Abstract:
Methods and systems for suppressing the electromagnetic interference (EMI) signature generated by a QKD station are disclosed. One of the methods includes generating two or more modulator drive signals corresponding to two or more of the n possible modulator states of the particular QKD protocol. The modulator drive signals are sent to a random number generation (RNG) unit, which randomly selects one of the two or more modulator drive signals and passes it to the modulator. Another method involves generating two modulator drive signals, wherein the voltage sum is constant. One signal is sent to the modulator while the other is sent to a circuit-terminating element, which can be a second modulator. The method suppresses the EMI signature associated with individual modulation states. This prevents an eavesdropper from gaining information about the modulator states via the EMI signature, which information could otherwise yield information about the exchanged key.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to quantum cryptography, and in particular relates to method and systems for enhancing the security of a quantum key distribution (QKD) system by suppressing (e.g., reducing, eliminating or obscuring) electromagnetic emissions.  
       BACKGROUND OF THE INVENTION  
       [0002]     Quantum key distribution involves establishing a key between a sender QKD station (“Alice”) and a receiver QKD station (“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 an 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 and thus reveal her presence.  
         [0003]     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). The basics of quantum cryptography are described in the article by Gisin et al, entitled “Quantum Cryptography,” Reviews of Modern Physics, Vol. 74, January 2002 (pages 145 to 195), which article is incorporated by reference herein as background material.  
         [0004]     Specific QKD systems are described in U.S. Pat. No. 5,307,410 (the &#39;410 patent) to C. H. Bennett, in the publication by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992), and in the book by Bouwmeester et al., entitled “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. All of the above-cited references are incorporated herein by reference as background information.  
         [0005]     In a typical QKD system, 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 in the &#39;410 patent 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.  
         [0006]     During the QKD process, Alice uses a true random number generator (TRNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding). She then sends this qubit to Bob, who randomly measures (modulates) the qubit. This process can loosely be referred to as “qubit encoding” at Alice and “qubit decoding” at Bob.  
         [0007]     In the typical QKD system, either polarization or phase modulators are used at each QKD station to respectively encode and decode the qubits. Such modulators are randomly driven by a modulator driver that sends the modulator a modulator drive signal. The modulator drive signals have different strengths (e.g., voltages, such as V[0], V[π], V[π/2] and V[3π/2]) corresponding the different modulation states (e.g., phase states of 0, π, π/2 and 3π/2) called for by the particular QKD protocol.  
         [0008]     The random activation of the modulators using different modulator drive signal strengths can, under certain circumstances, pose a security risk to an otherwise secure QKD system. With reference to  FIG. 1 , there is shown a schematic diagram of prior art version of a QKD station Alice for a one-way QKD system. Alice includes a light source  12  that emits coherent light pulses P 0 . Alice also includes a (polarization or phase) modulator MA downstream of light source  12  and optically coupled thereto via, e.g., an optical fiber section  16 . Modulator MA is coupled to a modulator driver  20 , which in turn is couple to a true random number generator (RNG)  30 . Alice also includes a controller  40  coupled to light source  12  and to RNG  30 . Alice further typically includes a housing H that encloses all of the above-described elements.  
         [0009]     In operation, controller  40  sends a control signal S 0  to light source  12  to initiate the emission of initial light pulse P 0 . Controller  40  also sends an activation signal S 1  to RNG  30  that causes the RNG to generate a random number. The random number is embodied in a control signal S 2  sent from RNG  30  to modulator driver  20 . Modulator driver  20  receives control signal S 2  and in response thereto generates a corresponding modulator drive signal (e.g., a voltage) S 3  and sends it to modulator MA. The modulator drive signal sets modulator MA to a corresponding modulator state for a time interval corresponding to the duration of modulator drive signal S 3 .  
         [0010]     The activation of modulator MA is timed (gated) to coincide with the arrival of initial light pulse P 0  by the synchronized operation of the controller. The result is a randomly modulated light pulse P 1  that leaves Alice and travels to Bob, e.g., via an optical fiber link FL connecting Alice to Bob (not shown).  
         [0011]      FIG. 2  is a close up schematic diagram of  FIG. 1  of modulator driver  20  as it generates modulator drive signal S 3 . The modulator drive signals S 3  vary in strength to correspond to one of the n possible modulator states. Also shown in  FIG. 2  is housing H, along with a first radiation detector (antenna) A 1  external to housing H, and a second antenna A 2  internal to housing H. Antennas A 1  and A 2  are tuned to received electromagnetic radiation and are assumed to have been surreptitiously placed in their respective locations by an eavesdropper (“Eve,” not shown) who is seeking to gain information about the state of modulator MA during the operation of the QKD system.  
         [0012]     When modulator driver  20  generates different drive signals S 3  (typically in the range of 0 to 5 volts or so for a phase modulator), it also emits corresponding electromagnetic radiation R 3  (dashed lines). This radiation, which differs in relation to the different modulator drive signals S 3 , can be picked up directly by Eve&#39;s internal antenna A 2 , or through housing H by external antenna A 1 . This radiation is sometimes referred to as electromagnetic interference (EMI). The detected radiation (i.e., EMI “signature”) can then be used by Eve to gain information about the state of modulator MA, and ultimately information about the keys exchanged between Alice and Bob. This eavesdropping technique, which is relatively easy to implement as compared to other eavesdropping techniques (such as a Trojan horse attack or man-in-the-middle attack) can result in a catastrophic security breach of an otherwise perfectly secure QKD system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic diagram of a prior art QKD station Alice for a one-way system illustrating the operation of the modulator in encoding qubits;  
         [0014]      FIG. 2  is a close-up of the QKD station Alice of  FIG. 1 , showing the modulator driver and modulator, along with the radiation (R 3 ) associated with the modulator driver;  
         [0015]      FIG. 3  is a schematic diagram of an example embodiment of a QKD station Alice similar to that of  FIG. 1 , but modified to eliminate or suppress the EMI signature from the modulator driver; and  
         [0016]      FIG. 4  is a schematic diagram of an example embodiment of a QKD station Alice similar to that of  FIG. 3 , but that further includes an additional RNG that allows for the modulator driver to send a random subset of the entire set of possible modulator drive signals to the RNG unit, which then randomly selects and passes one of the sent modulator drive signals;  
         [0017]      FIG. 5  is a schematic diagram of another example embodiment of a QKD station Alice similar to that of  FIG. 1 , wherein the controller is adapted to generate two modulator drive signals, wherein the first modulator drive signal (S 3 R) is provided to the “real” modulator (MA) and the second modulator drive signal S 3 F is a “fake” signal provided to circuit-terminating element (MF); and  
         [0018]      FIG. 6  is a detailed schematic diagram of the modulator driver of  FIG. 5 . 
     
    
       [0019]     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.  
       SUMMARY OF THE INVENTION  
       [0020]     A first aspect of the invention is a method of modulating light in a QKD system. The QKD system is presumed to have a modulator capable of being set to two or more modulator states according to a particular QKD protocol. The method includes simultaneously (or nearly simultaneously) generating two or more modulator drive signals corresponding to the two or more modulator states. The method also includes randomly passing one of the two or more modulator drive signals to the modulator to suppress the EMI signatures associated with each individual modulator setting.  
         [0021]     A second aspect of the invention is a method of modulating light in a QKD system having first modulator optically coupled to a laser source and capable of being set to two or more modulator states. The method includes generating first and second modulator drive signals having respective first and second voltages, wherein the sum of the first and second voltages is a constant. The method further includes passing the first modulator drive signal to the first modulator.  
         [0022]     A third aspect of the invention is a QKD station that operates under a QKD modulation protocol. The QKD station includes a modulator arranged to modulate light pulses passing therethrough. The modulator may be, for example, a polarization modulator or a phase modulator. The QKD station also includes a modulator driver adapted to simultaneously (or nearly simultaneously) generate two or more modulator drive signals. The QKD station further includes a random number generation (RNG) unit connected to the modulator and the modulator driver. The RNG unit is adapted to receive and randomly select one of the two or more modulator drive signals and pass the selected modulator drive signal to the modulator.  
         [0023]     A fourth aspect of the invention is a QKD station that operates under a QKD modulation protocol. The QKD station includes a first modulator arranged to modulate light pulses passing therethrough. A modulator driver is coupled to the first modulator and to a circuit-terminating element. The modulator driver is adapted to generate first and second modulator drive signals based on a random control signal provided thereto. The first and second modulator drive signals have respective first and second voltages, the sum of which is a constant. The first modulator drive signal is provided to the first modulator, and the second modulator drive signal is provided to the circuit-terminating element.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]      FIG. 3  is a schematic diagram of an example embodiment of a QKD station Alice similar to the Alice of  FIG. 1 , but modified to suppress (e.g., eliminate, reduce or otherwise obscure) the EMI signature associated with the different modulator driver voltages. Alice of  FIG. 3  includes many of the same elements as Alice of  FIG. 1 , and these elements have the same reference number in  FIG. 3 . Further, only the main differences between the Alice of  FIG. 1  and the Alice of  FIGS. 3 and 4  are described below.  
         [0000]     n Modulator Drive Signal Embodiment  
         [0025]     In the example embodiment of Alice of  FIG. 3 , modulator driver  20  is operatively connected to controller  40 , and an RNG unit  30 ′ is operably connected to the modulator driver via connection  50 . RNG unit  30 ′ is also operably connected to modulator MA via connection  52 . RNG unit  30 ′ is adapted to generate random numbers, and for each random number pass a corresponding one of the received modulator drive signals S 3 . Further, modulator driver  20  is adapted to simultaneously or nearly simultaneously provide two or more of the plurality n of modulator drive signals S 3  (e.g., S 3 A, S 3 B, . . . S 3   n ) to RNG unit  30 ′.  
         [0026]     In an example embodiment, all n of the modulator drive signals S 3  are generated simultaneously by modulator driver  20 . In another example embodiment, the modulator drive signals S 3  are generated by the modulator driver close enough in time to implement the invention, i.e., to suppress the EMI signature associated with the modulation process, wherein the unsuppressed EMI could otherwise reveal information about the modulation state. For the purposes of the description herein, these two embodiments relating to the timing of the generated modulator drive signals are respectively described by the phrases “simultaneously” and “nearly simultaneously.” 
         [0027]     In an example embodiment, multiple drive signals S 3  (S 3 A, S 3 B, . . . S 3   n ) are carried from modulator driver  20  to RNG unit  30 ′ via an embodiment of connection  50  that has n independent connections (i.e.,  50 A,  50 B, . . .  50   n ), where n is the number of possible modulation states. In an example embodiment, the independent connections are wires linking the modulator driver and the RNG unit. Four connections  50  ( 50 A- 50 D) are shown for the sake of illustration, corresponding to a QKD protocol requiring four possible modulator states (e.g., phase states of 0, π/2, π, 3π/2).  
         [0028]     In an example embodiment, connections  50  and  52  are adapted to allow each drive signal S 3  to propagate the same distance, regardless of whether RNG unit  30 ′ passes the signal to modulator MA. In an example embodiment, this is accomplished by providing suitable wiring W that allows the modulator drive signals not passed to the modulator to propagate for the same amount of time as the modulator drive signal sent to the modulator. For example, wiring W is made to have the same length as the connection length for connections  50  and  52  so that each of the signals S 3  starts and stops at the same time. This ensures that there is no lingering radiation from one of the signals that could be detected by Eve through antenna  1  and/or antenna  2 . In an example embodiment, wiring W is formed and terminated (e.g., connected to ground G) directly within (or partially within) RNG unit  30 ′, as shown.  
         [0029]     With continuing reference to  FIG. 3 , in response to activation signal S 1  from controller  40 , in an example embodiment modulator driver  20  generates all n of the modulator drive signals S 3  (S 3 A, S 3 B, . . . S 3   n ) of the particular QKD protocol. Each modulator drive signal is delivered to RNG unit  30 ′ via connection  50 . RNG unit  30 ′ then randomly selects one of the signals to be passed to modulator MA. This signal is identified in  FIG. 3  as SR. The process of passing signal SR to modulator MA is repeated for each light pulse P 0 .  
         [0030]     In an example embodiment, RNG unit  30 ′ acts in response to receiving the drive signals. In another example embodiment, RNG unit  30 ′ is connected to controller  40  and acts in response to a timed control signal S 4  provided by the controller.  
         [0031]     Associated with modulator driver  20  generating all n of the drive signals S 3  is corresponding radiation Rn. In an example embodiment, radiation Rn is emitted once for every light pulse P 0  to be modulated, and is the same each time modulator driver  20  is activated. Accordingly, an eavesdropper having access to information received by antenna A 1  and/or antenna A 2  will not receive any information about the actual modulation state of modulator MA. Thus, the radiation emitted by the modulator driver no longer provides information about the modulator state because all of the modulator drive signals are generated while only one is (randomly) passed to the modulator.  
         [0032]     Further, even if antennae A 1  and A 2  were sensitive enough to detect radiation generated by RNG unit  30 ′, such radiation would not contain any significant information about the modulator state, particularly in the case where the propagation lengths for drive signals S 3  are the same.  
         [0000]     m&lt;n Modulator Drive Signal Embodiment  
         [0033]     In the example embodiment of the present invention described above, the entire plurality (n) of modulator drive signals S 3  is sent to RNG unit  30 ′ to suppress, eliminate or otherwise obscure the EMI signature associated with the individual modulator drive signals. However, in another example embodiment, a random subset m (where 1&lt;m&lt;n) of the modulator drive signals S 3  is sent to the RNG unit, which then randomly passes one signal from the subset.  
         [0034]     With reference to  FIG. 4 , this is accomplished, for example, by coupling a RNG unit  60  to modulator driver  20  and controller  40 . An RNG signal S 5  corresponding to a random number is then provided to modulator driver  20  by the RNG unit  60 . In response thereto, modulator driver  20  provides a random subset m of the plurality n of possible modulator driver signals S 3  to RNG unit  30 ′.  
         [0035]     By way of example and as shown in  FIG. 4 , in one instance (i.e., for one of the pulses P 0 ), only signals S 3 A, S 3 C and S 3 D (i.e., m=3) of the total (n=4) possible modulator drive signals are sent to RNG unit  30 ′. In this manner, the EMI signature (radiation) Rm so generated and detected by antennae A 1  and/or A 2  is scrambled. This precludes Eve from obtaining any useful information about the actual modulator state.  
         [0000]     Two Modulator Drive Signal Embodiment  
         [0036]      FIG. 5  is a schematic diagram of a QKD station Alice similar to that of  FIG. 1 . Alice of  FIG. 5  has a modified modulator driver  20 ′, and includes a circuit-terminating element MF coupled to modulator driver  20 ′. In an example embodiment, circuit-terminating element MF is a modulator similar or identical to modulator MA. In other example embodiments, circuit-terminating element is a resistor (e.g., a 50 Ohm resister) or ground. Alice of  FIG. 5  also includes controller  40  coupled to RNG unit  30 , as in the Alice of  FIG. 3 .  
         [0037]      FIG. 6  is a detailed schematic diagram of modulator driver  20 ′. Modulator driver  20 ′ includes controller  200  coupled to two modulator drivers  202 R and  202 F. The output of modulator driver  202 R is a “real” signal SR that travels to and drives modulator MA, while the output of modulator driver  202 F is a “fake” signal SF that travels to circuit-terminating element MF.  
         [0038]     In operation, control signal S 2  from RNG  30  is received by controller  200  of modulator driver  20 ′. Controller  200  includes logic that identifies the voltage level of control signal S 2  and then passes the control signal to modulator driver  202 R. Controller  200  also is adapted to generate another voltage signal S 2 C (e.g., a complementary voltage signal as compared to signal S 2 ) that is sent to modulator driver  202 F.  
         [0039]     Modulator driver  202 R, in response to receiving signal S 2 C from controller  200 , generates a modulator drive signal S 3 R that sets modulator MA to a given phase. Likewise, modulator driver  202 F, in response to receiving signal S 2 F from controller  200 , generates a complimentary modulator drive signal S 3 F. In the example where circuit-terminating element is a modulator, modulator drive signal S 3 F sets this modulator to a setting complementary to that of modulator MA.  
         [0040]     Thus, in an example embodiment, if modulator drive signal S 3 R has a voltage V R  and the “fake” modulator drive signal S 3 F has a voltage V F , then V R +V F =constant. For example, the constant voltage might be a voltage V 3π/2  corresponding to the voltage for setting a modulator at a phase of 3π/2.  
         [0041]     Accordingly, an eavesdropper attempting to gain information about the settings of modulator MA via antennae A 1  and/or A 2  will only be able to detect a constant radiation R C  corresponding to an apparent constant modulator voltage.  
         [0042]     In the foregoing Detailed Description, various features are grouped together in various example embodiments for ease of understanding. For example, the above-description was described in connection with four possible modulator states for the sake of illustration, though the invention applies generally o two or more modulator states. Thus, 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.