Double phase encoding quantum key distribution

Abstract

A laser pulse representing a bit of a quantum key is split into two pulses. In addition to known round trip phase encoding schema, a secret phase key is modulated into one of the two pulses: P 1 and P 2 . The secret phase key is used to identify whether the returning pulses originated from the sender, i.e., whether the key distribution has been attacked by an eavesdropper. A secret key phase modulator randomly modulates pulse P 1 . An attenuator then reduces the average photon number of the modulated pulse P 1 to a selected level greater than one to increase the likelihood of efficient, successful transmission while reducing the possibility of eavesdropping, e.g., μ=10. Both pulses P 1 and P 2 are sent to the intended recipient and reflected to the sender. Pulse P 2 is modulated upon return to the sender using the same secret phase key previously modulated into pulse P 1 . Therefore, when both pulses meet together at a coupler/beamsplitter of the sender, both pulses should contain the same secret key in their phase and therefore exhibit no resulting phase difference if the photon pulse is the same pulse originated by the sender. If the returning pulse is not the pulse originated by the sender then phase differences indicative of a so-called intercept-resend attack applied by an eavesdropper EVE are indicated by a large quantum bit error rate (“QBER”) will be detectable. If EVE applies photon-split attack, the secret phase key modulated by the sender prevents Eve from knowing the encoded key information in the photon(s). Therefore, double phase encoding QKD enables use of multi-photon pulses without unacceptable loss of security, thereby enhancing QKD bit rate.

1 . A method of Quantum Key Distribution between a first node and a second node, comprising the steps of: 
 by the first node:    generating a pulse having multiple photons,    splitting the pulse into first and second sub-pulses;    modulating the phase of the first sub-pulse with a secret key;    transmitting both the first and second sub-pulses to the second node;    by the second node:    receiving the first and second sub-pulses from the first node;    modifying at least one of the first and second sub-pulses;    transmitting both the first and second sub-pulses back to the first node;    by the first node:    receiving the first and second sub-pulses from the second node;    modulating the phase of the second sub-pulse with the secret key; and    comparing the first and second sub-pulses to detect phase modulation mismatch.    
   
   
       2 . The method of  claim 1  wherein the modifying step includes the further step of the second node modulating a phase shift of the first sub-pulse selected randomly from bases B 1  and B 2  based on the second node's key bit.  
   
   
       3 . The method of  claim 2  wherein the modifying step includes the further step of the second node flipping the polarization of the first sub-pulse.  
   
   
       4 . The method of  claim 1  wherein the modifying step includes the further step of the second node flipping the polarization of the second sub-pulse.  
   
   
       5 . The method of  claim 1  including the further step of generating the secret key such that the secret key is random and equal to the Quantum Key in length.  
   
   
       6 . The method of  claim 1  including the further step of, prior to step of transmitting both the first and second sub-pulses to the second node, attenuating the first and second sub-pulses to reduce the number of photons to a selected number greater than one.  
   
   
       7 . The method of  claim 1  including the further step of the first node modulating a phase shift of the second sub-pulse selected randomly from bases B 1  and B 2  based on the first node's key bit.  
   
   
       8 . The method of  claim 1  including the further step of correlating Quantum Key bits of the first and second sub-pulses to facilitate quantum key distribution.  
   
   
       9 . A network architecture operable to distribute a Quantum Key, comprising: 
 a first device including: 
 a laser operable to generate a pulse;  
 a coupler operable to split the pulse into first and second sub-pulses, the first sub-pulse being sent to a long loop and the second sub-pulse being sent to a short loop;  
 a first modulator in the long loop operable to modulate the phase of the first sub-pulse with a secret key;  
 a port operable to transmit both the first and second sub-pulses to a second device, the second device being operable to modify at least one of the first and second sub-pulses;  
 a port operable to receive the first and second sub-pulses back from the second device;  
 a polarization beam splitter operable to send the first sub pulse to the short loop and to send the second sub-pulse to the long loop, where the phase of the second sub-pulse is modulated with the secret key, the first and second sub-pulses then being combined by the coupler; and  
 detectors operable to detect phase modulation mismatch of the first and second sub-pulses.  
   
   
   
       10 . The network architecture of  claim 9  wherein the second device includes a phase modulator operable to modulate a phase shift of the first sub-pulse selected randomly from bases B 1  and B 2  based on a key bit.  
   
   
       11 . The network architecture of  claim 10  wherein the second device further includes a Faraday mirror operable to flip the polarization of the first sub-pulse.  
   
   
       12 . The network architecture of  claim 9  wherein the second device further includes a Faraday mirror operable to flip the polarization of the second sub-pulse.  
   
   
       13 . The network architecture of  claim 9  further including logic operable to generate the secret key such that the secret key is random and equal to the Quantum Key in length.  
   
   
       14 . The network architecture of  claim 9  further including an attenuator operable to attenuate the first and second sub-pulses to reduce the number of photons to a selected number greater than one.  
   
   
       15 . The network architecture of  claim 9  including logic operable to correlate Quantum Key bits of the first and second sub-pulses to facilitate quantum key distribution.  
   
   
       16 . The network architecture of  claim 9  further including a phase modulator operable to modulate a phase shift of the second sub-pulse selected randomly from bases B 1  and B 2  based on Bob's key bit.