Abstract:
A cascaded modulator system ( 20 ) and method for a QKD system ( 10 ) is disclosed. The modulator system includes to modulators (M 1  and M 2 ) optically coupled in series. A parallel shift register ( 50 ) generates two-bit (i.e., binary) voltages (L 1 , L 2 ). These voltage levels are adjusted by respective voltage adjusters ( 30 - 1  and  30 - 2 ) to generate weighted voltages (V 1 , V 2 ) that drive the respective modulators. An electronic delay element ( 40 ) that matches the optical delay between modulators provides for modulator timing (gating). The net modulation (M NET ) imparted to an optical signal ( 60 ) is the sum of the modulations imparted by the modulators. The modulator system provides four possible net modulations based only on binary voltage signals. This makes for faster and more efficient modulation in QKD systems and related optical systems when compared to using quad-level voltage signals to drive a single modulator.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to modulation in quantum cryptography, and in particular relates to modulators used in quantum key distribution (QKD) systems. 
     BACKGROUND ART 
     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. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal 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 U.S. Pat. No. 5,307,410 to Bennett, 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. 
     Most QKD systems utilize modulators to randomly encode the quantum signals shared between Alice and Bob. The typical modulator is a standard lithium niobate phase modulator, such as is available from Covega Corp. of Jessup, Md. or EOspace Inc. of Redmond, Wash. However, as the quantum bit rates of QKD systems increase, it becomes more and more difficult to drive lithium niobate phase modulators at the higher speeds needed. In a QKD system, it is necessary to have the ability to quickly and cleanly jump between four distinct modulation voltage values that correspond to four phase modulations (e.g., +3π/4, +π/4, −π/4 and −3π/4). It is preferred, however, to employ bi-level (or binary) electrical signals for modulation rather than quad-level signals because they are relatively fast and inexpensive to implement as compared to quadrature-level signals. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention is an optical modulator system that includes first and second optical modulators optically coupled in series, and first and second voltage adjusters respectively electrically coupled to the first and second optical modulators. A parallel shift register is electrically coupled to the first and second voltage adjusters and provides respective binary voltages to the voltage adjusters. The voltage adjusters act on the binary voltages to create first and second voltages corresponding to desired phase modulation values for the first and second optical modulators. The result is four possible net modulations based on binary voltage signals rather than quadrature-level signals. 
     A second aspect of the invention is a method of optically modulating a quantum signal in a quantum key distribution (QKD) station of a QKD system. The method includes passing the quantum signal through a first optical modulator and then a second optical modulator. The method also includes activating the first and second optical modulators with respective first and second binary voltages that correspond to desired modulations imparted by the first and second modulators, respectively, so as to randomly impart one of four possible net modulations to each quantum signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a QKD system  10  that includes two QKD stations, ALICE and BOB, optically coupled by an optical fiber link; 
         FIG. 2  is a schematic diagram of an example embodiment of the modulator system of  FIG. 1  according to the present invention; and 
         FIG. 3  is schematic diagram illustrating the two voltage signals V 1  and V 2 , along with the corresponding net output modulation M NET  of modulators M 1  and M 2 , showing an example of how four different output modulations are generated based on binary voltage values for V 1  and V 2 . 
       The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment 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 QKD system  10  that includes two QKD stations, ALICE and BOB optically coupled using, for example, an optical fiber link  12 . ALICE and BOB each include a number of optical and electronic elements that are known in the prior art and thus not shown, along with respective identical modulator assemblies  20  according to the present invention and denoted as  20 A at ALICE and  20 B at BOB. 
       FIG. 2  is a schematic diagram of an example embodiment of modulator system  20  according to the present invention. Modulator system  20  includes a first optical modulator M 1  optically coupled to a second optical modulator M 2 , e.g., by an optical fiber section  24 . First and second modulators M 1  and M 2  are, for example, lithium niobate phase modulators, such as those commercially available from Eospace, Inc. of Redmond, Wash. or Covega, Inc. of Jessup, Md. An input optical fiber  26  is optically coupled to modulator M 1  and an output optical fiber  28  is optically coupled to modulator M 2 . In an example embodiment where modulator system  20  is at BOB as system  20 B, input optical fiber  26  is optical fiber link  12 . 
     Modulator system  20  includes voltage adjusters  30 - 1  and  30 - 2  respectively electrically coupled to modulators M 1  and M 2  via electrical lines  36 - 1  and  36 - 2 . Electrical line  36 - 1  includes an electrical delay element  40 , such as a coaxial delay line. In an example embodiment, delay element  40  is adjustable to adjust the amount of delay. Voltage adjusters  30 - 1  and  30 - 2  are respectively electrically connected to a low-skew, low-jitter parallel shift register  50  via electrical lines  52 - 1  and  52 - 2 . 
     In an example embodiment, modulator system  20  also includes a random number generator (RNG)  54  electrically coupled to a controller  56 , such as a field-programmable gate array. Controller  56  is electrically coupled to parallel shift register  50  and voltage adjusters  30 - 1  and  30 - 2 , and that is adapted (e.g., programmed) to control the operation of the modulator system. In the example embodiment where electrical delay element  40  is adjustable, controller  56  is electrically coupled thereto (dashed line) and adapted to adjust the amount of electrical delay via a control signal S 40 . 
     In the operation of modulator system  20 , an optical signal  60  to be modulated travels into modulator system  20  via input optical fiber  26 . In an example embodiment, optical signal  60  is a single-photon-level quantum signal (i.e., single photons or optical pulses having one photon or less, on average, such as  0 . 1  photons on average). In an example QKD system such as QKD system  10 , optical signal  60  needs to be modulated with four different phase modulations (e.g., +3π/4, +π/4, −π/4 and −3π/4) generated by four corresponding drive voltage values. 
     In the present invention, the four different drive voltage values are determined by a two-bit binary word L 1  and L 2  (i.e., “logic” or “binary” voltages) found at the output of parallel shift register  50 . The binary (voltage) levels L 1  and L 2  are then adjusted (either amplified or attenuated) by respective voltage adjusters  30 - 1  and  30 - 2  to generate weighted voltage outputs V 1  and V 2 . In an example embodiment, the weighting is at a nominal ratio of two to one, but this need to be the case. The weighting is adjusted so that four different net modulation values M NET  can be achieved using only two binary voltage levels L 1  and L 2 . 
     Weighted voltage signals V 1  and V 2  are provided to respective modulators M 1  and M 2  via respective electrical lines  36 - 1  and  36 - 2 . The optical propagation delay of optical signal  60  over optical fiber section  24  optically connecting the two modulators is compensated by electrical delay element  40 . Delay element  40  is adapted to have exactly the same delay as the optical delay between the modulators. In this manner, the low-skew output of the shift register can be used to best advantage. The delay also ensures that modulators M 1  and M 2  are independently activated (gated) precisely when optical signal  60  is passing through the particular modulator. 
     Thus, optical signal  60  traveling on input optical fiber  26  is first modulated by modulator Ml, thereby creating once-modulated optical signal  60 ′. The optical signal then travels over optical fiber section  24  to modulator M 2 , which modulates the once-modulated optical signal  60 ′, thereby creating a twice-modulated optical signal  60 ″, which exits modulator system via output optical fiber  28 . The net modulation M NET  imparted to optical signal  60 ″ is given by the sum of the modulations of modulators M 1  and M 2 , with the electrical delay line causing the optical delay to appear as if the modulators are acting at the same time rather than serially. 
       FIG. 3  is schematic diagram illustrating the two voltage signals V 1  and V 2 , along with the corresponding net output modulation M NET  of modulators M 1  and M 2 , showing an example of how four different output modulations are generated based on binary voltage values for V 1  and V 2 . In the example timing diagram, ±V 1  corresponds to phases ±π/4 and ±V 2  corresponds to phases ±3π/4. For voltage combination +V 1 -V 2 , the net modulation MNET is π/4. For voltage combination V 1 +V 2 , the net modulation MNET is π/2. For voltage combination V 2 -V 1 , the net modulation MNET is π/4. For voltage combination −V 1 -V 2 , the net modulation MNET is −π/2. 
     In an example embodiment where the modulation needs to be random, such as in establishing a key between BOB and ALICE in QKD system  10 , random number generator (RNG)  54  sends a random number in the form of a RNG signal S 54  to controller  56 . The random number represented by RNG signal S 54  is received by controller  56 , which then sends an RNG signal S 56  to parallel shift register  50 . RNG signal S 54  goes through controller  56  so that the controller can generate RNG signal S 56  having enhanced randomness relative to RNG signal S 54 . This is accomplished, for example, by XOR-ing the random numbers of RNG signals S 54  with a pseudorandom sequence stored in or provided to controller  56 . 
     In another example embodiment, controller  56  sends control signals S 57  to parallel shift register  50 . In an example embodiment, control signals S 57  correspond to the quantum key established between ALICE and BOB and stored in controller  56 . Control signals S 57  allow for modulator system  20  to encode messages using multi-photon pulses, as opposed to random phase modulation of single-photon-level pulses used to establish the quantum key. 
     With reference again to  FIG. 1 , ALICE and BOB respectively utilize modulator systems  20 A and  20 B to randomly encode quantum signals at Alice and measure the quantum signals at BOB to establish a quantum key between them using known techniques. In an example embodiment where ALICE and BOB have the ability to send non-quantum optical signals  100 , the quantum key is then used to drive modulator system  20 A and/or  20 B as described above (using control signals S 57 ) to encode non-quantum optical signals in order to exchange an quantum-encoded message. 
     While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.