the step of monitoring quantum bit error rate to detect potential eavesdropping.  
   
   
       16 . The method of  claim 15  further including the step of reducing the number of photons in the pulse.  
   
   
       17 . The method of  claim 11  further including the step of indicating, by the first node to the second node, which base types were used by the first node.  
   
   
       18 . The method of  claim 17  further including the step of comparing, by the second node, base types used by the first node with base types used by the second node, and indicating base type matches to the control logic and the first node.  
   
   
       19 . The method of  claim 18  further including the step of removing mismatched bits.  
   
   
       20 . The method of  claim 19  further including the step of indicating to the first node which bits are neither matched nor mismatched.