Abstract:
Systems and methods for multiplexing two or more channels of a quantum key distribution (QKD) system are disclosed. A method includes putting the optical public channel signal (SP 1 ) in return-to-zero (RZ) format in a transmitter (T) in one QKD station (Alice) and amplifying this signal (thereby forming SP 1 *) just prior to this signal being detected with a detector ( 30 ) in a receiver (R) at the other QKD station (Bob). The method further includes precisely gating the detector via a gating element ( 40 ) and a coincident signal (PN 1′ ) with pulses that coincide with the expected arrival times of the pulses in the detected (electrical) public channel signal (SP 2 ). This allows for the public channel signal to have much less power, making it more amenable for multiplexing with the other QKD signals.

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/607,540, filed on Sep. 7, 2004. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to and has industrial utility with respect to quantum cryptography, and in particular relates to and has industrial utility with respect to multiplexing different channels of a QKD system onto a single optical fiber. 
     BACKGROUND OF THE INVENTION 
     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) pulsed 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 and 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. 
     The performance of a QKD system can be degraded by noise in the form of photons generated by three different mechanisms. The first is forward Raman scattering, in which frequency-shifted photons are generated and co-propagate with the quantum signal photons. Raman scattering in an optical fiber limits the power that can be put into a single fiber because of a transfer of energy from a high power signal to the single-photon wavelength. 
     The second mechanism is Raman backscattering, in which frequency-shifted photons are generated and propagate in the opposite direction to the quantum signal photons. 
     The third mechanism is Rayleigh scattering, in which photons are elastically scattered back in the opposite direction of the quantum signal photons. 
     The scattering of light in an optical fiber—and particular forward Raman scattering—is problematic in multiplexing the different channels of a QKD system because of the noise it creates in the detection process. 
     Two simple solutions have been proposed to overcome the effects of light scattering in combining different channels onto a single optical fiber. The first solution is to use one fiber for the public discussion channel, possibly the sync channel as well, and a second fiber for the quantum channel. The second solution is to limit the fiber length so that the input power can be reduced, and so the scattering power transfer ratio is lower with the shorter distance. Both of these solutions, while simple, are also unappealing because they are not particularly robust and are ill-suited for a commercially viable QKD system. 
     The prior art relating to multiplexing the different channels associated with QKD includes U.S. Pat. No. 6,438,234 (“the &#39;234 patent”). In the &#39;234 patent, the sync signal is time-multiplexed with the quantum channel. The prior art also includes U.S. Pat. No. 5,675,648 (“the &#39;648 patent). The &#39;648 patent proposes the idea of having a “common transmission medium” (i.e., an optical fiber) for the quantum channel and the public channel, where the public channel also carries a calibration signal. 
     However, the prior art does not address the daunting problem of combining the relatively strong public and sync channels with the very weak quantum channel. In particular, the &#39;648 patent does not address how the public channel can be multiplexed with the quantum channel in the “common transmission medium” in a way that will not interfere with detecting the single-photons associated with the quantum channel. 
     Also, in the &#39;234 patent, a sample-and-hold type of phase lock loop needs to be implemented to hold the sync timing while working on single photons. However, the difficulties of multiplexing sync and quantum channel are less challenging than the task of multiplexing the public (data) channel and the quantum channel. The &#39;234 patent does not address the issue of transmitting the public channel and the quantum channel over the same optical fiber. 
     The publication “Eighty kilometer transmission experiment using an InGaAs/InP SPAD-based quantum cryptography receiver operating at 1.55 um” by P. A. Hiskett, G. Bonfrate, G. S. Buller, and P. D. Townsend, published in the  Journal of Modern Optics,  2001, vol. 48, no. 13, pp. 1957-1966, suggests an approach to combining the sync and quantum channels. The light from the transmitter laser is split into a quantum signal and a sync signal. The sync signal is sent over a separate fiber and upon entering the receiver is amplified by an erbium doped fiber amplifier (EDFA). After the amplified light signal is converted into electrical signal, the electrical signal is used to gate the receiver&#39;s detector. 
     It would be desirable to wavelength-multiplex 10 MHz Ethernet public discussion traffic (i.e., the public channel) onto the same fiber as the sync and quantum channels. However, the optical power of the Ethernet public channel signal must be significantly reduced to prevent scattering and other such interference that reduces the ability to detect the channels. Unfortunately, reducing the public channel power results in an unacceptably low signal-to-noise ratio for the public channel for any QKD system with a satisfactory distance or span. While the use of an optical fiber amplifier (e.g., an erbium-doped fiber amplifier or EDFA) can increase the amplitude of the optical signal and remove the need for a narrow band optical filter, its output will still have a very low signal to noise ratio. 
     SUMMARY OF THE INVENTION 
     The present invention includes systems and methods for multiplexing two or more channels of a quantum key distribution (QKD) system. The systems and method result in reduced detection noise for the public channel, thereby allowing weaker public channel signal to be used. Use of a weaker public channel signal enables multiplexing the public channel with the quantum channel and/or the sync channel on the same optical fiber for a commercially viable QKD system. 
     An aspect of the invention is a method that includes putting the optical public channel signal in return-to-zero (RZ) format and amplifying this signal just prior to it being detected. The method further includes precisely gating the detector to coincide with the expected arrival times of the pulses in the detected (electrical) public channel signal to reduce the detection noise. The method also includes forming a first signal comprising a train of pulses that is frequency-locked with the electrical public channel signal, and then applying a selective delay to the first signal so that the first signal coincides (in time) with the electrical public channel signal. The first signal is then used to gate the detector. 
     This method serves to drastically reduce the noise associated with the detection of the public channel, which in turn allows for a weaker public channel signal to be used. Use of a weaker public channel signal is what enables multiplexing the public channel with the quantum channel and/or the sync channel on the same optical fiber. 
     The method is generally applicable to detecting a weakened Ethernet signal that would otherwise be difficult to detect. 
     Another aspect of the invention is a method of combining a public channel signal (SP 1 ) of a first wavelength and a quantum channel signal (SQ) of a second wavelength on an optical fiber connecting first and second quantum key distribution (QKD) stations (Alice and Bob). The method includes providing signal SP 1  in a return-to-zero (RZ) format and multiplexing and transmitting signals SP 1  and SQ from Alice to Bob. The method also includes wavelength-demultiplexing signals SP 1  and SQ at Bob and optically amplifying signal SP 1  to form an optically amplified signal SP 1 *. The method also includes detecting signal SP 1 * to create a public channel electrical signal SP 2  and forming from this signal a signal PN 1 ′ that comprises electrical pulses that are frequency-locked and coincident (in time) with signal SP 2 . Finally, the method includes using signal PN 1 ′ to gate the detecting of signals SP 1 *. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmitter-receiver (T-R) system for use with QKD stations of a QKD system in order to effectively transmit the public channel along with the quantum channel and/or the sync channel on the same optical fiber; 
         FIG. 2  is a schematic diagram of a QKD system with two QKD stations Alice and Bob showing how the T-R system is used in a QKD system, wherein Alice has a transmitter T 1  and a receiver R 2 , and Bob has a transmitter T 2  and a receiver R 1 , so that two-way communication over the public channel is enabled by T 1 -R 1  and T 2 -R 2 . 
     
    
    
     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 
     The present invention relates to quantum cryptography, and in particular relates to systems and methods that allow for select channels of a QKD system, such as the public discussion channel, the synchronization (“sync”) channel and/or the quantum channel, to be multiplexed on to a single optical fiber. In the discussion herein, the quantum channel carries the quantum signal, which is a single-photon optical pulse. The term “single photon” is meant to encompass optical pulses having one photon or less on average. 
     The sync channel as discussed herein carries synchronization data (signals), and optionally, calibration data (signals) that allows for the coordinated operation between the two QKD stations, which are typically denoted as Bob and Alice. 
     Also in the discussion below, the terms “signal” and “pulse” are used interchangeably in a manner that will be apparent to those skilled in the art. Also, the terms “public channel signal” and “quantum signal” are each understood as including one or more pulses, e.g., a train of pulses. 
     For commercial QKD systems, there is a strong desire to use an existing optical fiber to carry two or more of the QKD channels between QKD stations. The present invention enables carrying all three of the above-mentioned channels on a relatively long optical fiber (e.g., 50 km to 100 km) normally associated with a commercially viable QKD system. 
     Note that in a typical QKD system, the two QKD station are referred to as “Alice” and “Bob,” and transmission occurs over the QKD channel in one direction, i.e., from Alice to Bob. However, in connection with an Ethernet public discussion channel, Alice and Bob are identical peers. That is, in order to support the Ethernet-related protocols (e.g. TCP/IP) over the public channel, bi-directional communication is required. This, in turn, means that Alice and Bob each contain a receiver R and a transmitter T, as discussed below. 
     QKD System 
       FIG. 1  is a schematic diagram of an example embodiment of a transmitter-receiver (T-R) system  2  according to the present invention. The T-R system  2  includes a QKD station transmitter T that is coupled to a QKD station receiver R by an optical fiber link FL.  FIG. 2  illustrates how the T-R system is incorporated into a QKD system as two systems T 1 -R 1  and T 2 -R 2  to achieve bi-directional public channel communication, as described in greater detail below. 
     The transmitter T includes three light source systems L 1 , L 2  and L 3  operating at respective wavelengths λ 1 , λ 2  and λ 3 . Light source systems L 1 , L 2  and L 3  are respectively adapted to generate corresponding quantum signal SQ, sync signal SS and public channel signal SP 1 . For example, light source system L 3  is adapted to provide the public channel signal SP 1  in a variety of formats, including return-to-zero (RZ) format. Light source systems L 1 , L 2  and L 3  are optically coupled with and wavelength-multiplexed onto fiber link FL via a wavelength-division multiplexer  5 . 
     In an example embodiment, the transmitter T includes an RZ encoder  6  that accepts an industry-standard 10 MHz Ethernet Manchester-encoded signal SE from an Ethernet port EP 1 . RZ encoder  6  converts signal SE to narrow, low-duty-cycle pulses S 6 . Signals S 6  are then used to drive light source system L 3  in order to generate relative low-power optical public channel signals SP 1  that have a 10-MHz-Ethernet RZ format. 
     With continuing reference to  FIG. 1 , receiver R includes a wavelength-division demultiplexer  8  optically coupled to optical fiber link FL. Demultiplexer  8  is adapted to separate optical signals SQ, SS and SP 1  with wavelengths λ 1 , λ 2  and λ 3 , into separate optical paths, e.g., separate optical fiber sections. The two optical paths associated with quantum signal SQ at wavelength λ 1  and sync signal SS at wavelength λ 2  are indicated by  9 . The third optical path associated with the public channel signal SP 1  and wavelength λ 3  is indicated by  10 . 
     Note that in  FIG. 1 , the details of quantum channel and the sync channel apparatus are not shown in transmitter T and receiver R and because they are not critical to the understanding of the invention and are based on known art. 
     T-R system  2  of  FIG. 1  further includes along optical path  10  (e.g., optical fiber section  10 ) downstream of wavelength-division multiplexer  8  an optical amplifier  20 , such as an erbium-doped fiber amplifier (EDFA). Optical amplifier  20  is adapted to optically amplify optical public channel signal SP 1  to form an amplified optical public channel signal SP 1 * just prior to or soon after signal SP 1  enters receiver R. Optical amplifier  20  is shown within the receiver in  FIG. 1  for the sake of illustration. 
     Downstream of optical amplifier  20  is a detector  30  (e.g., a PIN photodiode) operably coupled to the optical amplifier, and a gating element (“gate”)  40  (i.e., a fast on-off switch) downstream and operably coupled to detector  30 . The output of gate  40  is coupled to a filter  50 , which in the present example is a 10 MHz narrow-bandpass filter. 
     The output of filter  50  is operably coupled to one input of a high-speed comparator  60 . The other input of comparator  60  is provided with a threshold signal ST. The output of comparator  60  is coupled to a multi-vibrator  65  (e.g., a one-shot or mono-stable multi-vibrator). The output of multi-vibrator  65  is coupled to a variable delay  70 , which is controlled by a programmable controller  80  operatively coupled to the delay. In an example embodiment, controller  80  includes a field-programmable gate array (FPGA). The output of variable delay  70  is also coupled to gate  40  via line  72 . 
     One of the outputs from variable delay  70  is connected to one input port of a multiplier  90 , while an input line  82  is coupled to the other multiplier port. Line  82  carries the public channel signals (pulses) SP 2  that make it through gate  40 , as discussed below. The output of multiplier  90  is sent to the input of a low-pass filter  100 , whose output is connected to an input of controller  80 . Controller  80  then controls the variable delay  70 , which has an output to gate  40 . 
     As mentioned above, in an example of the present invention, public channel signal SP 1  is a 10 MHz Ethernet Manchester-encoded data stream re-coded into an RZ format with very narrow RZ pulses. This allows the output of optical amplifier  20  to be gated (or enabled) via variable delay  70  to the multiplier ( 90 ) only when the RZ pulses might be present. The presence of a narrow pulse represents a data bit of “1” and the lack of a narrow pulse represents a “0”. The narrow RZ pulses occur only on the Ethernet 10 MHz square wave edges. 
     The gating of the optical amplifier output significantly reduces the noise in the public channel signal detection process. However, such gating requires that the time slots where the narrow RZ pulses occur be known. Fortunately, the frequency of the public channel signals is known to within 100 PPM (Parts Per Million), as is consistent with the IEEE 802.3 standard. This information is used to produce the required detector gating signal in the manner described below. 
     Method of Operation 
     An example embodiment of the present invention uses a non-return-to-zero (NRZ) Manchester-encoded and industry-standard 10 MHz Ethernet signal and converts it to an RZ format using RZ encoder  6 . The resulting RZ public channel signal SP 1  is then sent over the public channel, as mentioned above. Public channel signal SP 1  is multiplexed with the quantum and sync channel signals SQ and SC via multiplexer  5 , and sent over to receiver R via optical fiber link FL. The public channel signal S 1  is then demultiplexed from the quantum signal and sync signals (not shown) by demultiplexer  8  and is amplified by optical amplifier  20  to form amplified public channel signal SP 1 *. The amplified signal SP 1 * is then detected by detector  30 , which converts this signal into a corresponding electrical public signal SP 2 . 
     The electrical public signal SP 2  passes through gate  40  (whose operation is discussed below) and travels to filter  50  (e.g., a 10 MHz bandpass filter). Filter  50  creates a (10 MHz) sine-wave signal S 3  that is frequency-locked to the incoming Ethernet RZ data (i.e., electrical public signal SP 2 ). 
     High-speed comparator  60  receives sine-wave signal S 3  at the “+” input and the threshold signal ST at the “−” input, and converts signal S 3  to a (10 MHz) square wave signal S 4  at the comparator output. The square-wave signal S 4  then passes to multi-vibrator  65 , which converts the signal to a train of narrow electrical signals (pulses) PN 1 . The pulse width of multi-vibrator  65  is preferably as great or slightly greater than the width of signal SP 2  that travels through gate  40 . 
     Pulses PN 1  enter delay  70 , whose delay is selectively controlled by programmable controller  80 . It is the job of controller  80  to impart a selective delay to pulses PN 1  so they fall directly on top of (i.e., are coincident in time with) the incoming narrow Ethernet RZ signals SP 2 . For the sake of clarity, the train of selectively delayed pulses created by delay  70  are referred to as signal PN 1 ′. 
     Signal PN 1 ′ from variable delay  70  is multiplied with the incoming RZ Ethernet pulses (i.e., electrical public channel signal SP 2 ) from input line  82  at multiplier  90 . Multiplier  90  creates a cross-correlation function signal SC from the two multiplier input signals. Signal SC is provided to controller  80  through a low-pass filter  100 . In an example embodiment, it is assumed that controller  80  makes changes to the delay values slowly, because quick changes could result in closed-loop instability. The controller only needs to initially acquire and then track the input pulse train (i.e., signal SP 2 ), neither of which requires a quick response. The low pass filter  100  removes all of the high-speed information which is of no value and that could destabilize the system. Also, note that statistically half of the RZ signal SP 2  (e.g., Ethernet RZ pulses) are missing (for logic “0&#39;s”); the low pass filter is need to “smooth over” these gaps. 
     In an example embodiment, an analog-to-digital (AD) converter  101  is arranged between low-pass filter  100  and controller  80  to create digital signals SC from analog signals SC in the case where controller  80  is a digital device. 
     Based on the information in signal SC, controller  80  controls variable delay  70  via a control signal S 5  to form coincident signal PN 1 ′. Signal PN 1 ′ is sent over line  72  to control the operation of gate  40 . In other words, coincident signal PN 1 ′ is used as a gating signal to control the operation of gate  40 . 
     If the output signals (pulses) PN 1 ′ from variable delay  70  and the RZ Ethernet pulses SP 2  are in phase, then the multiplier output signal SC will be at a maximum. In an example embodiment, the cross-correlation of multiplier  90  is averaged over a time period greater than a 10 MHz clock period (100 nanoseconds). 
     When signal SC is maximized, the pulses in delay output signal PN 1 ′ coincide with the Ethernet RZ pulses SP 2 . Controller  80  can therefore send these coincident pulses over line  72  to gate  40  to enable the gated detection of the optically amplified electrical public channel signal SP 2 . 
     If, during the gating signal PN 1 ′ at line  72 , a pulse is found at the output (line  82 ) of gate  40 , then the result is an Ethernet logical “1”. If, during the gating signal PN 1 ′ at line  72 , no pulse is found at the output of the gate, then the result is an Ethernet logical “0”. The train of Ethernet logical “1&#39;s” and “0&#39;s” are then serially combined to produce a Manchester-encoded signal SP 2  that can be processed by standard, commercially available Ethernet integrated circuits. 
     The conversion from the narrow RZ pulses to the wide Manchester-encoded pulses required by the 10 MHz Ethernet standard is performed by a decoder  110  coupled to the output of gate  40 . This describes the required receiver. Decoder  110 , in turn, is coupled to an Ethernet port EP 2  or other like device. 
     Bi-Directional Public Channel Communication 
       FIG. 1  shows an example of a T-R system  2  by which Manchester-coded public channel data flows from transmitter T to receiver R as signal SP 1 . However, for bi-directional operation of the public channel, another set of transmitters and receivers is needed to carry data the other way. 
     Accordingly,  FIG. 2  is a schematic diagram illustrating an example implementation of a QKD system with QKD stations Alice and Bob, each having a transmitter T and a receiver R as illustrated in  FIG. 1 , thereby enabling Alice and Bob have bi-directional public channel communication. Specifically, Alice has a transmitter T 1  and a receiver R 2 , and Bob has a transmitter T 2  and a receiver R 1 , so that two T-R systems—T 1 -R 1  and T 2 -R 2  are present. 
     Alice is coupled to Ethernet port EP 1  while Bob is coupled to Ethernet port EP 2 . In the example QKD system of  FIG. 2 , the quantum signal SQ and the sync signal SC travel in one direction from Alice to Bob, while the public channel signal SP 1  travels bi-directionally from Alice to Bob and from Bob to Alice. 
     With reference again to  FIG. 1 , in an example embodiment, controller  80  in receiver R includes programmable logic (e.g., a logic-programmed FPGA) adapted to determine the peak (maximum) of the averaged cross-correlation function signal SC. Once the peak is found, it keeps the delay matched to the incoming pulse train PN 1  so that the peak is maintained over time, even in the face of varying influences such as temperature fluctuations. 
     The coincident gating of the detection of the public channel signal serves to drastically reduce the amount of noise in the public channel detection process. This allows the optical power level of the optical public channel signal SP 1  to be reduced to the point that it can coexist on the same optical fiber as the quantum and/or the sync channels. 
     The present invention is described above in connection with a 10 MHz Ethernet signal as an example embodiment of public channel signal SP 1 . However, the present invention is applicable to most any kind of data transmission at most any data rate. For instance, Sonet, 100 MHz Ethernet, 1G Ethernet, etc. would all apply. Also mentioned above is Manchester encoded data, but the present invention is not so limited and would apply, for example, to 8B/10B coding and most any other type of coding. 
     Further, the present invention is generally applicable to QKD systems, (including one-way and two-way QKD systems) and generally to telecommunication applications. In particular, although the method is eminently suited for multiplexing weak (single-photon) optical signals with relatively strong Ethernet optical signals, it can be applied to cases involving Ethernet signals only. The present invention can be applied to situations wherein a standard Ethernet signal has to travel longer distances than anticipated, resulting in having to detect a relatively weak Ethernet signal. The present invention can thus be used to increase the detectability of the weakened Ethernet signal without the need for amplification. 
     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.