Abstract:
A system and a method for quantum key distribution over a multi-user wavelength division multiplexing (WDM) network are disclosed. The system comprises a tunable or multi-wavelength transmitter; a plurality of receivers, each assigned a receiving-wavelength; and a multi-user WDM network linking the transmitter to the receivers. The transmitter can select a receiver among the receivers to be communicated therewith and transmit quantum signals to the selected receiver over the WDM network. The quantum signals are at a wavelength equal to a receiving-wavelength of the receiver. Therefore the WDM network allows quantum signals to be communicated between the transmitter and the receivers by wavelength routing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for communicating encrypted data. In particular, the present invention relates to the technique known as quantum key distribution over multi-user wavelength division multiplexing (WDM) network with wavelength routing. 
     TECHNICAL BACKGROUND OF THE INVENTION 
     Quantum cryptography is believed to be a natural candidate to enhance conventional cryptographies because it can provide ultimate security by the laws of quantum theory. Most of research in this field is centered on point-to-point transmission between two users. At present, quantum cryptography has been successfully achieved in a point-to-point link in optical fiber and free space. However, there are limited achievements on quantum key distribution over network to date. There exist more problems for quantum key distribution over network than that over point-to-point transmission. In fact, it has been thought that it is a difficult problem to distribute quantum keys over network. 
       FIG. 1  shows a conventional configuration of quantum key distribution over a star network, which exploits four phase shifts of weak pulse strings based on BB84 protocol at transmitter and receiver. In this setup, a transmitter (Tx) and receivers (Rx 1 -Rx 3 ) use a phase modulator to encode and decode the phase shifts, and the transmitter launches a 3-photon pulse with a phase shift randomly chosen from four phases, (e.g. 0, π/2, π and 3π/2) into the fiber. The pulse is then equally split among the 3 receivers. For measurement, each receiver needs synchronization with the sent pulse. In addition, the setup cannot identify which user should receive the signal because all users in the depicted network can simultaneously receive signals from the sender even if she or he is not the intended receiver. That is to say, this system cannot establish a link just between two specific users to implement quantum key distribution. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention provides a communication system for quantum key distribution, in which a transmitter can communicate over a conventional optical communications network with a plurality of receivers by using a different secret key sent at a different wavelength for each different receiver. 
     The present invention also provides a communication system for quantum key distribution with a relatively simple structure and high communication efficiency. 
     The present invention provides a method of quantum key distribution between a transmitter and a plurality of receivers over a multi-user wavelength division multiplexing (WDM) network with wavelength routing which comprises: 1) assigning a different receiving-wavelength to each of the receivers, respectively; 2) selecting a receiver among the receivers to be communicated with the transmitter; and 3) transmitting quantum key signals from the transmitter to the selected receiver over the WDM network, wherein the quantum key signals are at a wavelength identical to the receiving-wavelength of the receiver. 
     The present invention further provides a communication system for quantum key distribution comprising a transmitter; a plurality of receivers, each having a distinct receiving-wavelength; and a multi-user WDM network linking the transmitter to the receivers, wherein the transmitter selects a receiver among the receivers to be communicated therewith and transmits quantum signals to the selected receiver over the WDM network. The quantum signals transmitted are at a wavelength equal to the receiving-wavelength of the receiver. 
     According to an aspect of the present invention, a wavelength routing technology is employed to implement quantum key distribution for a plurality of receivers. The wavelength routing can be realized by a wavelength division demultiplexer, which may be in the form of an array waveguide grating (AWG). Moreover, the system can use all-fiber connections, which is suitable for optical fiber network. 
     According to another aspect of the present invention, continuous wave light is employed in the system, which can improve the security of the system. Moreover, differential phase detection is also employed in the present invention, in order to overcome the influence of a temperature shift and phase shift in the system, which can further make the system simple and stable. Furthermore, the present invention employs a randomly phase-modulated light of weak coherent states, e.g. two non-orthogonal states with phase shifts 0 and π, which can improve the communication efficiency of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing the principle of quantum key distribution over star network in the prior art; 
         FIG. 2  is a schematic view showing principles of a wavelength routing technology employed in a communication system for quantum key distribution over a multi-user WDM network according to the present invention; 
         FIG. 3  is a schematic view of a communication system according to the present invention, which illustrates a structure of a channel between a transmitter and a receiver; and 
         FIG. 4  shows an experimental result of the key rate and crosstalk of the communication between the transmitter and 8 different receivers over an 8-user WDM network according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail with reference to the drawings. 
       FIG. 2  shows an embodiment of the communication system for quantum key distribution over a multi-user WDM network according to the present invention. As shown in  FIG. 2 , the communication system comprises: a transmitter  100 , a plurality of receivers  200  (8 receivers in this embodiment), each assigned a different receiving-wavelength (e.g. λ1-λn, where n=8), and an 8-user WDM network linking the transmitter  100  to the receivers  200 . 
     Wavelength routing technology is used to implement quantum key distribution among specific users in the invention. In the embodiment, the wavelength routing can be realized by a wavelength division demultiplexer  400 , such as an array waveguide grating (AWG). Therefore, the transmitter  100  can choose a wavelength to establish a channel (e.g. from channel 1 to channel n) with each receiver so that the transmitter  100  does not need to send signals to all the receivers. For example, if a receiver  210  having a receiving-wavelength, λ 1 , is selected to communicate with the transmitter  100 , the transmitter will transmit quantum signals with a wavelength equal to λ 1 . The signals, after passing through an optical fiber  300 , reach the array waveguide grating  400 , which can route the quantum signals to the receiver  210  only. In this manner, the transmitter  100  can communicate the quantum signals with only one selected receiver. By tuning the transmitter wavelength or selecting an appropriate wavelength of a multi-wavelength transmitter, one single transmitter can transmit quantum signals to each of the receivers  200  by using WDM technology. Receiving-wavelengths that can be assigned to each of the receivers in this embodiment are listed in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Receiving-Wavelength 
                 Percentage of 
                 Arrival Counts 
               
               
                 Receivers 
                 FWHM(nm) 
                 Used Gates 
                 (s −1 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                  1549.32 (0.018) 
                 8-9% 
                 558.305 
               
               
                 2 
                  1551.12 (0.016) 
                 7-8% 
                 477.7992 
               
               
                 3 
                  1552.4 (0.017) 
                 4-5% 
                 316.36 
               
               
                 4 
                 1554.145 (0.017) 
                 6-7% 
                 422.407 
               
               
                 5 
                 1555.689 (0.017) 
                 4-5% 
                 303.2212 
               
               
                 6 
                 1557.375 (0.017) 
                 3-4% 
                 245.2329 
               
               
                 7 
                 1558.996 (0.018) 
                 4-5% 
                 332.095 
               
               
                 8 
                 1560.615 (0.016) 
                 2-3% 
                 207.4174 
               
               
                   
               
             
          
         
       
     
     Now referring to  FIG. 3 , a configuration for transmitting quantum signals between a transmitter and an intended (selected) receiver over a multi-user WDM network according to the present invention is described. 
     As shown in  FIG. 3 , when a receiver  210  is selected to communicate with the transmitter  100 , a tunable laser  101  at the transmitter  100  emits a continuous wave (CW) light whose wavelength corresponds to the receiving-wavelength of the receiver  210 , into a phase modulator  102 . A random phase shift of 0 or π generated by a random data signal generator  104  is added to the CW light. Then the CW light with a random phase shift is attenuated to single photons, with an average photon number less than one within a measured gate period, at the exit of a variable optical attenuator  103 , which is coupled into a 8.5 km standard single mode optical fiber  300 . After that, the attenuated light signal is sent to the AWG  400 , to determine which user is selected via wavelength routing. The AWG  400  provides a plurality of output ports  401 - 40   n , each having a distinct central wavelength and a bandwidth that corresponds to the receiving-wavelength of each of the receivers. And then, through the AWG  400 , the attenuated signals arrive at the selected receiver  210  corresponding to the receiving-wavelength. 
     The receiver  210  provides an asymmetric Mach-Zehnder interferometer  218  to reconstruct a phase shift introduced by the transmitter  100 . Preferably, the asymmetric Mach-Zehnder interferometer  218  comprises a first 50/50 beam splitter  211 , a long arm  212 , a short arm  213 , and a second 50/50 beam splitter  214 . The beam splitter  211  is employed for splitting the incoming signals into two portions respectively entering the long arm  212  and the short arm  213 . The two split light signals are recombined by the beam splitter  214 , in which the time difference between the two arms  212  and  213  is set equal to a time interval of a phase modulation period. That is, the random data signal generator  104  of the transmitter  100  can synchronize the phase modulator  102  to modulate the light with the time interval of the phase modulation period equal to the time difference experienced by the light while traveling across the two arms. With the Mach-Zehnder interferometer  218 , an interference between the photons of the two arms occurs. The receiver  210  can detect single photons created by the constructively interfered signal by a single photon detector module  219  comprising two single photon detectors  215  and  216  respectively connected to two outputs of the beam splitter  214 . The detector module works in gated mode with 2.5 ns and 100 KHz. The data can be stored in a computer via data capture software. Moreover a time slot measurement device  217  can be provided in the single photon detector module  219  for measuring the time slots at which a photon is detected at the detectors. 
     After raw key transmission, the receiver  210  tells the transmitter  100  the time slots measured. From this time message and the modulation state of the photons, the transmitter knows which detector clicked in the receiver. Under an agreement that the click by the detector  215  denotes “0” and the click by the detector  216  denotes “1”, for example, the transmitter  100  and the receiver  210  will obtain an identical quantum key. 
     In this way, the transmitter  100  can choose a wavelength to establish a channel, channel 1, with the receiver  210  so that the transmitter  100  does not need to send the signal to the other receivers. 
     Moreover, since all the other receivers have the same configuration as the receiver  210 , except that the receiver-wavelength thereof is different, the transmitter  100  can communicate with any of the single receivers in a similar way described above. 
     The efficiency of the single photon detector module  219  is greater than 10%, and thus the count rate should be less than 10 KHz in order to guarantee a single photon in a measured time slot. To achieve a better performance, the laser of the present invention is a narrow band laser source, so that the light emitted from the laser has a bandwidth narrower than the bandwidth of each of the output ports (from output port  401  to  40   n ) of the array waveguide grating. According to the embodiment of the present invention, the laser  101  can be tuned from 1,475 nm to 1,600 nm. The receiving-wavelengths used in the embodiment are listed in Table 1. Lastly, the single photon signals have been measured in each channel and crosstalks due to other channels are also detected. The count rate of the experiment is less than 1×10 4  counts/s, which corresponds to less than 0.1 count in a measured slot at the transmitter, in order to guarantee a single photon in the modulation time slot. After about 12 dB transmission loss, there are about 6% single photons to arrive at a receiver. The percentage of the used gates in the receivers  200  is shown in Table 1, and the count rates of single photons arriving at the receivers  200  from the transmitter  100  are also shown in Table 1. The error rte in each channel of the 8-user network is listed in Table 2. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Channels 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
                   
               
             
          
           
               
                 Error Rates (%) 
                 1.93 
                 2.15 
                 2.75 
                 3.19 
                 2.76 
                 2.73 
                 2.24 
                 6.26 
               
               
                   
               
             
          
         
       
     
     This system can use all-fiber connections, which is suitable to optical fiber networks. The quantum key signal of the system is carried by the phase difference between two sequential phases. 
     An advantage to adopt differential phase detection is to overcome the influence of the temperature shift and phase shift in the system, which also makes the system simple. Another advantage is of high communication efficiency. In the previous schemes, at least two measurement bases are necessary when detection is done at receivers. In principle, there are only 50 percent of outcomes that are correct. Therefore, it will make a 3 dB loss, i.e. BB84 protocol has been used. However, one can use all of the measured outcomes in the present system. 
     Continuous wave light is employed in the system of the invention, which can improve the security of the system. Compared with a pulsed light, an eavesdropper, Eve, cannot measure the period of the phase modulation. Therefore, Eve cannot know the detailed information of the interferometer. In fact, single pulses will lead to leaking more information (interferometer structure parameter, which is very important to form quantum keys) to eavesdropper. If continuous light is adopted, then eavesdropper cannot know the difference between the two arms of the interferometer, which further improves the security of this system. 
     Compared with other automatic compensation schemes, the construct according to the invention has lower noise because there is no return signals (which can cause interference) in the current structure. That is also another advantage. 
       FIG. 4  shows that experimental key rates for the 8 channels vary from about 2 kb/s to more than 5 kb/s, which are shown in black diamond. Other dots in  FIG. 4  show the crosstalks caused by the referred channels. The crosstalk due to channel 1 is the largest because of its highest single photon rate. For security, there is an upper bound for the loss in the system, which includes fiber transmission loss, component insertion loss, loss from wavelength routing and some loss caused by imperfect alignment. The total loss of about 12 dB is much less than the secure upper bound of 31 dB for the mean photon number per bit of 0.1. The crosstalk is mainly caused by the wavelength demultiplexing device, AWG, and laser source. Form  FIG. 4 , it is obvious that quantum key distribution in the present architecture is feasible because the cont rates caused by crosstalk and dark counts are very small compared to single photon signal counts. 
     Therefore, quantum key distribution over multi-user network using wavelength routing is achieved experimentally, which overcomes the broadcasting problem of the tree network. Furthermore, a differential phase modulation is applied to continuous wave light, which can eliminate the variations caused by temperature and polarization fluctuations in the system. Moreover, the higher key generation efficiency with a simple configuration demonstrated in this disclosure is suitable to practical applications. 
     Although, for ease of illustration, only 8 receivers are described in the embodiments, it would be obvious to those skilled in the art that smaller or greater numbers of the receivers may be employed in practice networks. The number chosen can be varied according to the field of use. 
     It is appreciated that the scope of the invention should be defined by the appended claims and not be restricted by the description discussed in the summary and/or the detailed description of the preferred embodiments.