Abstract:
Methods and systems for generating calibrated optical pulses in a QKD system. The method includes calibrating a variable optical attenuator (VOA) by first passing radiation pulses of a given intensity and pulse width through the VOA for a variety of VOA settings. The method further includes resetting the VOA to maximum attenuation and sending through the VOA optical pulses having varying pulse widths. The method also includes determining the power needed at the receiver in the QKD system, and setting the VOA so that optical pulses generated by the optical radiation source are calibrated to provide the needed average power. Such calibration is critical in a QKD system, where the average number of photons per pulse needs to be very small—i.e., on the order of 0.1 photons per pulse—in order to ensure quantum security of the system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to quantum cryptography, and in particular relates to calibrating the intensity of optical pulses in quantum key distribution (QKD) systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     Quantum cryptography involves exchanging messages between a sender (“Alice”) and a receiver (“Bob”) by encoding a plain text message with a key that has been shared between the two using weak (e.g., 0.1 photon on average) optical signals (pulses) transmitted over a “quantum channel.” Such a system is referred to as a quantum key distribution (QKD) system. The security of QKD systems is based on the quantum mechanical principal that any measurement of a quantum system 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. Because only the key is transmitted in a QKD system, any information about the key obtained by an eavesdropper is useless if no message based on the key is sent between Alice and Bob.  
         [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). A “one-way” QKD system is described in U.S. Pat. No. 5,307,410 to Bennet (the &#39;410 patent). A two-way (i.e., folded) QKD system is described in U.S. Pat. No. 6,438,234 to Gisin.  
         [0004]     A crucial aspect of creating a commercially viable QKD system is ensuring that the optical pulses sent over the quantum channel have a known intensity. The average number of photons in a given pulse needs to be set to a known quantity, and needs to be less than one. To achieve such low-intensity pulses, a light source (e.g., a laser) is used to emit relatively. high-intensity pulses, and an optical attenuator is used to attenuate the pulses down to the single-photon level.  
         [0005]     While people generally understand that optical pulses need to be attenuated in QKD, the practical aspects of performing the needed attenuation tend to remain unappreciated and overlooked. In the quantum cryptography literature, when an attenuator is included as part of a QKD system, its operation is not described in any significant detail. This is because it is generally assumed that prior art attenuation methods, such as those used in optical telecommunications, can be directly applied to QKD systems to achieve optical pulse calibration.  
         [0006]     Such assumptions may be true for experimental or prototype. QKD systems, where the precise intensity of the pulses is not a major concern and the instrumentation is in a very well controlled environment. However, for a commercially viable QKD system, it is crucial that the optical pulses have well-controlled intensities in order to create a select number of photons per pulse on average (e.g., 0.1 photons per pulse.) over a long period of time, and under a wider set of environmental factors. If the pulses are too strong, they will no longer be at the single-photon level and the security of the QKD is compromised. On the other hand, if the pulses are too weak, then, many pulses will go undetected, which reduces the key transmission rate.  
         [0007]     A laboratory QKD system can be tuned to each individual test setup. A commercially viable QKD system will have sources of loss that arise from a number of internal and external factors, such as quantum channel inherent loss, environmental effects, fiber splices, fiber type and length, etc., that are different for each installation. This makes the process of providing pulses with a well-defined, small intensity quite daunting—to the point where the prior art methods for attenuating optical signals used in other optical technologies are not applicable to a high-performance, commercially viable QKD system.  
         [0008]     In addition, the self-discovery aspect of setting up a commercial QKD system is simplified by the ability to provide both strong and weak optical pulses. The use of stronger optical pulses for self-calibration and set-up of a QKD system is currently neglected in the prior art.  
       SUMMARY OF THE INVENTION  
       [0009]     The calibration systems and methods of the present invention take into account the low power levels (i.e., small average number of photons per pulse), and variations in the optical pulse width of the pulses used in QKD systems.  
         [0010]     A first aspect of the invention is a method of generating calibrated optical pulses for a quantum key distribution (QKD) system. The method includes generating first optical pulses having a fixed pulse width and a fixed power using an optical radiation source, and passing the first pulses through a variable optical attenuator (VOA) for different VOA settings. The transmitted powers of the first optical pulses are related to the respective VOA settings and the information is stored in the controller, e.g., as a look-up table. The method also includes setting the VOA to a maximum attenuation by operation of the controller, generating second optical pulses having varying pulse widths using the optical radiation source, and sending the second pulses through the VOA. The method further includes relating, respective transmitted powers of the second optical pulses to the respective varying pulse widths and storing the results in the controller. The method additionally includes determining an amount of average power needed to be incident a receiver of the QKD system, setting the VOA to a calibrated setting that would result in the receiver receiving the needed amount of average power via third radiation pulses, and then sending the third optical pulses from an optical radiation source through the VOA to create a calibrated set of optical pulses.  
         [0011]     A second aspect of the invention is a calibrated QKD system, which can be a one-way system or a two-way (autocompensated) system. The system comprises first and second stations optically coupled via an optical channel, and optical radiation source located in the first station and capable of generating optical pulses that travel in the optical channel between the stations. The system also includes a variable optical attenuator (VOA) arranged in the first station for a one-way system or in the second station for a two-way system. The system also includes a VOA driver operatively couple to the VOA, and an electrical meter operatively coupled to the VOA. A controller is operatively coupled to the VOA, the VOA driver, the optical radiation source and the electrical meter. The VOA is automatically set by the controller using a calibration table stored in the controller, and an average amount of power expected at the receiver. The receiver is located in the second station for a one-way system and is located in the first station for a two-way system. The result is the production of calibrated optical pulses from the optical pulses output by the optical radiation source. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a schematic diagram of a QKD system that includes a variable optical attenuator system, as configured for calibrating the variable optical attenuator;  
         [0013]      FIG. 2  is a flow diagram of the method of calibrating the attenuator system in the QKD system of  FIG. 1 ;  
         [0014]      FIG. 3  is a flow diagram of the method of generating calibrated optical pulses in the QKD system of  FIG. 1  using the calibrated attenuator system therein;  
         [0015]      FIG. 4  is a flow diagram of the method of ensuring continued calibration of the optical pulses in the QKD system of  FIG. 1  during system operation; and  
         [0016]      FIG. 5  is a schematic diagram of a two-way QKD system to illustrate that the method of calibration is general and applies to both one-way and two-way QKD systems. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The present invention is a system and method for optical pulse calibration in a QKD system. The systems and methods apply to both one-way and two-way systems. For the sake of convenience, the invention is first described in connection with a one-way system.  
         [0018]      FIG. 1  is a schematic diagram of a QKD system  10  having a first station Alice and a second station Bob. Alice and Bob are optically couple via an optical channel  16 , which may be an optical fiber or free space. Optical channel  16  includes first and second optical channel portions  16 A and  16 B connected by a connector  18 . Channel  16 A has an end  20  and channel  16 B has an end  22 . Connector  18  allows for the optical-channel to be separated downstream of a VOA (discussed below) and accessed in order to perform the calibration procedures of the present invention, as described below. In  FIG. 1 , optical channel  16  is shown disconnected at coupler  18 .  
         [0019]     Alice includes an optical radiation source  30  capable of generating optical pulses  32 . Optical radiation source  30  is capable of controlling the pulse widths w and pulse rate r of optical pulses  32 . In an example embodiment, optical radiation source  30  is a gain switched communications laser. In an example embodiment, the pulse widths of optical pulses  32  can range between 10 ps and 10 ns and the pulse rate varies from 100 kHz to 20 MHz.  
         [0020]     A variable attenuator (VOA)  40  is optically coupled to the optical radiation source and is arranged to receive and selectively attenuate optical pulses  32  to form attenuated pulses  32 ′. A driver  44  is operatively connected to VOA  40 . Driver  44  drives or otherwise sets VOA  40  to a select level of attenuation A i  within the range of possible attenuations of the VOA. In an example embodiment, VOA  40  includes a no-attenuation or a substantially no-attenuation setting.  
         [0021]     In example embodiments, VOA  40  is any one of a number of known VOAs, such as an electronically controlled LCD shutter or a mechanically controlled coupler, such as an optical fiber coupler that sets the alignment between to optical fibers to correspond to a given level of attenuation.  
         [0022]     In system  10 , it is convenient to identify a VOA calibration system  60 , which includes VOA  40  and driver  44 . VOA calibration system  60  also includes an electrical meter  50  connected to VOA  40  to measure the electrical feed back from the VOA.  
         [0023]     VOA calibration system  60  further includes an optical power meter  70 , temporarily coupled to channel portion end  20 , for measuring optical power (e.g., watts W) or intensity (Watts/cm 2 ) of optical radiation incident thereon. Power meter  70  need not be a single-photon detector. By measuring the power of the pulses with no attenuation, and measuring the attenuation with a strong pulse sent through the attenuator, the single-photon level power can be calculated without the sensitive equipment ordinarily required to make single-photon level measurements. This is particularly important because single-photon detectors only detect the arrival of a photon (as opposed to the actual number of photons) in given time interval.  
         [0024]     In an example embodiment, a single-photon detector  74 , which is internal to Alice and coupled (e.g., spliced) to optical channel portion  16 A, is used rather than a separate power meter  70 . The internal single-photon detector  74  can also be used during system operation to double check, that the calibration has not been adjusted either by accident or maliciously to leak information by creating multiple-photon optical pulses. In the case where single-photon detector  74  is used, optical pulses  32  need to be reflected so that they pass back through VOA  40 . This can be accomplished by replacing power meter  70  with a mirror, or by keeping optical channel  16  intact and reflecting the pulses back from a mirror (not shown) located within Bob.  
         [0025]     VOA calibration system  60  also includes a controller  80 , which also controls the operation of Alice. Controller  80  is operatively connected to optical radiation source  30 , VOA driver  44 , electrical meter  50 , detector  74 , and power meter  70 , and controls the operation of these components. Controller  80  is also coupled to a controller  80 ′ at Bob via a timing/synchronization link  84  so that the operation of the QKD system is synchronized between the two stations. In this sense, controller  80  and controller  80 ′ can be considered as a single controller. Controller  80 ′ is coupled to a detector  82  located in Bob that detects the weak optical pulses  32  after they have been polarization-modulated or phase-modulated by phase modulators PM and PM′ located in Alice and Bob, respectively.  
         [0026]     Thus, to summarize, attenuator  60  includes VOA  40 , driver  44 , electrical meter  50  and controller  80 .  
         [0027]     With continuing reference to  FIG. 1  and also to  FIG. 2  and flow diagram  200  therein, the general method of the present invention is now described. In  202 , optical channel  16  is disconnected and power meter  70  is optically coupled to channel portion  16 A at end  20 . In  204 , controller  80  sends a control signal to driver  44 , which in turn communicates with VOA  40  to set the VOA to its maximum attenuation A MAX . In  206 , controller  80  sends a control signal to optical radiation source  30  which sets the optical power output to a high, fixed power (E.g., maximum power P MAX ) and sets the pulse width w to obtain repeatable measurements on optical power meter  70 . Thus, the pulses emanating from the optical radiation source have maximum power, P MAX  and thus the maximum number of average photons per pulse m MAX .  
         [0028]     In  208 , VOA  40  is adjusted (e.g., swept or stepped) over a range of attenuation, e.g., from its maximum attenuation A MAX  to its minimum attenuation A MIN . In  210 , as VOA  40  is adjusted, the output optical power P T  of the optical pulses  32 ′ transmitted by VOA  40  is measured by power meter  70  for each VOA setting. Power meter  50  produces electrical signals corresponding to the measured power. The electrical signals are sent to controller  80 . Also in  210 , the electrical feedback from VOA  40  as measured by electrical meter  50  and that corresponds to the VOA settings is sent to controller  80  via electrical signals. Further in  210 , the information, in the electrical signals corresponding to the measured optical power transmitted by the VOA and the VOA settings are stored (recorded) in controller  80 .  
         [0029]     In  212 , controller  80  generates a table or curve that relates the relative power transmitted by the VOA  40  to the VOA position or setting. In  214 , controller  80  sends a control signal to driver  44  that causes driver  44  to set VOA  40  to its maximum attenuation A MAX . In  216 , controller  80  sends a control signal to optical radiation source  30  to cause the optical radiation source to emit optical pulses that vary in pulse width w over a range of pulse widths that vary from a minimum to a maximum usable pulse width.  
         [0030]     In  218 , power meter  70  receives and measures (detects) the optical pulses  32 ′ and sends electrical signals to controller  80  that correspond to the detected power P T  for each of the optical pulses. In an example embodiment, the pulse rate r is higher than that used in QKD system  10  since the QKD system rate is limited by the single-photon detectors, not the optical radiation source This raises the average power level so that a better measurement of the power in the optical pulses  32 ′ can be obtained by the power meter. Also in  218 , the information in signals from power meter  70  is recorded (stored) in controller  80 .  
         [0031]     In  220 , controller  80  generates a calibration table or curve that relates the optical pulse width w to the corresponding power level P T  measured for the optical pulses. In practice, the optimal (best) pulse width depends on the system operating conditions.  
         [0032]     In  222 , controller  80  calculates the greatest amount of attenuation A G  that might be required for a given system configuration or set of operating conditions. In an example embodiment, a fixed attenuator  40 F (dotted line,  FIG. 1 ) having a known attenuation, is added in series with VOA  40  to ensure that all system configurations can be met with the appropriate amount of attenuation in view of the possible adjustment range of the optical radiation source.  
         [0033]     Once step  222  is carried out, the calibration of attenuator system  60  needed to perform pulse calibration is complete. In  224 , optional fixed attenuator  40 F is removed, power meter  70  is disconnected from optical channel portion  16 A, and optical channel portions  16 A and  16 B are connected (e.g., using connector  18 ) to form an unbroken optical channel  16  between Alice and Bob.  
         [0034]      FIG. 3  is a flow diagram  300  of the method of using QKD system  10  to generate optical pulses having a desired average number of photons per pulse m (i.e., “calibrated optical pulses”) by using calibrated attenuator system  60 .  
         [0035]     In  302 , an average power P A  desired at the receiving detector  82  is decided upon. This average power may be, for example, the lowest power that can be consistently detected. The average power P A  depends on the pulse repetition rate r, wavelength λ of optical radiation emitted by optical radiation source  30 , and the desired average number of photons per pulse m, where m is typically less than 1, and further, is typically about 0.1  
         [0036]     In  304 , the average power P′ A  needed in each optical pulse outputted by optical radiation source  30  to achieve the desired average power P A  at receiving detector  82  is calculated, taking into account the system attenuation, losses, and the pulse width w of each pulse. In  306 , the amount of attenuation A i  needed to be added by VOA  40  to achieve the desired amount of average power P A  (or the desired average number of photons m) in each optical pulse  32 ′ at receiver  82  is calculated. In  308 , controller  80  directs driver  44  to set VOA  40  to the needed amount of attenuation A i  based on the calibration data (i.e., table or curve) as determined using the method illustrated in flow diagram  200  of  FIG. 2 . At this point, system  10  is set up to generate optical pulses  32 ′ have a well-defined (i.e., calibrated) average number of photons per pulse m C .  
         [0037]      FIG. 4  is a flow diagram  400  of a method according to the present invention of ensuring that the optical pulses remain calibrated during the operation of system  10 . In  402 , the average power P A  per optical pulse or average number of photons per optical pulse m is measured. This can be done in one of two preferred ways. In a first example embodiment, optical channel  16  is disconnected and power meter  70  is connected to end  20  of optical channel  16 A. This approach is used to measure the average power P A . In a second example embodiment, optical channel  16  is not disconnected and the average power in the optical pulses are measured by the (single-photon) receiving detector  82  at Bob or single-photon detector  74  in Alice. The second example embodiment is preferred in situations where QKD system  10  needs to stay intact or where it is otherwise advantageous not to disconnect optical channel  16 .  
         [0038]     In  404 , if the measured average power P A  or average of number of photons m differs from a desired (e.g., previously calibrated) value P D  or m D , then one or more of the following adjustments are made: (a) increasing or decreasing the integration time T I  of receiving detector  82 , (b) increasing or decreasing the pulse repetition rate r, (c) increasing or decreasing the optical pulse width w, and (d) increasing or decreasing the attenuation provided by VOA  40  by a select amount in accordance with the calibration table or curve stored in controller  80 .  
         [0039]     In  406 , the average number of photons per pulse m or average power P A  is measured after the one or more adjustments in  404 . In  408 , the measurements obtained in  406  are compared to a threshold value P TH  or m TH  for the average number of photons per optical pulse m (e.g., m TH =1 photon per pulse), above which the security of the transmitted keys in the QKD system is deemed to be compromised.  
         [0040]     In  410 , if the threshold value m TH  or P TH  is exceeded, an error condition is declared and any bits associated with the threshold violation are not used in the key. This error alarm function is correlated against the system measurement activity to ensure false alarms are not given.  
         [0041]     In  412 , if m&lt;m TH  has the calibrated value m C  (or if PA has the calibrate average power value P C ), then the re-calibration process is terminated. If m≠m C  (or P A ≠P C ), then the process returns to  404  and is repeated until m=m C  (or P A =P C ).  
         [0042]     It will be apparent to one skilled in the art that the above-described method applies to both one-way and two-way QKD systems.  FIG. 5  is a schematic illustration of a two-way QKD system  500 , such as described in U.S Pat. No. 6,438,234 to Gisin. In system  500 , Bob&#39;s optical radiation source  30  sends Alice two unmodulated optical pulses, which both reflect from a Faraday mirror FM at Alice. One pulse is then randomly phase-modulated by Alice by PM 1  on its way back to Bob, whereupon Bob phase encodes the remaining unmodulated pulse with his phase modulator PM 2 . The pulses are then combined (interfered) at Bob and detected to ascertain the phase differences in the two interfered pulses.  
         [0043]     For the purposes of optical pulse calibration, the only significant difference from a one-way system is that the VOA  40  is located at Alice, while the optical radiation source  30  is located at Bob. Thus, in an example embodiment, in a two-way system, the Faraday mirror at Alice is replaced with power meter  70 , and the calibration carried out using Alice&#39;s controller  80 ′ and/or Bob&#39;s controller  80 .  
         [0044]     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.