Abstract:
A system and a method for quantum key distribution between a transmitter and a receiver over wavelength division multiplexing (WDM) link are disclosed. The method includes providing one or more quantum channels and one or more conventional channels over the WDM link; assigning a different wavelength to each of the one or more quantum channels and each of the one or more conventional channels; transmitting single photon signals on each of the one or more quantum channels; and transmitting data on each of the one or more conventional channels. The data comprises either conventional data or trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. All channels have wavelengths around 1550 nm. The WDM link can be a 3-channel WDM link comprising two quantum channels for transmitting single photon signals and one conventional channel for transmitting conventional data or triggering signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a communication system and a method for communicating encrypted data. In particular, the present invention relates to the technique known as quantum key distribution over wavelength division multiplexing (WDM) links. 
     TECHNICAL BACKGROUND OF THE INVENTION 
     The purpose of cryptography is to exchange messages in perfect privacy between a transmitter and a receiver by using a secret random bit sequence known as a key. Once the key is established, subsequent messages can be transmitted safely over a conventional channel. For this reason, secure key distribution is a fundamental issue in cryptography. Unfortunately, the conventional cryptography provides no tools to guarantee the security of the key distribution because, in principle, classical signals can be monitored passively. The transmitter and receiver have no idea when the eavesdropping has taken place. 
     However, secure key distribution is possibly realized by using the technology of quantum key distribution (QKD). Quantum key distribution is believed to be a natural candidate to substitute conventional key distribution because it can provide ultimate security by the uncertainty principle of quantum mechanics, namely, any eavesdropping activities made by an eavesdropper will inevitably modify the quantum state of this system. Therefore, although an eavesdropper can get information out of a quantum channel by a measurement, the transmitter and the receiver will detect the eavesdropping and hence can change the key. 
     A variety of systems for carrying out QKD over an optical fiber system have been developed. Quantum cryptography has already been applied to the point-to-point distribution of quantum keys between two users. As shown in  FIG. 1 , quantum cryptography system in the prior art employs two distinct links. Of them, one is used for transmission of a quantum key by an optical fiber, while the other carries all data by internet or another optical fiber. 
     However, it is desirable to apply quantum cryptography in currently deployed commercial optical network. Yet only several studies on quantum key distribution over 1,300 nm network have been reported to date. One problem of the reported system is that it is difficult to transmit signals over a long distance at 1,300 nm in standard single mode fibers. Thus, quantum key distribution with wavelengths around 1,550 nm over the long distance is preferred. In addition, it is considered that no strong signals (e.g. conventional data) should exist in network with quantum channels or that a large spacing of wavelengths between a quantum channel and a conventional channel is needed to lower the interference from the strong signal. 
     However, this is not true in the installed commercial optical network because there are many strong signals that can cause severe interference to the quantum channel in the current optical fiber communications network employing WDM transmission. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention provides a communication system for quantum key distribution in which the quantum key distribution can be implemented in current commercial optical links by simply adding a wavelength for a quantum channel as quantum key distribution. 
     The present invention provides a method of quantum key distribution between a plurality of transmitting units and a plurality of receiving units over a wavelength division multiplexing (WDM) link, which comprises: 1) providing a plurality of WDM channels over the WDM link for coupling the transmitting units and the receiver units, respectively, the WDM channels comprising a plurality of quantum channels and a plurality of conventional channels; 2) assigning a different wavelength to each of the WDM channels; 3) transmitting single photon signals on each of the quantum channels; and 4) transmitting data on each of the conventional channels, the data comprising either conventional data or trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. 
     In preferred embodiments of the invention, the wavelengths assigned to the WDM channels are at around 1,550 nm. 
     The present invention further provides a communication system for quantum key distribution at wavelengths around 1,550 nm over a wavelength division multiplexing (WDM) optical link, which comprises a plurality of transmitting units comprising a plurality of quantum transmitting units and a plurality of conventional transmitting units; a plurality of receiving units comprising a plurality of quantum receiving units and a plurality of conventional receiving units; and a WDM link linking the transmitting units to the receiving units. Moreover, the WDM link comprises a plurality of WDM channels, and the WDM channels may further comprise a plurality of quantum channels for communicating single photon signals between the quantum transmitting units and the quantum receiving units, respectively; and a plurality of conventional channels for communicating data between the conventional transmitting units and the conventional receiving units, respectively. 
     In some embodiments of the invention, the data transmitted on the conventional channels comprises either conventional data or trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. Furthermore, each of the WDM channels is assigned a wavelength different from others so that the WDM channels are multiplexed in wavelengths over the WDM link. 
     According to an aspect of the present invention, it is possible to realize quantum key distribution between specific users (e.g. between a transmitter and a receiver) over a WDM link by using WDM technology. The transmitter may comprise one or more quantum transmitting units and one or more conventional transmitting units, the receiver may comprise one or more quantum receiving units corresponding to the one or more quantum transmitting units, respectively, and one or more conventional receiving units corresponding to the one or more conventional transmitting units, respectively. Moreover, the WDM link linking the transmitter and the receiver may comprise one or more quantum channels for communicating single photon signals between the one or more quantum transmitting units and the one or more quantum receiving units, respectively, and one or more conventional channels for communicating data between the one or more conventional transmitting units and the one or more conventional receiving units, and the data comprising either conventional data or trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. Furthermore, each of the conventional channels and the quantum channels may be assigned a wavelength different from others so that the conventional channels and the quantum channels can be multiplexed in wavelengths over the WDM link. 
     According to another aspect of the present invention, the WDM link of the communication system may be a 3-channel WDM link, which comprises two quantum channels and a conventional channel. The data transmitted over the conventional channel may include trigger signals for synchronizing the quantum channels. Thus, the conventional channel can also serve as a trigger channel to synchronize the system. Each of the conventional channels and the quantum channels is assigned a wavelength different from others, and the conventional channel and the quantum channels are multiplexed by wavelength at around 1,550 nm over the WDM link, which is suitable for long-haul transmission. 
     Based on the WDM technology which combines many different wavelengths into a single optical fiber provided by the WDM link, the quantum key distribution is easily conducted in the current commercial fiber links by sharing a common fiber with conventional communication signals. 
     Moreover, a differential phase modulation technology is employed in the present invention to overcome an influence of temperature shifts and phase shifts on the system, which also makes the system stable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a communication system for quantum key distribution in the prior art. 
         FIG. 2  shows a schematic view of a communication system for quantum key distribution over a multi-user WDM network according to the present invention. 
         FIG. 3  shows a schematic view of quantum key distribution over a WDM link according to the present invention. 
         FIG. 4  shows a schematic view of an embodiment of quantum key distribution over a 3-channel WDM link according to the present invention. 
         FIG. 5  shows an auto-compensation structure using a differential phase modulation technology employed in a quantum channel of the present invention. 
         FIGS. 6   a  and  6   b  show a detailed structure of the quantum key distribution over the 3-channel WDM link as shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail with reference to the drawings. 
     WDM is the key technology adopted in the present invention, which makes use of the parallel property of light to combine many different wavelengths into a single optical fiber. Thus it is possible to fulfill quantum key distribution over multi-user WDM network according to the present invention. By virtue of WDM, the system can establish simultaneously as many distinct secret keys as allowed by the number of wavelengths supported by the WDM network. 
     For example, a communication system for quantum key distribution over multi-user WDM network according to one embodiment of the present invention is shown in  FIG. 2 . The communication system includes N quantum channels assigned with wavelengths from λ 1  to λ N  for linking N quantum transmitting units  130  and N quantum receiving units  140  over a WDM link, and M conventional channels assigned with wavelengths from λ N+1  to λ N+M  for linking M conventional transmitting units  330  and M conventional receiving units  340  over the WDM link (where N and M are positive integers). The WDM link comprises array waveguide gratings (AWG)  402  and  401  and a single optical fiber  500 . In the embodiment, the quantum channels and the conventional channels with distinct wavelengths (from λ 1  to λ N+M ) are multiplexed into the single optical fiber  500  by using the AWG  401  and the AWG  402 . Thus, it is possible to realize quantum key distribution between specific quantum transmitting units and quantum receiving units by using WDM technology. 
       FIG. 3  shows an embodiment of quantum key distribution between specific users (e.g. between a transmitter and an intended receiver) among a plurality of users over a WDM link according to the present invention. As shown in the  FIG. 3 , the transmitter  711  has one or more quantum transmitting units and one or more conventional transmitting units, and the receiver  721  has one or more quantum receiving units, each of which corresponds to one of the one or more quantum transmitting units, respectively, and one or more conventional receiving units, each of which corresponds to one of the one or more conventional transmitting units. 
     The WDM link, linking the transmitter  711  and the receiver  721 , comprises an AWG  401 , an optical fiber  501  and another AWG  402 . The WDM link is provided for multiplexing one or more quantum channels between the quantum transmitting units and the corresponding quantum receiving units for communicating single photon signals, and one or more conventional channels between the conventional transmitting units and the conventional receiving units for communicating data. In the embodiment, the data further includes trigger signals for synchronizing the transmission of the single photon signals on the quantum channels. Moreover, each of the conventional channels and the quantum channels is assigned a wavelength different from others, and the conventional channels and the quantum channels are multiplexed by wavelengths at around 1,550 nm over the WDM link. 
       FIG. 4  shows an embodiment of quantum key distribution between a transmitter and a receiver over a 3-channel WDM link. The transmitter  712  comprises a first quantum transmitting unit  110 , a second quantum transmitting unit  210 , and a conventional transmitting unit  310 . The receiver  722  includes a first quantum receiving unit  120  corresponding to the first quantum transmitting unit  110 , a second quantum receiving unit  220  corresponding to the second quantum transmitting unit  210 , and a conventional receiving unit  320  corresponding to the conventional transmitting unit  310 . The 3-channel WDM link comprises an AWG  401 , an optical fiber  502  and another AWG  402 , for multiplexing two quantum channels  100  and  200  and one conventional channel  300 . The quantum channels  100  and  200  is provided between the two quantum transmitting units  110  and  210  and the two quantum receiving units  120  and  220  for transmitting single photon signals (quantum keys), respectively. The conventional channel  300  is provided between the conventional transmitting unit  310  and the conventional receiving unit  320  for transmitting data. In the embodiment, the data includes trigger signals S 1  which is transmitted to the quantum transmitting units  110  and  210 , and trigger signals S 2  which is transmitted to the quantum receiving units  120  and  220 , so as to synchronize the transmission of the single photon signals on the quantum channels  100  and  200 . The two quantum channels  100  and  200  and the conventional channel  300  are assigned with different wavelengths, λ 1 , λ 2  and λ 3 , respectively, at around 1,550 nm which is compatible with the standard optical links. 
     In this manner, quantum key distribution can be conveniently implemented in the current commercial optical links by simply adding another wavelength thereto for the quantum channel as quantum key distribution. Furthermore, at the optical wavelength of 1,550 nm, the fiber losses are 0.2 dB/km, which translates into a large increase in transmission distance when compared with that at 1,300 nm at the same bit rate for a quantum cryptographic system. 
     The BB84 protocol can be employed in the quantum channels  100  and  200 . In order to implement BB84 protocol, there must be four states in two non-orthogonal bases, each of which has two orthogonal states. For example, the four phases {0, π/2, π or 3π/2} can play the role of the four states. Moreover, {0, π} corresponds to one basis that can be realized via choosing measurement basis phase shift 0. Similarly, {π/2, 3π/2} is the other basis that corresponds to measurement choice of phase shift π/2. The four states can be expressed in the following,
 
For “0”, |‘0’           =1/√{square root over (2)}(|0         +|π/2         )
 
For “1”, |‘1’         =1/√{square root over (2)}(|π         +|3π/2         )

     From the wave functions, it is obvious that there is equal probability of 50% for phase shift 0 and π/2, respectively, for logic 0. So is logic 1. 
     An auto-compensation structure using a differential phase modulation technology is employed in the quantum channels of the present invention. As shown in  FIG. 5 , for example, in a quantum channel  100  (which is similar to a quantum channel  200 ), at the transmitter  712 , a phase shift, ΔA, provided by a phase modulator  112 , is added to a first pulse in two neighboring pulses both of which travel from the receiver  722  to the transmitter  712 . Another phase shift, ΔB, provided by a phase modulator  122 , at the receiver  722  will be also added to a second pulse when both the pulses return to the receiver side after being reflected by a Faraday rotating mirror  111 . When the first pulse and the second pulse delayed by a delay means  127  arrive at a beam splitter  123 , interference will happen, and the phase difference will be ΔA−ΔB. Therefore, only the phase difference has been retained. This arrangement enables the structure to compensate errors caused by temperature shifts, polarization changes and path variations experienced by the two pulses traveling in the interference section, because each of the two pulses, which will interfere at the receiver side of each quantum channel, experiences the same variations while traveling the same distance. Here we assume that another phase shift, δ, caused by the temperature shift, polarization variation and distance variations, is put on both of the pulses in the same channel. 
     The phase shift, δ, often changes at a different time for the variation by the factors mentioned above. However, it is nearly equal for the two neighboring pulses because they experience similar changes in the channel as those factors mentioned above vary relatively slowly within the time separation between the two neighboring pulses. For the first pulse, it has a phase shift, ΔA+δ, but there is a phase shift, δ+ΔB, for the second pulse. Hence, in the interfering section at the receiver side, the phase difference between the two returning pulses is ΔA−ΔB because the phase shift, δ, caused by the factors mentioned above will have been cancelled. Since the quantum channel  200  is similar to the quantum channel  100 , the scheme of the quantum channels  100  and  200  of the present invention can overcome fluctuations caused by temperature, polarization and distance variations. Theoretically, it can obtain perfect interference in the scheme. 
     A detailed structure and principles of the quantum key distribution over a 3-channel WDM link are described with reference to  FIGS. 6   a  and  6   b.    
     In the quantum channel  100 , at the receiver  722 , a laser  124  launches a pulse string with power of 0 dBm into the WDM link via a circulator  125 . Each pulse in the pulse string will be split into two pulses through a 50/50 beam splitter  123 , a first pulse and a second pulse. The first pulse passes through an upper path  1231  with a delay of 26 ns set by a delay means  127  (e.g. a delay line of an optical fiber) before hitting a polarization beam splitter  121 . A phase modulator  122  in the upper path  1231  is not used until a second pulse returns from the transmitter. A second pulse passes through a lower (shorter) path  1232  directly to the input port of the splitter  121 . 
     After passing through the splitter  121 , the two pulses with orthogonal polarizations and a delay of 26 ns between them are obtained. These two pulses then enter into an array waveguide grating (AWG)  402 , propagate through a single-mode fiber  502  of e.g. 8.5 km, enter into another array waveguide grating (AWG)  401 , and then exit from the AWG  401  in channel  100  at the transmitter  712 . 
     The pulses are again split by a 90/10 beam splitter  115 , and the photons coming out from the 90 percent port of the splitter  115  are detected by a detector  113  for controlling a variable attenuator  114  to attenuate the returning pulses to obtain single-photon pulses. The two pulses coming out of the 10% port of the splitter  115  will pass through the attenuator  114  first without attenuation. They will then arrive at a Faraday mirror  111  through a phase modulator  112 . The polarizations of the two pulses are rotated by 90° after they are reflected by the Faraday rotating mirror  111 . 
     A random phase shift of 0, π/2, π or 3π/2 generated by a random data signal generator (not shown) is then inserted into the first of the two return pulses by the phase modulator  112 . The two return pulses are next attenuated to yield a single photon within a pulse when they pass through the attenuator  114  again. A trigger signal S 1  generated from a detector  313  is used to synchronize with the phase modulator  112  to modulate the first return pulse from the Faraday mirror  111  and with an attenuation control signal from the detector  113  to attenuate both return pulses into single photons. Here the trigger signal S 1  from the detector  313  should have an appropriate delay to synchronize the phase shift single from the data signal generator with the first return pulse. Also, the signal from detector  113  used to control attenuator  114  has an electrical delay in order to attenuate both light pulses when they pass through it in their return trip. Finally the two pulses return to the receiver  722  via opposite paths between the polarization beam splitter  121  and the beam splitter  123  after passing through the AWG  401 , the 8.5 km standard single mode fiber  502  and the AWG  402 . Hence, they can arrive at the beam splitter  123  at the same time and generate constructive or destructive interference at the beam splitter  123  to enable single photons to be detected by a single-photon detector  126 . 
     The receiver  722  can randomly and independently select a measurement basis through setting a phase shift of 0 or π/2 in the phase modulator  122 , which is synchronized by a trigger signal S 2  derived from the pulse returning from the mirror  311  in the conventional data channel  300 . The outcomes are stored in a computer  600 . All fibers on receiver&#39;s side are polarization-maintaining fibers, which is necessary for the system to guarantee the polarizations of the two single photon pulses that will interfere are invariant after passing through the different paths of the interferometer. 
     A second quantum channel  200 , similar to the quantum channel  100 , comprises a Faraday mirror  211 , a phase modulator  212 , a detector  213 , a variable attenuator  214 , a 90/10 beam splitter  215 , an AWG  401 , a fiber  500 , an AWG  402 , a polarization beam splitter  221 , a phase modulator  222 , a beam splitter  223 , a laser  224 , a circulator  225 , a single photon detector  226  and a delay means  227 . For the reason that the configuration and principles of channel  200  is similar to the quantum channel  100 , except that a time delay set by the delay means  227  of the quantum channel  200  is 21 ns and an independent measurement basis and random phase shifts that are independent of channel  100 , the detailed description of the quantum channel  200  is omitted. 
     In the conventional channel  300 , a common laser  324  emits a pulse with the power of 2 dBm into a 50/50 beam splitter  321 , on receiver&#39;s side. The pulse then enters AWG  402  after passing the 50/50 beam splitter  321 , travels in the 8.5 km single-mode fiber  500  and then through AWG  401 , after which one-half of the pulse will be detected by a detector  313 . The detected signal is used as a first trigger signal S 1  to synchronize the phase modulators  112  and  212  with their respective pulses in quantum channels  100  and  200  through appropriate delays. The other half of the pulse will be reflected by a mirror  311  to return to the receiver, and will be detected by a detector  326  to generate a second trigger signal S 2  to trigger the single photon detectors  126  and  226  to measure the interference of the quantum signals and to trigger the phase modulators  122  and  222  to select a measurement basis on the receiver&#39;s side, respectively. 
     The data communication channel  300  may also function as a regular optical communication channel which has high laser powers, e.g., 2 dBm emitted by the laser  324  in this embodiment. The wavelengths and the pulse widths of the three channels are listed in Table 1. 
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 wavelength and pulse width 
               
             
          
           
               
                   
                 Channel 
               
             
          
           
               
                   
                 100 
                 200 
                 300 
               
               
                   
                   
               
             
          
           
               
                   
                 Wavelength (nm) 
                 1549.33 
                 1551.18 
                 1557.35 
               
               
                   
                 Pulse width (ns) 
                 2.5 
                 2.5 
                 20 
               
               
                   
                   
               
             
          
         
       
     
     BB84 protocol has been implemented in this system. We use 100 kHz signals for phase modulation and synchronization. The pulse widths are 2.5 ns for quantum channels  100  and  200 , and 20 ns for conventional channel  300 , shown in Table 1. In order to reduce the crosstalk among the channels, especially between weak quantum channels  100  and  200 , and the strong signal channel (the conventional channel  300 ), the wavelengths have to be arranged carefully. Here the spacing between quantum channel  100  and conventional channel  300  is about 8 nm, and that between quantum channel  200  and conventional channel  300  is about 6 nm. 
     The single photon detectors  126  and  226  are employed in the embodiment to measure the single photons. The dark count of the single photon detectors  126  and  226  is 40 Hz in the gated mode of 100 kHz with a measurement width of 2.5 ns, so the probability of measuring the dark count is 4.0×10 −4 . The efficiency of the single photon detectors  126  and  226  is more than 10%. On the transmitter&#39;s side, the average photon count per pulse should be less than 0.1 in order to guarantee that a single photon is obtained in each pulse in the embodiment when the pulse passes through the variable attenuator  114  again. For an overall transmission loss of 17 dB, about 2% of single photons can be detected. On considering the 3 dB loss due to BB84 protocol, about 1% of single photons can be used for quantum key distribution theoretically. 
     
       
         
               
             
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Experimental Results 
               
             
          
           
               
                   
                 Channel 
                   
               
             
          
           
               
                   
                 100 
                 200 
               
               
                   
                   
               
             
          
           
               
                   
                 Key rate (kb/s) 
                 0.75 
                 0.49 
               
               
                   
                 Error probability (%) 
                 2.2 
                 4.396 
               
               
                   
                   
               
             
          
         
       
     
     Experimentally, the count rate of the single photon detector  126  and  226  is 100 k counts/s and its efficiency is above 10%. In order to guarantee single photon in a pulse, the average photon count per pulse should be below 0.1 in the embodiment of the present invention. Therefore, count rate should be below 10 k/s at variable attenuators  114  and  214 . According to the embodiment, the experimental count rate obtained is 7.67 k/s. After considering the transmission efficiency, error rate and detector efficiency, a 0.75 kbps quantum key has been obtained in channel  100 , where the crosstalk causes an error probability of 2.2 percent, mostly derived from channel  300  and much less from channel  200  because the single photon signal in channel  200  is very weak. Similarly, in channel  200 , the quantum key rate is 0.49 kbps and the crosstalk also causes an error probability of 4.396%. The crosstalk in channel  200  is larger than that in channel  100  because its wavelength is closer to that of the conventional communication channel than is the wavelength of channel  100 . 
     While this invention has been described in conjunction with a few embodiments thereof, it will be understood for those skilled in the art to put this invention into practice in various other manners. It is appreciated that the scope of the invention is defined by the appended claims and should not be restricted by the description discussed in the summary and/or the detailed description of the preferred embodiments.