Abstract:
A QKD system having QKD link redundancy between two sites, with the system having only one QKD station at each site, and with two or more QKD links operably coupled to the QKD stations. The QKD stations have respective optical switches that are optically coupled to both QKD links and that are controlled by respective controllers in each of the QKD stations. If one of the QKD links fails or has trouble transmitting optical signals, the QKD switches are switched so that the optical path between the QKD stations uses the remaining QKD link. This arrangement requires only two QKD stations rather than the four QKD stations as presently taught in the prior art.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of priority under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application Ser. No. 60/880,975, filed on Jan. 18, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to quantum key distribution (QKD), and in particular relates to systems and methods for providing communication link redundancy between QKD stations of a QKD system without having to add additional QKD stations. 
       BACKGROUND ART 
       [0003]    QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. Consequently, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits introduces errors that reveal her presence. 
         [0004]    The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett (which patent is incorporated herein by reference), and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. 
         [0005]    The simplest form of QKD system for providing encrypted communication between two different sites has a first QKD station Alice at the first site and a second QKD station Bob at the second site. Alice and Bob are operably coupled to one another by a single optical fiber link. 
         [0006]    thas been proposed that doubling the encryption bandwidth while also providing redundancy between the sites can be achieved by providing two Alices (Alice  1  and Alice  2 ) at the first site and two Bobs (Bob  1  and Bob  2 ) at the second site. A first communication link connects Alice  1  and Bob  1  (the first QKD station pair) and a second communication link connects Alice  2  and Bob  2  (the second QKD station pair) Thus, if one of the communication links fail, the QKD station pair and its corresponding link provides redundancy. However, this approach is expensive because it requires a total of four QKD stations. 
       SUMMARY OF THE INVENTION 
       [0007]    One aspect of the invention is to provide a QKD system having QKD link redundancy between two sites by providing two QKD links operably coupled to a single transmitting QKD station Alice and a single receiving QKD station Bob. Alice and Bob are optically coupled to respective optical switches that are also optically coupled to both QKD links. The QKD switches are adapted to switch between the QKD links so that optical communication between Alice and Bob is maintained even if one of the QKD links fails. This arrangement requires only two QKD stations rather than the four QKD stations as presently taught in the prior art. 
         [0008]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is schematic diagram of a QKD system having a first QKD station Alice at a first site (Site A) and a second QKD station at a second site (Site B), with the two QKD stations optically coupled by two communication links; 
           [0011]      FIG. 2  is a close-up schematic diagram of an example embodiment of the QKD station Alice of the QKD system of  FIG. 1 ; and 
           [0012]      FIG. 3  is a close-up schematic diagram of an example embodiment of the QKD station Bob of  FIG. 1 . 
       
    
    
       [0013]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. Whenever possible, the same reference numbers or letters are used throughout the drawings to refer to the same or like parts. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]      FIG. 1  is schematic diagram of a QKD system  10  having a first transmitting QKD station Alice at a first site (Site A) and a second receiving QKD station Bob at a second site (Site B), with the two QKD stations optically coupled by two communication links (“links”) L 1  and L 2 . For the purposes of discussion herein, link L 1  is considered the “primary” link and link L 2  is considered the “secondary” QKD link. In an example embodiment of the present invention, one or both of links L 1  and L 2  are or include optical fibers. In another example embodiment, links L 1  and L 2  are free-space links. 
       Alice 
       [0015]      FIG. 2  is a close-up schematic diagram of an example embodiment of the QKD station Alice of QKD system  10  of  FIG. 1 . Alice includes a light source  12 A adapted to generate either single photons or weak photon pulses P 0 . An encoding optical system  20 A having an input end  22 A and an output end  23 A is optically coupled to light source  12 A at input end  22 A. Encoding optical system  20 A is adapted to form encoded (e.g., phase- or polarization-encoded) single-photon-level light pulses P 1  from incoming light pulses P 0 . In an example embodiment, encoding optical system  20 A is or includes an interferometer loop such as those used in the aforementioned U.S. patent to Bennett. In the example embodiment shown in  FIG. 2 , encoding optical system  20 A generates two coherent pulses P 1  from each initial pulse P 0 , and encodes one of the pulses P 1  to form an encoded pulse, indicated as P 1 ′. In an example embodiment, encoding optical system  20 A includes a modulator (not shown), such as a polarization modulator or a phase modulator. 
         [0016]    Alice also includes an optical switch  30 A that has an input port  31 A and two output ports  32 A and  34 A. Optical switch  30 A is optically coupled to output port  23 A of encoding optical system  20 A at optical switch input port  31 A. Optical switch  30 A is adapted to switch between outputs  32 A and  34 A, allowing the QKD system (or the QKD system user) to select link L 1  or L 2  in the optical path between Alice and Bob. 
         [0017]    Alice also includes two wavelength-division multiplexers (WDMs)  40 A and  50 A. WDM  40 A has an input end  42 A and an output end  44 A, while WDM  50 A has an input end  52 A and an output end  54 A. Input end  42 A of WDM  40 A is optically coupled to output port  32 A of optical switch  30 A. Likewise, input end  52 A of WDM  50 A is optically coupled to output port  34 A of optical switch  30 A. The respective output ends  44 A and  54 A of WDMs  40 A and  50 A are optically coupled to respective links L 1  and L 2 . 
         [0018]    Alice also includes a framing/synchronization (F/S) light source  60  optically coupled to a beamsplitter  60 A that has two output ends  62 A and  64 A. Beamsplitter output end  62 A is optically coupled to input end  52 A of WDM  50 A, while beamsplitter output end  64 A is optically coupled to input end  42 A of WDM  40 A. F/S light source  60  is adapted to provide classical (i.e., non-quantum) light pulses (F/S signals) PS for synchronization and framing of the single-photon-level quantum signals used in establishing a key between Alice and Bob. Alice also includes two public discussion channel interfaces  70 A and  72 A that are respectively optically coupled to respective WDM input ends  42 A and  52 A. WDM  40 A and  50 A operate in both directions for PD signals to support bi-directional public discussion. 
         [0019]    Alice also includes a controller CA operably coupled to light source  12 A, encoding optical system  20 A, optical switch  30 A, F/S light source  60 , and pubic discussion channel interfaces  70 A and  72 A. In an example embodiment, controller CA is a computer or field-programmable gate array (FPGA). Controller controls light source  12 A via control signals SA 1 , encoding optical system  20 A via control signals SA 3 , optical switch  30 A via control signals SA 2 , FS light source  60  via control signals SA 4 , and public discussion channel interfaces via control signals SA 5  and SA 6 . Controller CA is adapted to receive and process signals PD send over the public discussion channels. 
       Bob 
       [0020]      FIG. 3  is a close-up schematic diagram of an example embodiment of the QKD station Bob of  FIG. 1 . Bob includes WDMs  40 B and  50 B with respective input ends  42 B and  52 B respectively optically coupled to links L 1  and L 2 . Bob also includes an optical switch  30 B similar (if not identical) to optical switch  30 A, but arranged so that port  31 B is an output port and ports  32 B and  34 B are input ports that are selected by changing the state of the optical switch. WDM  40 B is optically coupled at its output end  44 B to optical switch input port  32 B and WDM  50 B is optically coupled at its output end  54 B to optical switch input port  34 B. Bob also includes two public discussion channel interfaces  70 B and  72 B that are respectively optically coupled to the output ends  44 B and  54 B of WDMs  40 B and  50 B so that they can communicate with their counterparts  70 A and  72 A at Alice. WDM  40 B and  50 B operate in both directions for PD signals to support bi-directional public discussion. 
         [0021]    Bob further includes an encoding optical system  20 B similar if not identical to Alice&#39;s encoding optical system  20 A, and having an input end  22 B and an output end  23 B. Optical switch output port  31 B is optically coupled to input end  22 B of encoding optical system  20 B. Encoding optical system  20 B is adapted to modulate encoded quantum signals sent from Alice. In an example embodiment, encoding optical system  20 B is adapted to modulate one of the quantum signals P 1  and P 1 ′ and then interfere these signals to form an interfered quantum signal that includes information about the encoding applied by Alice and Bob. 
         [0022]    Bob further includes a single-photon detector (SPD) unit  80  that includes in an example embodiment two SPDs  82  and  84 . SPD unit  80  is optically coupled to output end  23 B of encoding optical system  20 B and adapted to receive and detect optical signals (e.g., the interfered optical signal) from the encoding optical system. The interfered optical signal arrives either at one SPD (say, SPD  82 ), resulting in qubit value  0  or arrives at the other SPD (SPD  84 ), resulting in qubit value  1 . 
         [0023]    Bob further includes a framing/synchronization (F/S) detector unit  90  optically coupled to the respective output ends  44 B and  54 B of WDMs  40 B and  50 B so as to be in optical communication with F/S light source  60  via links L 1  and L 2 . In an example embodiment, F/S detector unit  90  includes separate detectors  92  and  94  corresponding to WDMs  40 B and  50 B and thus links L 1  and L 2 , respectively. 
         [0024]    Bob also includes a controller operably coupled to optical switch  30 B, public discussion channel interfaces  70 B and  72 B, SPD unit  80 , and F/S detector unit  90 . Bob uses control signals SB 3 , SB 4 , SB 5  and SB 6  to control optical switch  30 B, encoding optical system  20 B, and public discussion channel interfaces  70 B and  72 B, respectively. Bob also receives an SPD unit signal S 80  and a F/S detector unit signal S 90  from the SPD unit  80  and the F/S detector unit  90 , respectively. Controller CB also adapted to receive and process signals PD send over the public discussion channels between Alice and Bob. 
       Method of Operation 
       [0025]    In an example embodiment, QKD system  10  operates as usual, with the optical switches  30 A and  30 B at Alice and Bob set so that the optical path associated with the primary link L 1  is selected (e.g., as the default link). Alice transmits identical F/S pulses PS over both links L 1  and L 2 , and pulses PS are detected at F/S detector unit  90  (e.g., in respective detectors  92  and  94 ). The F/S pulses are converted to F/S detector unit signals S 90 , which are received and processed by controller CA and CB. F/S pulses PS are thus used to establish the timing and synchronization of the encoding and detection of the quantum signals P 1  so that the QKD protocol can be carried out. 
         [0026]    Each link L 1  and L 2  also carries public discussion signals PD generated by public discussion channel interfaces  70 A and  70 B (link L 1 ) and  72 A and  72 B (link L 2 ) over their respective public discussion channels. These public discussion signals PD are converted to electrical signals SP by the respective interfaces  70 A,  70 B and  72 A,  72 B, and are processed by controllers CA and CB in carrying out the particular QKD protocol. 
         [0027]    When both links L 1  and L 2  operate without failure or transmission problems, both public discussion channels are available for use with the particular QKD protocol, and either channel may be used. This mode of operation of QKD system  10  essentially identical to that for single-QKD-link architecture. 
       Failure of a QKD Link 
       [0028]    In the operation of QKD system  10 , primary link L 1  used to communicate quantum signals QS (i.e., signals P 1 ) between Alice and Bob is also called the active link, while the unselected link L 2  is called the standby link. 
         [0029]    Bob detects F/S signals PS for both the primary link L 1  and the secondary link L 2 . If correct framing/synchronization patterns are not detected for a pre-determined period of time T 1 , Bob declares a failure of the corresponding link. In another example embodiment, the QKD link status of the public discussion channel is used as the link-failure indicator. The choice depends on the speed and reliability of the failure indication. For the purpose of illustration, the framing/synchronization method is used and discussed. The failed status of the link is cleared after receiving correct framing/synchronization patterns from F/S pulses PS for a time T 2 . 
       Switching Links 
       [0030]    As discussed above, controllers CA and CB are adapted to control the state (switching position) of their respective optical switches  30 A and  30 B via control signals SA 3  and SB 3  so that the optical path between Alice and Bob uses either link L 1  or L 2 . 
         [0031]    In an example embodiment, the rules for the switching optical switches  30 A and  30 B are as follows:
       1. If the active link (L 1 ) fails and the standby link (L 2 ) has not failed, make the standby link the new active link.   2. If the failed primary link (L 1 ) recovers from failure:
           a. If the system is set to a revertive mode and the currently active link is the secondary link (L 2 ), then switch back to the primary link (L 1 ).   
           3. If the link protection is disabled by a user, do not switch over.   4. If a user issues a manual switch over, switch to the standby link if it has not failed.   5. If a user issues a “forced” switch over, switch to the standby link unconditionally.       
 
         [0038]    Alice and Bob must agree to select the same link. Since QKD requires the public discussion channel to be in operation at all times, it is most flexible to use the public discussion channel to coordinate the action of both stations. The following simple protocol accomplishes the goal.
       1. If the standby public discussion channel has not failed, select the standby link for the public discussion. Otherwise select the active link.   2. The receiver Bob decides the proposed new active link, new_active_link, to be primary (L 1 ) or secondary (L 2 ).   3. The receiver Bob sends a “switch to new_active_link” message to the transmitter Alice.   4. The transmitter Alice replies with “switch_accept” or “switch_deny” message. After sending the switch_accept message, the transmitter Alice switches to the new_active_link immediately. If the switch is denied, the reason is included in the reply message.   5. The receiver Bob switches after receiving the switch_accept reply from the transmitter Alice. Otherwise the switch-over is aborted.       
 
         [0044]    An advantage of the QKD system  10  of the present invention is that it does not require two transmitting and two receiving QKD stations to have redundant encrypted communication between Site A and Site B. Redundancy is not only provided with respect to the quantum signals, but is also included in the QKD stations with respect to the frame/synchronization channel and the public discussion channels. While this requires substantial modifications to the two direct-link QKD stations, the modifications obviate the need for additional QKD stations to accomplish system redundancy. 
         [0045]    Note that in another example embodiment of QKD system  10 , optical switch is a 1×N switch, wherein N is 2 or greater, and the number of links between Alice and Bob is two or greater. Extension of the above-described QKD system from two links L 1  and L 2  to more than two links follows directly from the teaching provided herein. 
         [0046]    It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.