Abstract:
The invention relates to a polarization-controlled encoding method, encoder and quantum key distribution system, which is characterized in that polarization maintaining light path or 90 degree rotation Faraday mirror are used inside the encoder to keep the polarization of the output pulses same, and that in the quantum key distribution system employing the polarization-controlled encoder the pulse emitted from transmitter is unidirectional-transmitted to receiver and then quantum key distribution is implemented using interference in the pulses according to the quantum key distribution protocol. The quantum key distribution system using the polarization-controlled encoder of the invention has the ability of avoiding the wiretapping to transmitter, receiver and quantum channel. Detection units each of which separates reversed photon from other photons are added at the out port of the transmitter and the in port of receiver, respectively, so that Trojan horse is prevented from entering and photons with phase modulated information are prevented from leaving the safe area in receiver. Unconditionally safe key distribution can be accomplished by using the quantum key distribution system of the invention.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the field of secure optical communication, and more particularly to an encoding method and device in quantum key distribution system.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     Early encoding scheme of quantum key distribution is based upon photon polarization. This scheme is fit for free-space communication instead of fiber communication. That&#39;s because the symmetry of normal fiber is not good and the interference in transmission path disturbs the polarization state, the photon polarization will be random changed during transmission in it. U.S. Pat. No. 5,307,410 disclosed a phase encoding scheme of quantum key distribution which bases on a pair of unbalanced Mach-Zehnder interferometers. Because the optical pulses pass through the different arms of the different interferometers in transmitter and receiver, the disturbance they suffered is not exactly identical to each other and can&#39;t be counteracted. The stability and anti-jamming ability of the system is bad. Also because two photon pulses pass through the different arms of transmitter&#39;s interferometer, the polarization relation between two photon pulses is uncertain when they enter the quantum channel. This system is sensitive to the disturbance in quantum channel, especially for long distance quantum key distribution. A lot of variations of this scheme have no essential improvement.  
         [0003]     In order to improve the stability of above scheme, a publication in Applied Physics Letters (vol 77 (No. 7), 793 (1997)) discloses a solution that uses Faraday mirrors in receiver to reflect optical pulses back to the transmitter so that the interfering pulses follow exactly the same optical path. This will get self-balancing effect. This scheme assumes that the disturbance is same when optical pulses pass through the same position twice and then they are counteracted to achieve the object of anti-jamming and stability. In fact, it works only for short distance and low disturbance frequency. When transmission distance increases, the time slot in which the optical pulses pass through the same position twice also increases, and the anti-jamming ability decreases. Furthermore, because the optical pulses pass through the same quantum channel twice, the loss of total optical path should be double. This can be solved by method with strong coming laser pulse and attenuating it to single photon level after reflecting. But, it can be used only for attenuated laser source. The ideal optical source for quantum key distribution is single photon source. When the single photon source becomes available, the maximum transmission distance should be half of the unidirectional scheme. On the other hand, an eavesdropper can attenuate the strong coming laser pulse before it reach the communication receiver, and then combine it with a spy laser pulse whose wavelength is close to the original one to make the total power as same as before, that will make power monitor of no effect. When the signals come back, the spy signal can be separated to get the information of communication receiver, and the original signal will be still transmitted to transmitter through an “ultra-low loss channel” so that the eavesdropper won&#39;t be detected. This is deadly for information security.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention provides a polarization-controlled quantum key distribution encoding method, encoder and system. It can provide unconditional secure key distribution between two users.  
         [0005]     The polarization-controlled encoding method comprises the steps of: splitting an injected optical pulse into two optical pulses traveling along two different paths; and relatively delaying the two optical pulses; recombining the delayed pluses in one output path; wherein the method further comprises a step of phase modulating at least one pulse after the splitting step or recombining step according to a quantum key distribution protocol, and a step of controlling the polarization states of two pulses during the course from the splitting to the recombining to make the polarization states same after recombining.  
         [0006]     Preferably, the step of controlling comprises: keeping the polarization states of two optical pulses fixedness during the course from the splitting to the recombining.  
         [0007]     Preferably, the step of controlling comprises: making the two optical pulses reflected odd times by 90 degree Faraday mirrors separately, and passing them through the same path even times respectively.  
         [0008]     Preferably, the step of controlling comprises: making one optical pulse outputted directly and another optical pulse reflected even times by 90 degree Faraday mirrors, and passing the reflected pulse through the same path even times.  
         [0009]     First polarization-controlled encoder constructed by using the method comprises: a first polarization-maintained beam splitter for splitting an optical pulse into two optical pulses traveling along two different paths; a delay line for relatively delaying the two optical pulses; a second polarization-controlled beam splitter for recombining the delayed pluses in one output path; and at least one phase modulator arranged on at least one path of the two different paths and the output path, wherein the two different paths are polarization-maintained paths.  
         [0010]     Second polarization-controlled encoder constructed by using the method comprises: a polarization-maintained beam splitter for splitting an optical pulse into two optical pulses traveling along two different paths; a polarization-maintained delay line arranged on one of the two different paths for relatively delaying the two optical pluses; at least one mirror for reflecting the delayed pluses to the polarization-maintained beam splitter to recombine the delayed pulses in one output path; and at least one phase modulator arranged on at least one of the two different paths and the output path, wherein the two different paths are polarization-maintained paths.  
         [0011]     Third polarization-controlled encoder constructed by using the method comprises: a beam splitter for splitting an optical pulse into two optical pulses traveling along two different paths; a delay line arranged on one of the two different paths for relatively delaying the two optical pluses; two 90 degree Faraday mirrors for respectively reflecting the delayed pluses back the beam splitter to recombine the delayed pulses in one output path; and at least one phase modulator arranged on at least one of the different paths and the output path.  
         [0012]     Fourth polarization-controlled encoder constructed by using the method comprises: a variable polarization-maintained beam splitter for splitting an optical pulse into two optical pulses traveling along two different paths and outputting one optical pulse directly; a first mirror for reflecting another optical pulse back the variable polarization-maintained beam splitter and passing it through the variable polarization-maintained beam splitter; a second mirror for reflecting the pulse passed through the variable polarization-maintained beam splitter back, wherein the variable polarization-maintained beam splitter recombines the reflected pulse with the outputted pulse in one output path; a polarization-maintained delay line arranged on the paths between the first and second mirrors; and at least one phase modulator arranged on at least one of the two different paths and the output path, wherein the paths between the first and second mirrors are polarization-maintained paths.  
         [0013]     Fifth polarization-controlled encoder constructed by using the method comprises: a beam splitter for splitting an optical pulse into two optical pulses traveling along two different paths and outputting one optical pulse directly; a first 90 degree Faraday mirror for reflecting another optical pulse back the beam splitter and passing it through the beam splitter; a second 90 degree Faraday mirror for reflecting the pulse passed through the beam splitter back, wherein the beam splitter recombines the reflected pulse with the outputted pulse in one output path; a delay line arranged on the paths between the first and second mirrors; and at least one phase modulator arranged on at least one of the paths between the first and second mirrors and the output path.  
         [0014]     A quantum key distribution system comprises: a transmitter side polarization-controlled encoder for splitting an optical pulse emitted from a pulse light source into two optical pulses traveling along two different paths, relatively delaying the two optical pulses and recombining the optical pulses in one output path, wherein at least one of the optical pulses is phase-modulated according to a quantum key distribution protocol; at least one quantum channel for unidirectional-transmitting the optical pulses output from the transmitter side polarization-controlled encoder; a receiver side polarization-controlled encoder for receiving the optical pulses from the quantum channel, splitting each of the optical pulses into two optical pulses which form a group and travel along two different paths, relatively delaying the two optical pulses on the basis of the quantum key distribution protocol, and recombining the two optical pulses in one output path, wherein at least one of the received optical pulses, the split optical pulses, the delayed pulses is phase-modulated before recombined in one output path according to the quantum key distribution protocol; and a single photon detector for measuring at least one superposition interference of two pulses come from different groups and distributing a quantum key according to the quantum key distribution protocol.  
         [0015]     Preferably, the system further comprises a return photon separating and detecting unit that is added in the output of a transmitter or in the input of a receiver, the return photon separating and detecting unit comprises an optical circulator and a single photon detector, wherein an input port of the unit connects with an output port of the encoder, and an output port of the unit connects with the quantum channel, and a reverse output port of the unit connects with an input port of the single photon detector.  
         [0016]     Preferably, the return photon separating and detecting unit further comprises a band pass filter arranged before the input of the circulator.  
         [0017]     The quantum channel may be optical waveguide, fiber, free space, separating optical component or propagation channel made up of above.  
         [0018]     Comparing with existing encoder bases on Mach-Zehnder interferometer, the encoder of present patent controls polarization state of optical pulse inside itself. This makes it insensitive to disturbance on itself so that the environment requirements of quantum key distribution system reduced largely. Because two pulses injected into the quantum channel are controlled at same polarization state, the disturbance they suffered when transmitting is compensated before they enter the receiver. That means signal transmitting is independent of channel disturbance. The stability of system is improved greatly. In the encoder with 90 degree Faraday mirrors according to the invention, the pulse passes through the phase modulator twice with orthogonal polarization. If the duration of phase modulating signal is larger than the time slot during which the pulse pass through modulator twice, the modulation will be independent of polarization. So the polarization-dependent modulator can be used to achieve polarization-independent modulation. The requirement of modulation rate will also be reduced. In the encoder with polarization-maintained beam splitter according to the invention, since polarization of optical pulses is maintained, polarization-dependent modulator can be used.  
         [0019]     In quantum key distribution system according to the invention, return photon separating and detecting unit can be added to separate and detect any reverse transmitted photons. This will not only prevent spy photons from entering the encoder to eavesdrop information but also detect whether there is an eavesdropper. Since single photon detector and circulator have a certain range of response wavelength, optical band pass filter can be added in the quantum key distribution system so that optical pulses with wavelength in working spectrum can pass through the filter, pulses with wavelength out of working spectrum can not pass through the filter. This can compensate the disadvantage that single photon detector and circulator don&#39;t have enough response band. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a block diagram of Mach-Zehnder interferometer based polarization-controlled with polarization-maintained paths.  
         [0021]      FIG. 2  is a block diagram of polarization-controlled encoder with mirrors and polarization-maintained paths.  
         [0022]      FIG. 3  is a block diagram of polarization-controlled encoder with 90 degree Faraday mirrors.  
         [0023]      FIG. 4  is a block diagram of another type of polarization-controlled encoder with 90 degree Faraday mirrors.  
         [0024]      FIG. 5  is a block diagram of polarization-controlled encoder with 90 degree Faraday mirrors in which the modulator is located in the output path.  
         [0025]      FIG. 6  is a block diagram of polarization-controlled encoder with variable beam splitter, mirrors and polarization-maintained paths.  
         [0026]      FIG. 7  is a block diagram of polarization-controlled encoder with variable beam splitter and 90 degree Faraday mirrors.  
         [0027]      FIG. 8  shows one return photon separating and detecting unit.  
         [0028]      FIG. 9  shows another return photon separating and detecting unit which has band pass filter.  
         [0029]      FIG. 10  is a block diagram of quantum key distribution system whose kernel part is polarization-controlled encoder and the quantum channel is optical fiber. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     First Embodiment  
       [0030]     The first configuration of polarization-controlled encoder in quantum key distribution system is shown in  FIG. 1 . It includes two 2×2-3 dB polarization-maintained beam splitters  3  and  6 , one polarization-maintained phase modulator  5  and one polarization-maintained delay line  4 . These form a Mach-Zehnder interferometer. Any one of the two ports  1  and  2  which are in the same side of 3 dB beam splitter  3  can be the input port of polarization-controlled encoder. Any one of the two ports  7  and  8  which are in the same side of 3 dB beam splitter  6  can be the output port. The modulator  5  and delay line  4  (no sequencing) can be inserted into one arm of the Mach-Zehnder interferometer or two arms separately. When the system works, an optical pulse is injected into port  1  or  2  of beam splitter  3  and divided into two pulses. One pulse passes through the modulator  5 . Another one pass through the delay line  4 .  
         [0031]     Then they are recombined by beam splitter  6  and output from port  7  or  8 . Because all the paths are polarization-maintained, the output pulses have same polarization state. When the modulator  5  and delay line  4  are in the same arm of Mach-Zehnder interferometer, the result is the same.  
       Second Embodiment  
       [0032]     The second configuration of polarization-controlled encoder in quantum key distribution system is shown in  FIG. 2 . It includes one 2×2-3 dB polarization-maintained beam splitter  11 , two mirrors  13  and  15 , one polarization-maintained phase modulator  12  and one polarization-maintained delay line  14 . These two ports  9  and  10  which are in the same side of 3 dB beam splitter  11  are input and output ports of polarization-controlled encoder respectively. One of the two ports which are in the other side of 3 dB beam splitter  11  connects with polarization-maintained phase modulator  12  and mirror  13  in turn. The other port connects with polarization-maintained delay line  14  and mirror  15  in turn. A variation of above configuration is to connect one port of beam splitter with modulator  12 , delay line  14  and mirror in turn, and connect the other port of beam splitter with only a mirror. When the system works, an optical pulse is injected into port  9  of polarization-maintained beam splitter  11  and divided into two pulses. One pulse pass through the delay line  14  and reflected back by mirror  15 . The other pulse pass through the modulator  12  and reflected back by mirror  13 . Then they are recombined by beam splitter  11  and output from port  10 . Because all the paths are polarization-maintained, the output pulses have same polarization state. The variation of above configuration has same result. Either port  9  is input and port  10  is output or reverse have same result.  
       Third Embodiment  
       [0033]     The third configuration of polarization-controlled encoder in quantum key distribution system is shown in  FIG. 3 . It includes one 2×2-3 dB beam splitter  18 , two 90 degree Faraday mirrors  20  and  22 , one phase modulator  19  and one delay line  21 . The two ports  16  and  17  which are in the same side of 3 dB beam splitter  18  are input and output ports of polarization-controlled encoder respectively. One of the two ports which are in the other side of 3 dB beam splitter  18  connects with phase modulator  19  and 90 degree Faraday mirror  20  in turn. The other port connects with delay line  21  and 90 degree Faraday mirror  22  in turn. When the system works, an optical pulse is injected into port  16  of beam splitter  18  and divided into two pulses. One pulse pass through the delay line  21  and reflected back by 90 degree Faraday mirror  22 . The other pulse pass through the modulator  19  and reflected back by 90 degree Faraday mirror  20 . Then they are recombined by beam splitter  18  and output from port  17 . Because the two pulses are reflected by 90 degree Faraday mirrors and pass through their own path even times, the output pulses have same polarization state. Either port  17  is input and port  16  is output or reverse has same result.  
       Fourth Embodiment  
       [0034]     The configuration of a variation of the third embodiment is shown in  FIG. 4 . The difference from the third embodiment is one port of beam splitter connects with modulator  19 , delay line  21  (position of them can be exchanged) and 90 degree Faraday mirror  22  in turn, and the other port of beam splitter connects with a 90 degree Faraday mirror  20 . When the system works, an optical pulse is injected into port  16  of beam splitter  18  and divided into two pulses. One pulse pass through the phase modulator  19  and delay line  21  (no sequencing) and reflected back by 90 degree Faraday mirror  22 . The other pulse reflected back by 90 degree Faraday mirror  20 . Then they are recombined by beam splitter  18  and output from port  17 . Because the two pulses are reflected by 90 degree Faraday mirrors and pass through their own path even times, the output pulses have same polarization state. Either port  17  is input and port  16  is output or reverse has same result.  
       Fifth Embodiment  
       [0035]     The fourth configuration of polarization-controlled encoder in quantum key distribution system is shown in  FIG. 6 . It includes one 2×2 polarization-maintained variable beam splitter  25 , two mirrors  23  and  27 , one polarization-maintained phase modulator  24  and one polarization-maintained delay line  26 . One of the two ports which are in the same side of variable beam splitter  25  is output port  28  of polarization-controlled encoder, and the other port connects with delay line  26  and mirror  27  in turn. One of the two ports which are in the other side of variable beam splitter  25  is input port  29  of polarization-controlled encoder, and the other port connects with polarization-maintained phase modulator  24  and mirror  23  in turn. A variation of above configuration is to connect one port of beam splitter with delay line  26  and modulator  24  (no sequencing) in turn, and connect the other port of beam splitter with only a mirror. When the system works, an optical pulse is injected into port  29  of polarization-maintained variable beam splitter  25  and divided into two pulses. One pulse exits directly from port  28  of polarization-maintained variable beam splitter  25 . The other pulse pass through the delay line  26  and reflected back by mirror  27 , and then pass through beam splitter  25  again and continue passing through modulator  24  and reflected by mirror  23 , and then pass through the beam splitter  25  at third time and output from port  28 . When splitting the optical pulse, the splitting ratio of variable beam splitter is regulated to make the two pulses exit from port  28  have equal amplitude. Because all the paths are polarization-maintained, the output pulses have same polarization state. The variation of above configuration has same result. Either port  28  is input and port  29  is output or reverse has same result.  
       Sixth Embodiment  
       [0036]     The fifth configuration of polarization-controlled encoder in quantum key distribution system is shown in  FIG. 7 . It includes one 2×2 variable beam splitter  32 , two 90 degree Faraday mirrors  30  and  34 , one phase modulator  31  and one delay line  33 . One of the two ports which are in the same side of variable beam splitter  32  is output port  35  of polarization-controlled encoder, and the other port connects with delay line  33  and 90 degree Faraday mirror  34  in turn. One of the two ports which are in the other side of variable beam splitter  32  is input port  36  of polarization-controlled encoder, and the other port connects with phase modulator  31  and 90 degree Faraday mirror  30  in turn. A variation of above configuration is to connect one port of beam splitter with delay line  33  and modulator  31  (no sequencing) in turn, and connect the other port of beam splitter with only a 90 degree Faraday mirror. When the system works, an optical pulse is injected into port  36  of variable beam splitter  32  and divided into two pulses. One pulse outputs directly from port  35  of variable beam splitter  32 . The other pulse pass through the delay line  33  and reflected back by 90 degree Faraday mirror  34 , and then passes through beam splitter  32  again and continue passing through modulator  31  and reflected by 90 degree Faraday mirror  30 , and then passes through the beam splitter  32  at third time and exits from port  35 . When splitting the optical pulse, the splitting ratio of variable beam splitter is regulated to make the two pulses output from port  35  have equal amplitude. Because the two pulses are reflected by 90 degree Faraday mirrors and pass through their own path even times, the output pulses have same polarization state. The variation of above configuration has same result. Either port  35  is input and port  36  is output or reverse have same result.  
       Seventh Embodiment  
       [0037]     In above five configurations of polarization-controlled encoder, the phase modulator can be moved to output path to get the same result. For example, a polarization-controlled encoder with phase modulator in output path is shown in  FIG. 5 . It includes one 2×2-3 dB beam splitter  18 , two 90 degree Faraday mirrors  20  and  22 , one phase modulator  19  and one delay line  21 . Port  16  of the two ports which are in the same side of 3 dB beam splitter  18  is input port of polarization-controlled encoder, and the other port connects with modulator  19  as output port  17  of encoder. One of the two ports which are in the other side of 3 dB beam splitter  18  connects with 90 degree Faraday mirror  20 , and the other port connects with delay line  21  and 90 degree Faraday mirror  22  in turn. When the encoder is used in receiver, the modulator which is located in output path must be moved to input path. For example, modulator  19  in encoder  49 - 5  must connect with port  16  instead of port  17 . When the system works, an optical pulse is injected into port  16  of beam splitter  18  and divided into two pulses. One pulse pass through the delay line  21  and reflected back by 90 degree Faraday mirror  22 . The other pulse is reflected back by 90 degree Faraday mirror  20 . Then they are recombined by beam splitter  18 , pass through modulator  19  and exit from port  17 . Because these two pulses are reflected by 90 degree Faraday mirrors and pass through their own path even times, the output pulses have same polarization state. When this encoder is used in receiver, the modulator  19  which is located in output path must be moved to input path. Either in output path of transmitter or input path of receiver, the modulator need not be polarization-maintained.  
       Eighth Embodiment  
       [0038]     The return photon separating and detecting unit in the quantum key distribution system is made up of optical circulator  38  and single photon detector  37 . The input port  39  of optical circulator is the input port of return photon separating and detecting unit. The output port  40  of optical circulator is the output port of return photon separating and detecting unit. The return photon injected into port  40  will be separated by circulator  38  and detected by single photon detector  37 . The configuration of the unit is shown in  FIG. 8 . When the system works, photons injected into port  39  of circulator pass through the circulator directly and exit from port  40 . If there are photons injected into port  40 , they will be prevented from exiting from port  39  but guided to single photon detector  37  by circulator  38  in order to detect whether there are spy photons or not. When this unit is used in receiver, single photon detector  37  could detect signal photon to get useful key information.  
       Ninth Embodiment  
       [0039]     Since optical circulator and single photon detector have limited response spectrum, there may be photons whose wavelength is out of response spectrum enter the polarization-controlled encoder. A variation of the return photon separating and detecting unit is shown in  FIG. 9 . The difference from the above one is that an optical band pass filter  41  is added after input port  39 . When the system works, photons in response spectrum pass through the optical band pass filter freely, but other photons are blocked. This will improve the system&#39;s ability of resisting eavesdropping.  
       Tenth Embodiment  
       [0040]     A quantum key distribution system including the polarization-controlled encoders, return photon separating and detecting unit, single photon detectors and optical pulse source is shown in  FIG. 10 . The transmitter is made up of single photon source  42  (it can be replaced by simulative single photon source which is made up of laser and attenuator), polarization-controlled encoder  43  and return photon separating and detecting unit  44 . The input port of encoder  43  (it can be encoder  49 - 1 ,  49 - 2 ,  49 - 3 ,  49 - 6 ,  49 - 7  or any variation of them) connects with the output port of single photon source  42 . The output port of encoder  43  connects with the input port of return photon separating and detecting unit  44  (it can be return photon separating and detecting unit  50 - 1  or  50 - 2 ). The output port of unit  44  connects with quantum channel  45 .  
         [0041]     The receiver is made up of polarization-controlled encoder  47  (it can be encoder  49 - 1 ,  49 - 2 ,  49 - 3 ,  49 - 6 ,  49 - 7  or any variation of them, wherein  49 - 3  or  49 - 7  is better), return photon separating and detecting unit  46  (it can be return photon separating and detecting unit  50 - 1  or  50 - 2 ) and single photon detector  48 . Quantum channel  45  connects with the input port of unit  46 . The output port of unit  46  connects with the input port of encoder  47 . The output port of encoder  47  connects with single photon detector  48 .  
         [0042]     The process of quantum key distribution will be described hereinafter. One single photon pulse (it can be obtained by attenuating high power laser to at most one photon per pulse in fact) which is emitted from single photon source  42  enters polarization-controlled encoder  43 , and then divided into two pulses and delayed. One of these two pulses is modulated according to quantum key distribution protocol. The output two pulses pass through return photon separating and detecting unit  44 , and then enter quantum channel  45  to be transmitted to the receiver. When arrived at receiver, these two pulses pass through return photon separating and detecting unit  46  and enter polarization-controlled encoder  47 . After that, each pulse is divided into two pulses again and delayed. Then these pulses are modulated according to quantum key distribution protocol and interfere. One of output interfering pulses is sent to single photon detector  48  to be measured (in order to reduce the dark count and disturbance from un-interfering pulses, the detector should work at gate mode, and the control signal of gate could be obtained from classical signal which is sent by transmitter), and the other passes through the return photon separating and detecting unit and be measured. According to the result of measurement and record of modulation, transmitter and receiver will get one bit of key. Repeat above steps, they will get any bit of keys with unconditional security. Because there are polarization-maintained paths and 90 degree Faraday mirrors in polarization-controlled encoder, two output pulses have same polarization state. That makes the system has good ability of anti-jamming. When the input and output ports are the same in transmitter and receiver, circulator and Y beam splitter should be added to separate input and output signal. When the encoders in transmitter and receiver are  49 - 6  or  49 - 7 , splitting ratio of beam splitter should be regulated to make two output pulses have equal amplitude. This will reduce the error rate of final keys.