Patent Publication Number: US-2006018475-A1

Title: Kd systems with robust timing

Description:
CLAIM OF PRIORITY  
      This patent application claims priority from U.S. Provisional Patent Application No. 60/445,805, filed on Feb. 7, 2003. 
    
    
     TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to quantum cryptography, and in particular relates to quantum key distribution (QKD) systems with robust timing systems and methods for performing QKD.  
     BACKGROUND ART  
      Quantum key distribution (QKD) 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 principal 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 inherently 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). A specific QKD system is described in U.S. Pat. No. 5,307,410 to Bennet (the &#39;410 patent).  
      The Bennett-Brassard article and the &#39;410 patent each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization of single photons, and Bob randomly measures the polarization of the photons. The one-way system described in the &#39;410 patent is based on a two-part optical fiber Mach-Zehnder interferometer. Respective parts of the interferometer 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. The &#39;410 patent discloses a separate “timing channel” to convey timing signals from a sender to a receiver. However, the timing systems and methods necessary for practical operation the system are not disclosed in the &#39;410 patent.  
      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 is autocompensated for polarization and thermal variations.  
      While the two-way QKD system of the &#39;234 patent has certain advantages over a one-way system, this system is like the &#39;410 system in that it cannot operate without a timing system that synchronizes the sending and receiving of optical pulses. However, as with the &#39;410 patent, such a timing system is not disclosed in the &#39;234 patent.  
      U.S. Pat. No. 5,675,648 (the &#39;648 patent) to Townsend discloses a QKD system that uses a “common transmission medium” for the quantum and public channels. The &#39;648 patent includes a description of a timing system that employs a system clock to avoid timing errors in transmitting and detecting a weak optical pulse. The timing function is performed during calibration of the interferometer. With reference to  FIG. 4  of the &#39;648 patent, the amplified output from the public channel detector is input into a clock registration module. This module contains an electronic filter that produces an oscillating signal at the pulse repetition frequency which is used to lock a local oscillator to the optical source or master clock frequency. This local oscillator is then used to provide the timing information required by the receiver during the quantum transmission stage of the protocol. Each time the transmitted system is recalibrated via the public channel, the local oscillator is re-timed to avoid the accumulation of any timing errors.  
      Thus, the timing system of the &#39;648 patent operates in a switched mode rather than in a continuous mode, which is not an efficient way to maintain timing synchronization and control jitter in the timing signal. Also, the system is designed so that the timing is controlled by only one of the stations. Further, the system as designed is not programmable to operate in the variety of operational modes needed in the field. These and other shortcomings of the &#39;648 Patent system are disadvantageous in a commercially viable QKD system.  
      Accordingly, there is a need for robust timing systems and methods that allow for the manufacture and deployment of commercially viable QKD systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a high-level schematic diagram of the symmetric QKD system of the present invention, showing the quantum, classical (data) and timing channels connecting Alice and Bob, along with each QKD station (Bob and Alice) including a quantum channel optics layer (“quantum transceiver”), a public data transceiver, an optical modem, a random number generator unit, and a controller;  
       FIG. 1B  is a schematic diagram similar to  FIG. 1A , but that includes a WDM in each QKD station and a single communication link for the quantum, data and timing channels;  
       FIG. 2A  is a schematic diagram of an example embodiment of the quantum transceiver for Alice for a one-way QKD system;  
       FIG. 2B  is a schematic diagram of an example embodiment of the quantum transceiver for Bob for a one-way QKD system for use with the quantum transceiver of  FIG. 2A ;  
       FIG. 2C  is a schematic diagram of an example embodiment of the quantum transceiver for Bob for a two-way QKD system;  
       FIG. 2D  is a schematic diagram of an example embodiment of the quantum transceiver for Alice for a two-way QKD system for use with the quantum transceiver of  FIG. 2C ;  
       FIG. 2E  is a schematic diagram of an example embodiment of an optical time-domain reflectometer (OTDR) that uses the optical pulse transmitter and quantum transceiver of  FIG. 2C ;  
       FIG. 3A  is a schematic diagram of the optical modems of the timing system of  FIG. 1A , wherein each optical modem has optical circulator, an optical transmitter, an optical receiver and associated phase lock loops, and illustrating the connections between the receive time domains (RTDs) and the transmit time domains (TTDs), as well as the sync signals that travel in each direction over the timing channel connecting the modems;  
       FIG. 3B  is a schematic diagram similar to  FIG. 3A  but illustrating another example embodiment of the modems;  
       FIG. 3C  is a close-up schematic diagram of a portion of the optical modem for Bob as it is used in OTDR mode;  
       FIG. 3D  is a close-up schematic diagram of the phase lock loops connected to an optical receiver in the optical modem;  
       FIG. 4  is a timing diagram illustrating the waveform of the sync signals sent across the optical modem of the timing system, wherein the sync signal includes sync pulses that include timing information as well as synchronization and control data;  
       FIG. 5A  is a schematic diagram of a clock-based digital pulse generator (DPG) implemented in the FPGA of the controller as used to generate trigger pulses for the drivers;  
       FIG. 5B  is a schematic diagram of an example embodiment of a pulse timing generator used in  FIG. 5A  and illustrates how the circuit of  FIG. 5A  is grouped and duplicated for each timing signal;  
       FIG. 6  is a schematic diagram of an example embodiment of an RNG unit that includes multiple data sources and a data source selector coupled to a modulator driver;  
       FIG. 7A  is a schematic diagram of a fine delay block placed after the output of each digital comparator to allow the pulses outputted therefrom to be adjusted in finer steps, and to allow adjustment of output pulse width;  
       FIG. 7B  is an electrical signal diagram that shows the relative timing of the pulses outputted from the digital comparator, the delay blocks, the logic gate and the fine delay block;  
       FIG. 8  is a schematic diagram of an example QKD system suitable for applying the timing systems and methods of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention relates to quantum cryptography, and in particular relates to QKD systems with robust timing systems and methods for performing QKD. QKD systems have industrial utility only to the extent that they can operate and be adjusted in the field and not just in a laboratory or other artificial environments. To this end, it is critical that a commercially viable QKD system be designed to operate in combination with a robust timing system that allows for the synchronous operation of the various elements of the QKD system. In particular, the QKD system needs to operate in the field so that weak quantum signals can be generated and detected in order to exchange a secure key in a variety of real-world environments.  
      An overview of various embodiments of QKD systems according to the present invention is first set forth. This is followed by a more detailed explanation of the structure and operation of example timing systems corresponding to the various modes of operation of the example QKD systems.  
      In the discussion below, “randomly modulating” means randomly selecting a modulation from a select and finite group of possible modulations, such as two or four select phase modulations. Also, “encoding” means imparting a random phase or polarization to a quantum signal.  
      In addition, the term “quantum transceiver” is used to describe the optical layer used to transmit, receive, or both transmit and receive quantum signals over the quantum channel. Further, “quantum signals” are signals that travel over the quantum channel between quantum transceivers. One skilled in the art will understand that in certain instances the quantum signals are relatively strong, while at other times the quantum signals are weak, i.e., less than a photon per pulse, on average.  
      I. QKD System Overview  
      A. Multi Communication Link Embodiment  
       FIG. 1A  is a high-level schematic diagram of a quantum key distribution (QKD) system  10  according to the present invention. System  10  includes two QKD stations, referred to as “Alice” and “Bob.” Alice includes a quantum channel optics layer (“quantum transceiver”)  20 A for preparing, transmitting and/or receiving a quantum signal S 1  sent to or receive from Bob over a quantum channel  24 , which is coupled to Bob. Alice also includes a random number generator (RNG) unit  30 A coupled to quantum transceiver  20 A. RNG unit  30 A provides random numbers to quantum transceiver  20 A so that it can randomly set either the polarization or phase of quantum signal S 1  based on a select set of polarizations or phases.  
      Alice also includes a public data transceiver (PDT)  40 A coupled to a classical (data) channel  44 , which is coupled to Bob. PDT  40 A is adapted to acquire and process classical signals S 2  used to publicly transmit and receive data (e.g., encrypted messages) between Alice and Bob. PDT  40 A is coupled to RNG unit  30 A and to quantum transceiver  20 A.  
      Alice also includes an optical modem unit  50 A coupled to a timing channel  54 , which is also coupled to Bob. Optical modem unit  50 A is adapted to transmit and receive optical signals S 3  sent over timing channel  54  necessary for carrying out the timing operations, described below, and necessary for QKD system  10  to function properly.  
      Alice further includes a controller  60 A which is coupled to quantum transceiver  20 A, RNG unit  30 A, PDT  40 A and optical modem unit  50 A. Controller  60 A is adapted to coordinate the timing of operation of the above-mentioned components, as described below.  
      The basic constitution of Bob corresponds identically to that of Alice, i.e., Bob includes a quantum transceiver  20 B, RNG unit  30 B, PDT  40 B, optical modem unit  50 B and a controller  60 B, all arrange essentially as in Alice. Quantum transceiver  20 B, PDT  40 B and optical modem unit  50 B are coupled to their respective counterparts quantum transceiver  20 A, PDT  40 A and optical modem unit  50 A in Alice, via quantum channel  24 , classical channel  44  and timing channel  54 , respectively. Controllers  60 A and  60 B, optical modem units  50 A and  50 B, and timing channel  54  are collectively referred to herein as the timing system”  70  for system  10 . The symmetry between Bob and Alice has great industrial utility in that it makes it easier to produce a QKD system by having the same layout for each QKD station. In particular, essentially the same circuit boards can be used in Bob and Alice, with the circuit boards being programmable to operate in a number of different modes (e.g., timing controlled by either Bob or Alice, OTD operation as explained below, etc.).  
      With continuing reference to  FIG. 1A , in the operation of system  10 , optical timing signals S 3  and S 3 ′ are exchanged between controllers  60 A and  60 B over timing channel  54 . The optical timing signals S 3  and S 3 ′ are processed via optical modem units  50 A and  50 B, which convert the optical timing signals into corresponding electrical timing signals S 4 A and S 4 B, and vice versa.  
      B. Double and Single Communication Link Embodiments  
      In the example embodiment of QKD system  10  illustrated in  FIG. 1A , all three channels  24 ,  44  and  54  are shown as using different physical communication links—for example, a first optical fiber for the quantum channel  24 , a second optical fiber timing channel  54 , and an Ethernet channel or fiber optic channel or single-wavelength fiber optic channel for the classical data channel  44 . In such a case, quantum channel  24  and timing channel  54  are synchronous channels, while classical data communication channel can be an asynchronous or a synchronous channel.  
      In the example embodiment of QKD system  10  of  FIG. 1B , the various communication channels  24 ,  44  and  54  are optically and/or electronically multiplexed into a single channel  76 . For example, as shown in  FIG. 1B , the optical channels can be combined into a single optical F 1  using respective wavelength-division multiplexers (WDMs)  78 A and  78 B at the output ends of Alice and Bob.  
      By way of example, classical data channel  44  and timing channel  54  are multiplexed into one electrical channel first and then sent through one optical channel, which can be separate single-wavelength optical fiber, and the quantum channel  24  is sent through a separate dedicated optical fiber. This example embodiment is well-suited for the situation when the quantum channel needs to be substantially noise-free, i.e., substantially free from back-scattering or back-reflections that can arise, for example, from Raman and Rayleigh scattering, or from reflections from fiber imperfections and/or splices.  
      C. Example One-Way Quantum Transceiver  
       FIG. 2A  is a schematic diagram of an example embodiment of Alice&#39;s quantum transceiver  20 A, along with Alice&#39;s other components, for use in combination with the example embodiment of Bob and his transceiver  20 B, illustrated in  FIG. 2B  described below.  
      Alice&#39;s quantum transceiver  20 A includes a laser source  100 A, such as a 1.5 μm laser, for generating an initial photon pulse signal S 0 . In an example embodiment, signal S 0  includes one or more light pulses each having hundreds or thousands of photons and having a temporal width of about 400 picoseconds (ps). Quantum transceiver  20 A also includes, in order from the laser source, a variable optical attenuator (VOA)  102 A, an optical delay  110 A, and a phase modulator  112 A. VOA  102  can also be located downstream of phase modulator  112 A or elsewhere in the system. VOA  102  is used to control the intensity of the optical pulse signal S 0  to form a weak quantum signal S 1 , i.e., a signal having, on average, less than a photon, and preferably about 0.1 photon or less. This weak quantum signal S 1  is sent to Bob over the quantum channel  24  ( FIG. 1A ).  
      With continuing reference to  FIG. 2A , Alice&#39;s RNG unit  30 A is also coupled to controller  60 A, which is coupled to a phase modulator driver  112 A-D, which in turn is coupled to a phase modulator  112 A. Controller  60 A is also coupled to VOA  102 A to control the amount of attenuation in creating quantum signal S 1  from signal S 0 . Controller  60  also includes a field programmable gate array (FPGA)  132 A.  
      Also illustrated in  FIG. 2A  is optical modem unit  50 A connected to optional WDM  78 A. Optical modem unit  50 A is as shown in greater detail in  FIG. 2C , and explained in greater detail below in connection with  FIGS. 3A through 3D . The optical modem unit  50 A is an integral part of the timing system used for data communication and clock synchronization between Alice and Bob.  
      Also illustrated in  FIG. 2A  is the connection of PDT  40 A to WDM  78 A. PDT  40 A encrypts data associated with the user and public discussion layer and transmits optically on the fiber F 1  via WDM  78 A. PDT  40 A also receives data split off by the WDM  78 A, and decrypts the data.  
      D. Example One-Way Quantum Transceiver  
       FIG. 2B  is a schematic diagram of an example embodiment of Bob and his quantum transceiver  20 B as used in a one-way QKD system in conjunction with Alice&#39;s quantum transceiver  20 A as shown in  FIG. 2A . In such a system, Alice&#39;s transceiver  20 A acts as a transmitter, and Bob&#39;s quantum transceiver  20 B acts as a receiver. Bob&#39;s quantum transceiver  20 B includes an optical delay  110 B and a phase modulator  112 B. The quantum transceiver also includes a first single-photon detector  114 B coupled to polarization compensator  118 B, a PM beamsplitter  106 B, and a second single-photon detector  116 B coupled to PM circulator  104 B. Single-photon detectors  114 B and  116 B are respectively coupled to discriminators  120 B and  122 B, which are coupled to controller  60 B. RNG unit  30 B is also coupled to controller  60 B, which is coupled to a phase modulator driver  112 B-D, which in turn is coupled to a phase modulator  112 B. Controller  60 B is also coupled to polarization compensator  118 B to correct polarization errors caused by the environmental changes in the transmission medium. Controller  60  also includes a field programmable gate array (FPGA)  132 B.  
      Also illustrated in  FIG. 2B  is optical modem unit  50 B and PDT  40 B each connected to a WDM  78 B similar to that illustrated in  FIG. 2A  in connection with Alice.  
      E. Example Two-Way Quantum Transceiver  
       FIG. 2C  is a schematic diagram of an example embodiment of Bob and his quantum transceiver  20 B as used in a two-way QKD system. For a two-way QKD system, a classical (i.e., non-quantum) optical pulse transmitter is required to transmit light from the receiver to the quantum transceiver (which in this case is Alice).  
      Quantum transceiver  20 B includes a laser source  100 B, such as a 1.5 μm laser, for generating an initial photon pulse signal SO consisting of one or more pulses of light each having hundreds or thousands of photons, and having a temporal pulse width of 400 ps. Quantum transceiver  20 B also includes, in order from the laser source, a variable optical attenuator (VOA)  102 B to control the intensity of the optical pulse signal S 0  to create quantum signal S 1 , which is ultimately introduced into the quantum channel  24  ( FIG. 1A ). Quantum transceiver  20 B also includes a polarization-maintaining (PM) circulator  104 B, a PM beamsplitter  106 B, a 45° polarizer  108 B, an optical delay  110 B, and a phase modulator  112 B. The quantum transceiver  20 B also includes a first single-photon detector  114 B coupled to PM beamsplitter  106 B, and a second single-photon detector  116 B coupled to PM circulator  104 B. Single-photon detectors  114 B and  116 B are respectively coupled to discriminators  120 B and  122 B, which are coupled to controller  60 B.  
      RNG unit  30 B is also coupled to controller  60 B, which is coupled to a phase modulator driver  112 B-D, which in turn is coupled to a phase modulator  112 B. Controller  60 B is also coupled to VOA  102 B to control the amount of attenuation provided by VOA  102  in attenuating photon signal SO to create the quantum signal S 1 . Controller  60  also includes a field programmable gate array (FPGA)  132 B.  
      Also illustrated in  FIG. 2C  is optical modem unit  50 B connected to optional WDM  78 B. Optical modem unit  50 B includes a timing/synch laser (i.e., an optical transmitter)  200 B (e.g., operating at 1.3 μm) and a timing/synch detector (i.e., an optical receiver)  202 B both coupled to a circulator  204 B. Laser  200 B and detector  202 B are coupled to FPGA  132 B in controller  60 B, as explained in greater detail below in connection with  FIG. 3A .  
      F. Example Two-Way Quantum Transceiver  
       FIG. 2D  is a schematic diagram of an example embodiment of Alice&#39;s quantum transceiver  20 A as used in combination with the example embodiment of Bob&#39;s transmitter as illustrated in  FIG. 2C  and discussed immediately above. Alice&#39;s quantum transceiver  20 A includes WDM  78 A, a phase modulator  112 A coupled to a phase modulator driver  112 A-D, and a Faraday mirror  160  arranged optically downstream of the phase modulator. RNG unit  30 A is coupled to controller  60 A, which is coupled to phase modulator driver  112 A-D. Also shown schematically in  FIG. 2D  is optical modem unit  50 A, which in an example embodiment similar to optical modem unit  50 B described above. Optical modem unit  50 A is coupled to WDM  78 A and to controller  60 A. Controller  60  also includes an FPGA  132 A.  
      G. Example OTDR Function With Two-Way Quantum Transceiver  
      An example embodiment of the present invention includes a system for performing optical time domain reflectometry (OTDR).  FIG. 2E  is a schematic diagram similar to Bob&#39;s quantum transceiver  20 B of  FIG. 2C , but that omits components not needed to perform OTDR functions.  
      The initial optical pulse signal SO is generated by laser  100 B and passes through VOA  102 B, the polarization-maintaining (PM) circulator  104 B, a PM beamsplitter  106 B, phase modulator  112 B, and WDM  78 B, and into the fiber F 1  as quantum signal S 1 . Here, quantum signal S 1  is a relatively strong signal as is needed for performing OTDR.  
      A portion of quantum signal S 1  is reflected back from a reflection point RP 1  in fiber F 1 . Reflection point RP 1  may arise from the presence of an element added to optical fiber F 1 , such as an optical tap placed by eavesdropper Eve between Bob and Alice, or from some other source of scattering or reflection in the fiber. Light reflecting from reflection point P 1  passes back to Bob through WDM  78 B. This light then passes through phase modulator  112 B and PM beamsplitter  106 B, where it is split into two orthogonal polarizations. The light continues to one of two single-photon detectors  114 B and  116 B coupled to discriminator  120 B and  122 B, respectively.  
      Single-photon detectors  114 B and  116 B and discriminators  120 B and  122 B are coupled to controller  60 B. Single-photon detectors  114 B and  116 B can be changed to operate in linear mode. Discriminators  120 B and  122 B can be changed to operate in proportional digital data mode to measure higher levels of light with high resolution.  
      Controller  60 B is coupled to VOA  102 B to control the output light intensity of the OTDR, and to optical modem  50 B. The operation of optical modem  50 B in OTDR mode is discussed below in connection with  FIG. 3C .  
      H. General Operation of the Example One-Way QKD System  
      With reference again to  FIG. 2A , laser source  100 A emits an initial light pulse signal S 0 . Light pulse S 0  then travels through VOA  102 A, which attenuates the pulses to a desired level, e.g., from hundreds or thousands of photons, to a level such that the final quantum signal S 1  introduced into the quantum channel has an intensity below a single photon, on average. The attenuated light pulse signal S 0  then travel through an optical delay  110 A which splits signal S 0  into two orthogonally polarized light pulses S 1 A and S 1 B.  
      Pulses S 1 A and S 1 B then pass through phase modulator  102 A. At this point, controller  60 A sends a gating pulse GPA- 1  to phase modulator driver  102 A-D. Gating pulse GPA- 1  is timed so that phase modulator driver  102 A-D causes phase modulator  102 A to randomly modulate the phase one of the S 1 A and S 1 B (say, S 1 B). The phase modulation applied is chosen randomly based on random numbers provided by RNG unit  30 A from a set of phase modulations. The light pulses then pass to WDM  78 A and travel to Bob as optical signal S 1 .  
      In the example embodiment of  FIG. 2B , the pulses travel to Bob by traversing optical F 1 , which carries the quantum channel  24 , the public channel  44  and the timing channel  54 . In the example embodiment of  FIG. 1A , these channels are shown as being separate.  
      In an example embodiment, data on the public (data) channel  44  is transmitted and received by public data transceiver PDT  40 B, which can be transmitted via a commonly available commercial optical transmitter with suitable wavelength to be combined by WDM  78 A at Alice. Also, optical modem  50 A transmits timing information from Alice&#39;s quantum transceiver  20 A to Bob&#39;s quantum transceiver  20 B relating to the transmission of pulses S 1 A and S 1 B, as explained in greater detail below.  
      With continuing reference to  FIG. 2B , the optical pulses S 1 A and S 1 B of quantum signal S 1  arrive at Bob through WDM  78 B and pass through polarization compensator  118 B and phase modulator  112 B. Controller  60 B, based on information received from Alice relating to the generation and transmission of pulses S 1 A and S 1 B, sends a gating pulse GPB- 1  to phase modulator driver  112 B-D timed so that it causes the phase modulator  112 B to modulate the remaining unmodulated pulse (in this case, pulse S 1 A). The phase modulation is randomly selected based on random numbers provided by RNG unit  30 A from a select number of phase-modulations.  
      The phase-modulated pulses S 1 A and S 1 B then enter optical delay  110 B, where they are combined and interfere with one another. The recombined pulse is then detected at single-photon detector  114 B or  116 B, depending on the relative phases imparted to the pulses. In response, single-photon detector  114 B or  116 B generates an electrical signal that passes to the respective discriminator  120 B or  122 B and back to controller  60 B. The timing coordination and synchronization of the gating pulses and other signals used to coordinate the operation of the various elements in the QKD system is discussed below.  
      Once a desired number of quantum pulses SI are exchanged between Bob and Alice, a shared key is derived using known techniques. For example, by Alice and Bob publicly compare the basis of their measurements (e.g., via public channel  44 ) and only keep the measurements (bits) corresponding to the same measurement basis. This forms a sifted key. They then choose a subset of the remaining bits to test for the presence of an eavesdropper (Eve) and then discard these bits.  
      The act of eavesdropping on optical path F 1  by Eve intercepting or otherwise attempting to measure the weak optical pulses being transmitted between Bob and Alice will necessarily introduce errors in the key due to the quantum nature of the photons being exchanged. If there are no errors in the sifted key due to the presence of an eavesdropper Eve, then the transmission is considered secure, and the quantum key is established.  
      I. General Operation of the Example Two-Way QKD System  
      The present invention applies to a two-way autocompensated system as well. Thus, with reference again to  FIG. 2C  and Bob&#39;s quantum transceiver shown therein, laser source  100 B emits initial light pulse S 0 . Light pulse S 0  travels through VOA  102 B, which reduces the intensity of the initial light pulses so that they each have hundreds or thousands of photons. Attenuated light signal S 0  passes through PM circulator  104 B and PM beamsplitter  106 B, and is polarized at 45° by polarizer  108 B. Light pulse S 0  then proceeds to optical delay  108 , which forms two orthogonally polarized pulses S 1 A and S 1 B (making up quantum signal S 1 ) from light pulse S 0 . In this embodiment, quantum signal S 1  heading out from Bob are not quantum signals per se, i.e., they are not optical pulses having a less than a single photon on average. Rather, they are relatively strong pulses having, for example, hundreds or thousands of photons per pulse. However, as mentioned above, it is still referred to as a “quantum signal” herein because it is transmitted over the quantum channel.  
      Pulses S 1 A and S 1 B then pass through WDM  78 B and travel over an optical fiber F 1  to Alice. In the example embodiments of  FIGS. 2C and 2D , optical fiber F 1  carries the quantum channel  24 , the public channel  44  and the timing channel  54 . Data on the public channel  44  can be transmitted on any commonly available commercial optical transmitter with suitable wavelength to be combined by a WDM. Optical Modem  50 A transmits timing information from Alice  20 A to Bob  20 B, as explained in more detail in a later section.  
      With reference again to  FIG. 2D  and Alice&#39;s transceiver  20 A shown therein, the optical pulses S 1 A and S 1 B arrive at Alice through WDM  78 A and pass through phase modulator  112 A to Faraday mirror  160 . Faraday mirror  160  reflects pulses S 1 A and S 1 B and rotates the polarization of each pulse by 90°. Controller  60 A then sends a gating pulse GPA- 1  to phase modulator driver  112 A-D. Gating pulse GPA- 1  is timed so that is causes phase modulator  112 A to randomly modulate one of pulses S 1 A and S 1 B (say, S 1 A) from a set of phase-modulations based on random numbers supplied by RNG unit  30 A. Pulses S 1 A and S 1 B are attenuated to down to single-photon level or below on average by VOA  102 A.  
      With reference again also to  FIG. 2C , the attenuated (i.e., weak) quantum pulses S 1 A and S 1 B then travel back to Bob over optical fiber F 1 , where controller  60 B generates a timing signal in the form of gating pulse GP 1 -B timed to phase-modulate the remaining unmodulated pulse (pulse S 1 B) in the manner described above.  
      The quantum pulses then enter optical delay  110 B, where they are combined and interfere with one another. The recombined pulse is then detected at single-photon detector  114 B or  116 B, depending on the relative phases imparted to the pulses. The single-photon detector that detects the recombined pulse generates an electrical signal that passes to the respective discriminator  120 B or  122 B and back to controller  60 B. The timing coordination and synchronization of the gating pulses and other signals used to coordinate the operation of the various elements in the QKD system is discussed below.  
      Once a desired number of pulses are exchanged between Bob and Alice, a shared key is derived using known techniques. For example, by Alice and Bob publicly comparing the basis of their measurements (e.g., via public channel  44 ) and only keeping the measurements (bits) corresponding to the same measurement basis. This forms a sifted key. They then choose a subset of the remaining bits to test for the presence of an eavesdropper (Eve) and then discard these bits. The act of eavesdropping on optical fiber F 1  by Eve intercepting or otherwise attempting to measure the weak optical pulses being transmitted between Bob and Alice will necessarily introduce errors in the key due to the quantum nature of the photons being exchanged. If there are no errors in the sifted key due to the presence of an eavesdropper Eve, then the transmission is considered secure, and the quantum key is established.  
      J. General Operation of the Example OTDR Mode Using Two-Way QKD System  
      With reference again to  FIG. 2E , an initial optical pulse signal S 0  is generated by laser  100 B. The width of the signal S 0  can be varied from about 400 ps to about 10 ns and the attenuation of the VOA  102 B can be reduced to near zero to generate a strong optical pulse.  
      The pulse S 0  travels through the polarization-maintaining (PM) circulator  104 B, a PM beamsplitter  106 B, phase modulator  112 B (without modulation), and WDM  78 B, and into the fiber F 1 . Light pulse S 0  travels down the fiber F 1  and portions S 1 B the original pulse S 0  return to due to scattering and reflections, generally indicated by one or more reflection points RP 1 . Various pulses S 1 B return from the fiber, varying in amplitude and time delay according to the amount of scattering and reflection and the round trip distance traveled by the returning signals.  
      Light pulses S 1 B travels back through WDM  78 B, phase modulator  112 B, and PM beamsplitter  106 B. The quantum transceiver  20 B includes a first single-photon detector  114 B coupled to PM beamsplitter  106 B, and a second single-photon detector  116 B coupled to PM circulator  104 B. Single-photon detectors  114 B and  116 B are respectively coupled to discriminators  120 B and  122 B, which are coupled to controller  60 B. Single-photon detectors  114 B and  116 B can be changed to operate in linear mode. Discriminators  120 B and  122 B can be changed in mode to give proportional digital data to measure levels of light beyond single photons.  
      Controller  60 B is coupled to detectors  114 B and  116 B, discriminators  120 B and  1220 B, and VOA  102 B to control the amount of light in signal S 0 , which serves to vary the working range of the OTDR. The controller measures the light level and time delay between the outgoing pulse S 0  and returning pulses S 1 B to determine the characteristics of the fiber, e.g., the location of reflection points RP 1 .  
      Optical modem  50 B is used to connect the transmit portion of the OTDR, starting with laser  100 B, to the receiver portion, including discriminators  120 B and  122 B.  
      II. Timing System  
      The above description of the various embodiments of the operation of QKD system  10  presupposes that a timing system is in place to coordinate the sending of the initial quantum signal pulse S 0 , the modulation of pulse S 1 A by Alice&#39;s phase modulator  112 A, the modulation of pulse S 1 B by Bob&#39;s phase modulator  112 B (thereby forming quantum signal S 1 ), and the detection of the combined pulse at either of the single photon detectors  114 B and  116 B.  
      Further, the above description presupposes that the timing system can account for variations (e.g., drifts and jitter) in the timing signals, and account for variations in the arrival times of the quantum signals.  
      The timing system  70  ( FIG. 1A ) is adapted to accomplish the above.  FIGS. 3A through 3C  are detailed schematic diagrams of an example embodiment of the timing system  70  of  FIG. 1A , for the specific cases of a one-way QKD system ( FIG. 3A ), a two-way QKD system ( FIG. 3B ), and an OTDR ( FIG. 3C ).  
      A. Example Timing Operation of One-Way Quantum Transceiver  
       FIG. 3A  is a schematic diagram of an example embodiment of the timing system  70  of  FIG. 1A  that uses two optical circulators and further illustrates the controller clock domains and their connections to the phase lock loops (PLLs) and clocks.  
      With reference to  FIG. 3A , system  70  includes at Alice a circulator  204 B coupled at one port to an optical transmitter  200 A and at another port to an optical receiver  202 A. Optical receiver  202 A is coupled to a receive PLL  216 A- 2 , which is coupled to the receive time domain RTD. Optical transmitter  200 A is coupled to a fixed clock oscillator (“transmit OSC”)  216 A- 1 .  
      Likewise, system  70  includes at Bob an arrangement similar to that at Alice describe immediately above, but with a circulator  204 B, optical transmitter  200 B, an optical receiver  202 B, an transmit PLL  216 B- 1  and receive PLL  216 B- 2 . Note that at Bob the optical receiver  202 B is coupled to both the receive PLL  216 B- 2  and the transmit PLL  216 B- 1 .  
      Transmit OSC  216 A- 1  generates a fixed operating frequency in the capture range of the system phase locked loops (PLL),  216 A- 2 ,  216 B- 1  and  216 B- 2 .  
      The output signal of transmit OSC  216 A- 1  is converted to an optical synchronization (“sync”) signal S 3  by optical transmitter  200 A. Sync signal S 3  is then coupled through circulator  204 A, carried by fiber F 1  to circulator  204 B, where it is directed to and received by optical receiver  202 B at Bob.  
      Control signals from Alice to Bob are extracted from the detected sync signal S 3  of the optical receiver  202 B and are synchronized to the receive time domain (RTD) of Bob.  
      The receive PLL  216 B- 2  and transmit PLL  216 B- 1  recover the signal from optical receiver  202 B, and in effect make a locked copy of the RTD and transmit time domain (TTD) on Bob that is synchronized to the TTD on Alice.  
      The TTD on Bob is sent back to Alice via an optical sync signal S 3 ′ generated by the optical transmitter  200 B, which passes through circulator  204 B, fiber F 1 , circulator  204 A, and is received by optical receiver  202 A.  
      The receive PLL  216 A- 2  at Alice recovers the corresponding electronic sync signal S 4 A generated by the received optical sync signal S 3  at optical receiver  202 A, so that the RTD on Alice is locked to the transmit timing domain (TTD) on Bob. Control signals from Bob to Alice are extracted from the output signal of the optical receiver  202 A and are synchronized to the RTD on Alice. Explanation of the RTD and TTD follows in a later section.  
      B. Example Timing Operation of Two-Way Quantum Transceiver  
       FIG. 3B  is a schematic diagram similar to  FIG. 3A  that illustrates another example embodiment of the timing system  70 .  FIG. 3B  illustrates that a single topology that can be used to implement the circuits of Alice and Bob in  FIG. 3A .  
      With reference to  FIG. 3B ,  216 B- 1  is now the transmit OSC that generates a fixed operating frequency in the capture range of all other system PLLs,  216 A- 1 ,  216 A- 2  and  216 B- 2 .  
      The output of OSC  216 B- 1  is converted to sync signal S 3  by optical transmitter  200 B. Sync signal S 3  is then coupled through circulator  204 B, carried by fiber F 1  to circulator  204 A, and received by optical receiver  202 A on Alice.  
      The receive PLL  216 A- 2  and transmit PLL  216 A- 1  recover (in electronic form) the optical sync signal S 3  received by optical receiver  202 A, and in effect make a locked copy of the RTD and TTD on Alice that is synchronized to the TTD on Bob.  
      The output of transmit PLL  216 A- 1  is converted by optical transmitter  200 A to a corresponding sync signal S 3 , which passes through circulator  204 A, to fiber F 1 , to circulator  204 B, and is received by optical receiver  202 B, which generates an electrical signal S 4 B corresponding to the received sync signal S 3 .  
      The receive PLL  216 B- 2  is locked to the output electrical signal S 4 B of optical receiver  202 B so that the receive timing domain (RTD) on Bob is locked to the transmit timing domain (TTD) on Alice.  
      Control signals are extracted from the sync signal S 3  outputted by optical receiver  202 B already synchronized to the RTD on Alice.  
      Explanation of the usage of the RTD and TTD follows in a later section.  
      C. Example Timing Operation of OTDR  
       FIG. 3C  is a close-up schematic diagram of a portion of timing system  70  as used in OTDR mode. With reference to  FIG. 3C , fixed clock oscillator (OSC)  216 B- 1  generates a fixed operating frequency in the capture range of the other system PLL  216 B- 2 .  
      The receive PLL  216 B- 2  locks to the signal from OSC  216 B- 1 , allowing the controller to operate with the RTD and TTD acting as a single timing domain such that delay times between the events on the RTD and TTD can be measured. The measurement of delay times between pulses is then used to determine the position(s) of reflecting points RP 1  in the link (e.g., fiber F 1 ) connecting Bob to Alice.  
      D. Generalized Timing Circuit Operation  
       FIG. 3D  is a close-up schematic diagram of a generalized example embodiment of Alice&#39;s portion of the timing system  70  as presented in  FIGS. 3A-3C , but that includes more details of the PLLs therein. With reference to  FIG. 3D , timing system  70  implements the timing functions for either Bob or Alice in the modes required in  FIGS. 3A through 3C .  
      The timing system  70  of  FIG. 3D  includes a generalized optical receiver  202  (i.e., either optical receiver  200 A or  200 B), a phase comparator  600  coupled to the receiver a low pass filter (LPF)  602  coupled to the phase comparator, and voltage controlled oscillator (VCO)  604  coupled to LPF  602 , that together comprise the PLL (e.g., Rx PLL  216 A- 2 ) for the receive timing domain (RTD). Likewise, a phase comparator  601 , a low pass filter (LPF)  603  and voltage controlled oscillator (VCO)  605  together form the PLL (e.g., Tx PLL  216 A- 1 ) for the transmit timing domain (TTD).  
      RTD Operation  
      In operation, for the RTD, phase comparator  600  measures the phase difference between two clock signals from receiver  202  and produces a voltage proportional to the input phase difference. One the clock signal input is always in the RTD, the fed back output from the RTD VCO  604 . The other clock signal can be selected by switch SW 1  that connects phase comparator  600  to either optical receiver  202  at “A” or phase comparator  601  at “B” so that the phase comparator  600  measures the RTD clock versus the input from the optical receiver or measures the RTD clock versus the TTD clock.  
      LPF  602  shapes the output of the phase comparator  600  so that the PLL is stable from a feedback control system point of view, and so that high-order frequency content is removed from the input to the VCO  604 .  
      VCO  604  is an oscillator producing an output clock with the frequency controlled by an input voltage control.  
      By feeding the output of VCO  604  back to the phase comparator  600 , the input to the VCO  604  is adjusted by the LPF  602  to increase of decrease the output frequency of VCO  604  until the frequency and phase of the two inputs to the phase comparator are equal.  
      By changing the position of SW 1  from position A to position B, the output of VCO  604  can be made to match the clock input from optical receiver  202  or VCO  605 .  
      TTD Operation  
      For the TTD, phase comparator  601  measures the phase difference between two clock signals and produces a voltage proportional to the input phase difference. One clock signal input is always in the TTD, the fed back output from the TTD VCO  605 . The other clock signal is always input from the optical receiver  202 .  
      The Low Pass Filter  603  shapes the output of the phase comparator  601  so that the phase lock loop is stable from a feedback control system point of view, and so that high order frequency content is removed from the input to the VCO  605  through SW 2 .  
      VCO  605  is an oscillator producing an output clock with frequency controlled by an input voltage control.  
      When switch SW 2  is in its A position, phase comparator  601 , LPF  603  and VCO  605  form a phase locked loop, in a manner similar to that described above for the RTD. By feeding the output of VCO  605  back to the phase comparator  601 , the input to the VCO  605  is adjusted by the LPF  603  to increase of decrease the output frequency of VCO  605  until the frequency and phase of the two inputs to the phase comparator are equal.  
      By changing the position of SW 2  from its position A to its position B, the output of VCO  604  can be made to produce a fixed frequency output clock in the TTD that is the original timing clock that other PLL blocks must lock to.  
      Table 1 below shows the positions of switches SW 1  and SW 2  that allow the generalized circuit to perform necessary timing as shown in  FIGS. 3A through 3C .  
               TABLE 1                          Switch position for generalized timing circuit                             FIG.   Function   SW1 position   SW2 position               3A   Alice   A   B       3A   Bob   A   A       3B   Alice   B   A       3B   Bob   A   B       3C   OTDR   B   B                  
 
      By changing the position of SW 1  and SW 2 , all required timing generation can be performed by a single physical circuit realization.  
      III. FPGA-Based Controller Functions  
      A. Communication Between Bob and Alice  
      Timing synchronization must be performed not only to link the timing domains from transmitter to receiver, but to synchronize the frames of data so that both transmitter and receiver know they are operating on the same photon crossing the data communications channel even though the transmitter and receiver are not aware of the state of the receiver and transmitter on the other end of the transmission.  
      With reference again to  FIG. 2C , a series of electrical synchronization (sync) pulses, collectively represented by electrical sync signal S 4 A- 1 , are created by the FPGA  132 B in the controller  60 B and communicated to optical transmitter  200 B in optical model  50 B. Optical transmitter  200 B converts the electrical pulses to optical pulses, collectively represented by sync signal S 3 ′, which pass through circulator  204 B and through WDM  78 B and travel over timing channel  54  (e.g., optical fiber F 1 ). Sync signals S 3 ′ are received at the corresponding optical receiver  202 A in optical modem  50 A.  
       FIG. 4  shows the series of sync pulses P 3  that make up optical sync signal S 3 . In an example embodiment, the leading edges of the pulses stay on a regular time interval Toclk nominally in the 10 ns to 50 ns range, while the width w of the pulses is used to encode data. An example of encoded data is where the pulse width that defines a logical  0  or  1  identifies the signal sender as Bob or Alice (i.e., authentication). Sufficient bandwidth is available to transmit the pulses of varying width such that the pulse width can be detected at the receiver.  
      In an example embodiment, sync signals S 3  and S 3 ′ are sent continuously, i.e., there is no interruption of the quantum signal S 1  in order to send the sync signals. This optimizes the bandwidth of quantum channel  24 .  
      The pulses P 3  in sync signals S 3  run continuously during operation at an optical clock period Toclk. Starting with the frame sync pulse PA, both controllers  60 A and  60 B co-ordinate their timing so they know they are operating on the same bit. Frame sync pulse PA can be repeated if necessary to ensure a unique pattern is recognizable at the receiver.  
      Following the frame pulses PA are multiple data pulses of arbitrary number, labeled PB through PN. Each pulse PB through PN can send frame synchronous data between receiver and transmitter parties on each link of the optical modem to help co-ordinate tasks such as declaring the state of each party.  
      Pulses with no meaningful data are labeled P 0  and are inserted to fill the time between a subsequent frame sync pulse, again labeled PA.  
      The leading edge positions of the pulses are used by the phase comparators  600  and  601  in  FIG. 3D . The data encoded in the pulse widths is encoded and inserted by FPGAs  132 A and  132 B. The time interval between frame sync pulses PA is given by T FSYNC .  
      B. Pulse Timing Generation  
       FIG. 5A  is a schematic diagram of a clock-based digital pulse generator (DPG)  700  implemented in the FPGA as used to generate trigger pulses for the drivers. DPG  700  includes a counter  704  that increments on each positive edge of the input clock. The input clock is either a clock signal on the receive timing domain (RTD) or transmit timing domain (TTD).  
      DPG  700  also includes a digital comparator  708  that compares the value of counter  704  with a terminal count  702 . When the terminal count is reached, the counter is reset to its initial state. The output of digital comparator  708  (qclock) pulses once every time a quantum pulse is transmitted over the QKD system (i.e., between Alice and Bob). Signal qclock is used below to trigger the operation of circuits that operate once per quantum pulse.  
      DPG  700  also include a digital comparator  710  that compares the value of the counter  704  with an FPGA register that determines the position at which an output pulse is generated.  
      With reference to  FIG. 5B , the circuit of  FIG. 5A  is grouped and duplicated for each timing signal to be generated. Transmit timing machine (TTM)  720  groups all signals that are synchronized to the transmitted optical pulse on the quantum channel, and creates them at the same time with multiple copies of DPG  700 . Only a single set of counter register  704 , terminal count  702  and digital comparator  708  is required, and is shared by all members of DPG  700  in TTM  720 .  
      Each output of TTM  720  has a separate register  706 , grouped as control registers  740  and digital comparators inside TTM  720  to allow output pulses to be generated in different moments in time to drive external devices  760 .  
      With continuing reference to  FIG. 5B , the receive timing machine (RTM)  730 , which has control registers  750 , groups all signals that are synchronous to the received optical pulse on the quantum channel, and creates them at the same time with multiple copies of DPG  700 . Only a single set of counter register  704 , terminal count  702  and digital comparator  708  is required, and is shared by all members of DPG  700  in RTM  730 .  
      Each output of RTM  730  has a separate register  706 , grouped as control registers  750  and digital comparators  752  inside RTM  730  to allow output pulses to be generated in different moments in time to drive external devices  770 .  
      External devices grouped in block  760  (e.g., laser  100 B and optical transmitter  200 B) can be dynamically changed depending on the system timing application. With reference also to  FIG. 2C , laser  100 B and optical transmitter  200 B are both driven by the TTM, and are in the TTD. Single-photon detectors  114 B and  116 B, discriminators  120 B and  122 B, and modulator driver  112 B-D, as external devices  770 , are all driven by the RTM, and are in the RTD.  
      The selection of whether a driver should be in the TTD or RTD is determined by the timing compensation required. When the transmission time along optical path F 1  is changed due to environmental factors, both the flight time of the signals on quantum channel  24  and timing channel  54  along the optical path F 1  see the same environmental factors, and they change flight time by the same amount. Signals in the TTD do not get delayed as they do not travel along optical path F 1 . On the other hand, signals in the RTD are delayed by the optical path flight time. By using a separate RTD, the time variation in optical path flight time is automatically accounted for with reference also to  FIG. 2E , for OTDR functions the RTD and TTD are locked together, so that the time delay of a pulse traveling from the TTD-based laser pulse  100 B to the RTD-based photon detectors  114 B and  116 B can be measured. This allows the time variation in optical path flight time to be measured, rather than being removed. The time variation is then used to deduce information regarding the location reflection points RP 1 , which may be optical taps set up by an eavesdropper.  
      C. Random Number Generation and Insertion  
      With continuing reference to  FIG. 5B , outputs  770  in the RTD are triggers for events that occur for every quantum pulse. For example, modulator driver  112 B-D must be set to a different random value as part of the quantum key distribution process for every pulse that passes through modulator  112 B.  
      At other times, such as when setting up and calibrating the QKD system, a consistent non-random value is applied to the modulator.  
      With reference now to  FIG. 6 , in an example embodiment RNG unit  30 A includes a plurality (e.g., four) of individual data sources  30 A- 1  through  30 A- 4  that supply a new value at a low to high level transition of the qclock signal.  
      As an example embodiment, data source  30 A- 1  can be a hardware-based true random number generator, data source  30 A- 2  can be a linear shift register based pseudo random number generator, data source  30 A- 3  can be a fixed short length data output that produces non-random numbers, and data source  30 A- 4  can be an externally input data source. All data sources must update the values of their outputs during the low to high level transition of the qclock signal.  
      The example embodiment of RNG unit  30 A of  FIG. 6  includes a data source selector RNG-S that selects one of the plurality of data sources for presentation to modulator driver  112 A-D. A copy of the data used to drive modulator  112 A is buffered for later use to implement well-known QKD exchange mechanisms.  
      With reference again to  FIG. 5A , DPG  700  controls when data from RNG-S is output by modulator driver  112 A-D to drive modulator  112 A, so that it is present on the modulator no longer than necessary to encode the photon passing therethrough.  
      D. External Peripheral Control  
      DPG  700  of  FIG. 5A  is limited in delay resolution based on the frequency of clock (CLK) input to counter  704 , typically in the 100 MHz to 200 MHz range. With reference to  FIG. 7A , a fine delay block  728  is placed after the output of each digital comparator  710  to allow the pulses outputted therefrom to be adjusted in finer steps, and to allow adjustment of output pulse width. In fine delay block  728 , the output of digital comparator  710  is connected to delay blocks  722  and  724 , each of which is connected to separate inputs of logic gate  726 . The output of logic gate  726  is adjusted by the resolution of delay blocks  722  and  724 .  
      Controller  60 A is coupled to delay blocks  722  and  724  and programs them to set their delay. Typical delay blocks can delay inputs from 0 to 20 ns with better than 10 ps steps.  
      With reference now to  FIG. 7B , digital output of block  710  shows an output pulse limited in time and width w to an interval of CLK input to block  704 .  
      Output of delay  722  is delayed by interval td 1  and output of delay  724  is delayed by interval td 2 . Logic gate  726  combines outputs of delay  722  and delay  724  to form a pulse with a start at td 2  and an end at td 1 +w, with width td 1 −td 2 +w.  
      In an example embodiment, w is 8.00 ns, td 1  is 5.72 ns and td 2  is 8.40 ns. Output pulse of logic gate  726  is 8.40 ns after 5.32 ns, with adjustment capability of 10 ps of each edge.  
      Other logic in logic gate  726  can be used. Output  728  shows an alternative logic gate (A and (not B)) that results in an output pulse with start at td 1  and end at td 2 . The pulse width is then td 2  -td 1 , with no dependency on the variation of width w.  
      E. Single-Photon Data Collection and Processing  
       FIG. 8  is a schematic diagram of an example QKD system  800  of the present invention. In QKD system  800 , RTM  730 A and RTM  730 B are synchronized as described above, and now the data transfer required to implement known QKD algorithms in QKD processors  806 A and  806 B needs to be carried out.  
      Frame sync pulses included in sync signals S 3  sent between optical modems  50 A and  50 B over timing channel  54  ensure QKD engines  806 A and  806 B operate on data from the same photons as they pass through the QKD system.  
      Input data for processor  806 A required to distribute quantum keys includes the state of phase or delay encoded on to single photons, as has been stored in RNG buffer  802 A, and basis measurement information provided over public data channel  44  from QKD processor  806 B.  
      Discriminators  120 B and  122 B indicate whether or not a single photon has been received by SPD  114 B and SPD  116 B in each of two orthogonal polarization. With each update from the qclock output of RTM  730 B, the single-photon state is stored in SPD buffer  804 B, and the modulator state is store in RNG buffer  802  in a synchronized fashion.  
      Input data for  806 B required to distribute quantum keys includes the state of phase or delay encoded on to single photons from RNG buffer  802 B, and results from SPD  114 B and  116 B from SPD buffer  804 B. Basis measurement information is provided over public data channel  44  to QKD processor  806 A.