Abstract:
A quantum key distribution (QKD) cascaded network with loop-back capability is disclosed. The QKD system network includes a plurality of cascaded QKD relays each having two QKD stations Alice and Bob. Each QKD relay also includes an optical switch optically coupled to each QKD station in the relay, as well as to input ports of the relay. In a first position, the optical switch allows for communication between adjacent relays and in a second position allows for pass-through communication between the QKD relays that are adjacent the relay whose switch is in the first position.

Description:
CLAIM OF PRIORITY 
     This patent application claims priority from U.S. Provisional Patent Application Ser. No. 60/584,970, filed on Jul. 2, 2004. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to quantum cryptography, and in particular relates to quantum key distribution (QKD) system networks and QKD stations for use therein. 
     BACKGROUND ART 
     Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence. 
     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 publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography.”  J. Cryptology , (1992) 5: 3-28, and 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. 
     The above-mentioned publications by Bennett describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper and incorporated by reference herein is based on a shared interferometric system. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The signals (pulses) sent from Alice to Bob are time-multiplexed and follow different paths. As a consequence, the interferometers need to be actively stabilized to within a few tens of nanoseconds during transmission to compensate for thermal drifts. 
     U.S. Pat. No. 6,438,234 to Gisin (the &#39;234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that employs an autocompensating interferometer first invented by Dr. Joachim Meier of Germany and published in 1995 (in German) as “Stabile Interferometrie des nichtlinearen Brechzahl-Koeffizienten von Quarzglasfasern der optischen Nachrichtentechnik,” Joachim Meier.—Als Ms. gedr.—Düsseldorf: VDI-Verl., Nr. 443, 1995 (ISBN 3-18-344308-2). Because the Meier interferometer is autocompensated for polarization and thermal variations, the two-way QKD system based thereon is less susceptible to environmental effects than a one-way system. 
     It will be desirable to one day have multiple QKD links woven into an overall QKD network that connects its QKD endpoints via a mesh of QKD relays or routers. Example QKD networks are discussed in the publication by C. Elliot, New Journal of Physics 4 (2002), 46.146.12, and also in PCT patent application publication no. WO 02/05480, which publication and PCT patent application are incorporated by reference herein. 
     When a given point-to-point QKD link within the relay fails—e.g. by a fiber being cut or from too much eavesdropping or noise—that link is abandoned and another used instead. This type of QKD network can be engineered to be resilient even in the face of active eavesdropping or other denial-of-service attacks. 
     Such QKD networks can be built in several ways. In one example, the QKD relays only transport keying material. After relays have established pair-wise agreed-to keys along an end-to-end point, e.g., between the two QKD endpoints, they employ these key pairs to securely transport a key “hop by hop” from one endpoint to the other. The key is encrypted and decrypted using a onetime-pad with each pairwise key as it proceeds from one relay to the next. In this approach, the end-to-end key will appear in the clear within the relays&#39; memories proper, but will always be encrypted when passing across a link. Such a design may be termed a “key transport network.” 
     Alternatively, QKD relays may transport both keying material and message traffic. In essence, this approach uses QKD as a link encryption mechanism, or stitches together an overall end-to-end traffic path from a series of QKD-protected tunnels. 
     Such QKD networks have advantages that overcome the drawbacks of point-to-point links enumerated above. First, they can extend the geographic reach of a network secured by quantum cryptography, since wide-area networks can be created by a series of point-to-point links bridged by active relays. Links can be heterogeneous transmission media, i.e., some may be through fiber while others are free-space. Thus, in theory, such a network could provide fully global coverage. 
     Second, they lessen the chance that an adversary could disable the key distribution process, whether by active eavesdropping or simply by cutting a fiber. A QKD network can be engineered with as much redundancy as desired simply by adding more links and relays to the mesh. 
     Third, QKD networks can greatly reduce the cost of large-scale interconnectivity of private enclaves by reducing the required (N×(N−1))/2 point-to-point links to as few as N links in the case of a simple star topology for the key distribution network. 
     Such QKD networks do have their own drawbacks, however. For example, their prime weakness is that the relays must be trusted. Since keying material and—directly or indirectly—message traffic are available in the clear in the relays&#39; memories, these relays must not fall into an adversary&#39;s hands. They need to be in physically secured locations and perhaps guarded if the traffic is truly important. In addition, all users in the system must trust the network (and the network&#39;s operators) with all keys to their message traffic. Thus, a pair of users with unusually sensitive traffic must expand the circle of those who can be privy to it to include all machines, and probably all operators, of the QKD network used to transport keys for this sensitive traffic. 
     U.S. patent application Ser. No. 11/152,875, entitled “QKD System Network,” filed on Jun. 15, 2005, (the &#39;875 application), and also filed as a corresponding PCT Patent Application on Jul. 28, 2005, and incorporated by reference herein, discloses a QKD network system that includes a cascaded arrangement of QKD stations that utilize switches. The switches allow for a choice of pathways between points in the network. The &#39;875 application also describes approaches for communicating keys between stations in the network. In QKD networks, such as those described in the &#39;875 application, it would be useful to have a way to perform a check of the Alice and Bob units in each box without fear of outside interference through the externally accessible fiber links, while also allowing pass-through communication between relays in a QKD-based network. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention is a QKD cascaded network with loop-back capability. The QKD system network includes a plurality of cascaded QKD relays each having two QKD systems Alice and Bob therein. Each QKD relay also includes an optical switch. The optical switch is optically coupled to each QKD station in the relay, as well as to the input ports of the relay. In a first position, the optical switch allows for communication between adjacent relays. In a second position, the optical switch allows for pass-through communication between the QKD relays that are adjacent the relay whose switch is in the first position. Also in the second position, the optical switch allows for communication between the QKD stations within the relay. This “loop back” configuration allows for diagnostic measurements to be made of one or both of the QKD stations via an optical path that is entirely within the relay station enclosure. 
     These and other aspects of the invention are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a 50 km link as part of a QKD cascaded network, wherein each QKD relay (box)  10  and  30  includes an Alice A and a Bob B; 
         FIG. 2  is a schematic diagram of the QKD cascaded network of  FIG. 1 , but with a QKD box  20  similar to boxes  10  and  30  interposed between boxes  10  and  30 ; 
         FIG. 3  is a schematic diagram of the QKD cascaded network of  FIG. 2 , showing details of QKD box  20  with a 2×2 optical switch shown in a first (open) position that allows for cascaded QKD communication between boxes  10 ,  20  and  30 ; and 
         FIG. 4  is a schematic diagram similar to  FIG. 3 , but wherein with the 2×2 optical switch of QKD box  20  is shown in a second (closed) position that allows for cascaded QKD communication between boxes  10  and  30 , while also allowing for a loop-back self-check of Alice and Bob within box  20 . 
     
    
    
     The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of a 50 km link as part of a QKD cascaded network  5 , wherein the network includes QKD relays (“boxes”)  10  and  30  that each includes QKD stations Alice A and Bob B. QKD boxes  10  and  30  are operably connected via fiber link F 1 . In the operation of the QKD network, the Alice of one box communicates with the Bob in the adjacent box in the cascaded network. 
     If the QKD network  5  requires redundancy and self-checking, then another QKD box  20  is added in between boxes  10  and  30 , as illustrated in  FIG. 2 . Now, fiber link F 1  is divided into two sections F 1 A and F 1 B. For the purpose of discussion, it is assumed that the distance from box  10  to box  20  and the distance from box  20  to box  30  is 25 km. Also, in an example embodiment, boxes  10 ,  20  and  30  have respective enclosures  12 ,  22  and  32 , and the boxes are designed to be tamperproof. 
     With reference now to  FIGS. 3 and 4 , a 2×2 optical switch  50  is added to each QKD box  10 ,  20  and  30  and is optically coupled to input ports PI of each box. In an example embodiment, 2×2 optical switch is a prism-based switch made by Dicon Fiberoptics, such as the optical switch described at: http://www.diconfiberoptics.com/products/scd0009/index.htm. Note that each box  10 , and  30  includes the same internal components and that only the internal components of box  20  are shown in detail for ease of illustration. 
     In an example embodiment, optical switch  50  has a single control input that switches the device between two configurations (states) based on a 0V or 5V input signal S 50 . In an example embodiment, optical switch  50  is operatively connected to either controller CA and/or controller CB of Alice A or Bob B, respectively, via an electrical line  51 . Controller CA or controller CB provides input signal S 50  to optical switch  50  to control the network system configuration. 
     Optical switch  50  has four ports, P 1 , P 2 , P 3  and P 4 . Optical switch  50  is connected to Alice via fiber link  52  at port P 1  and to Bob via fiber link  54  at port P 2 . The remaining two ports P 3  and P 4  are connected to optical fiber sections F 1 A and F 1 B, respectively. 
     Optical switch  50  has two positions, as shown respectively in  FIG. 3  and  FIG. 4 . With reference first to  FIG. 3 , in a first (open) position, the switch allows for cascaded communication between adjacent boxes in the network, as illustrated by the double-ended arrows  70 . In  FIG. 3 , QKD stations A and B share access to their key databases and are now communicating them to the adjacent QKD boxes  10  and  20 , with QKD station A in box  30  communicating with QKD station B in box  10  and QKD station B in box  20  communication with QKD station A in box  30 . 
     With reference to  FIG. 4 , in a second (closed) position, switch  50  acts to bypass box  20 , so that box  10  can communicate directly to box  30  through box  20 , as indicated by double-ended arrow  80 . At the same time, box  20  can perform diagnostics on its QKD stations A and B without fear of outside interference from the externally accessible fiber links. This optical connection between Alice A and Bob B within QKD relay  20  is referred herein as “loop back,” and is indicated by double-ended arrow  90 . In the loop-back configuration associated with the second position of optical switch  50 , Alice A and Bob B are optical coupled via fiber links  52  and  54  through the optical switch. 
     In an example embodiment, controllers CA and CB are connected to similar controllers in boxes  10  and  30  (not shown) and coordinate the mode of operation (i.e., the position of optical switch  50 ). For example, the network may be configured so that on a regular basis optical switch  50  is placed in bypass mode for a given diagnostic time period pre-agreed in the network. After the diagnostic time elapses, optical switch  50  is returned to the position shown in  FIG. 3 . 
     In another example embodiment, information about the desired position of optical switch  50  is transmitted from a controller in box  10  (not shown) to controller CA, then via electrical line  51  to controller CB, from controller CB to a controller in box  30  (not shown), etc. Controllers CA and CB can be connected to similar controllers in boxes  10  and  30  (not shown) and coordinate the mode of operation (i.e., the position of optical switch  50 ). For example, the network may be configured so that on a regular basis optical switch  50  is placed in bypass mode for a given maintenance time period pre-agreed in the network. After the maintenance time elapses, optical switch  50  is returned to the position shown in  FIG. 3 . 
     In particular, the diagnostic loop-back testing of the Alice (A) and Bob (B) QKD stations within the QKD relay includes, for example, checking the function of one or more of the various elements (not shown) of each QKD box, such as the output power of the laser, the calibration of both variable optical attenuators (VOA&#39;s), confirming the function and calibration of a watchdog detector at Alice, the calibration of the modulators, and the calibration and operation of single photon detectors. All of these functions are normally set during a system turn up. However, when the fiber path is in loop back mode, there is no access to the fiber for an eavesdropper to insert herself into the optical loop. 
     In an example of performing diagnostic testing in the loop-back configuration, the laser output is calibrated as a function pulse width. This is accomplished, for example, by using a calibrated PIN diode detector pre-installed into the QKD stations, and placing all of the VOAs in each box to minimum attenuation. With a wide laser pulse, each VOA is varied to check the calibration. The average photon level μ is then calculated and used to calibrate the single-photon detectors. 
     When QKD relay  20  is added as an extra node, one disadvantage is extra network cost. However, when QKD relay  20  is not bypassed (i.e., when the optical switch is in the first position), the key rate is increased, so that the added cost provides an added benefit. Also, the optical switch can be configured so that when the electronics in one QKD relay totally fail, as in the case of a power failure, the system places the optical switch in the second position so that the failed QKD relay is bypassed without active intervention. Also, the ability to perform diagnostic testing and/or calibration of each box is an important aspect of creating a commercially viable QKD network system. 
     The present invention allows for two other network redundancies to be realized. With time multiplexing, the same fiber link F 1 A can be used to connect system  10  to system  20  and to connect system  10  to system  30 . Secondly, since multiple boxes work with keys transmitted in the same path, more information is available to remotely determine whether a fault is in a system on the fiber or in the fiber itself. 
     In the foregoing Detailed Description, various features are grouped together in various example embodiments for ease of understanding. The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction, operation and example embodiments described herein. Accordingly, other embodiments are within the scope of the appended claims.