Abstract:
Systems and methods for performing quantum key distribution (QKD) using one or more high-altitude platforms (HAPs) are disclosed. The system includes a second QKD station (Alice) supported by the HAP so as to be in free-space communication with the first QKD station (Bob) over an optical path (OP) via an optical quantum communication channel that carries quantum signals (P 1 ′), an optical synchronization channel that carries synchronization signals (PS) and optionally classical communication signals (PC), an optical beacon channel that carries beacon signals (PB), and a radio-frequency (RF) channel that carries RF signals. The beacon signals are used to detect changes in the optical path and correct the synchronization signals (PB) so as to gate the first and second SPD pairs to correspond to arrival times of the quantum signals at said first and second SPD pairs. The system does not require a Pockels cell for quantum signal modulation, thereby improving the security of the system.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/843,640, filed on Sep. 11, 2006. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to quantum cryptography, and in particular relates to quantum key distribution (QKD) systems that use high-altitude platforms. 
       BACKGROUND ART 
       [0003]    QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. Consequently, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits introduces errors that reveal her presence. 
         [0004]    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. 
         [0005]    U.S. Pat. No. 5,966,224 to Hughes et al. (“the &#39;224 patent”), which patent is incorporated by reference herein, discloses a Pockels-cell-based optical system for providing secure communications between an earth station and a low-orbit spacecraft. The optical system of the &#39;224 patent enables secure long-range communication through space to provides secure satellite-based telemetry using the principles of QKD. 
         [0006]    Satellite-based QKD presents serious technical challenges. For instance, the ground-satellite link is normally in the range from 100-400 km. The faint quantum signal encounters turbulence, weather and scattering from airborne particles, particularly over the 10-20 km closest to the ground. These factors result in ˜30-50 dB loss. Accordingly, the system of the &#39;224 patent would require quantum signals having a relatively high average number of photons per pulse in order to have a reasonable data rate. This leads to a decrease in the overall security of the system. Further, use of a Pockels cell for quantum signal modulation presents a security risk because they have a pronounced electromagnetic interference (EMI) signature that could be detected by an eavesdropper and used to discern the quantum signal modulations. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The present invention includes systems and methods for performing QKD using one or more high-altitude platforms (HAPs). The system includes a second QKD station supported by the HAP so as to be in free-space communication with the first QKD station over an optical path via an optical quantum communication channel that carries quantum signals, an optical synchronization channel that carries synchronization signals, an optical beacon channel that carries beacon signals, and a radio-frequency (RF) channel that carries RF signals. The beacon signals are used to detect changes in the optical path and correct the synchronization signals so as to gate the first and second SPD pairs to correspond to arrival times of the quantum signals at said first and second SPD pairs. The system does not require a Pockels cell for quantum signal modulation, thereby improving the security of the system over prior art systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of an example embodiment of a QKD station ALICE suitable for use in a HAP QKD configuration; 
           [0009]      FIG. 2  is a schematic diagram of an example embodiment of a QKD station BOB suitable for use in combination with QKD station ALICE described immediately above and shown in  FIG. 1 , for use in a HAP configuration; 
           [0010]      FIG. 3  is a schematic diagram of an example embodiment of a HAP-based QKD system that employs a HAP in the form of a zeppelin; 
           [0011]      FIG. 4  is a schematic diagram illustrating a second example embodiment of a HAP-based QKD system used to transfer quantum keys between QKD stations ALICE, BOB- 1  and BOB- 2 , wherein BOB- 1  and BOB- 2  at different locations L 1  and L 2  and ALICE resides in the HAP; 
           [0012]      FIG. 5  is a schematic diagram of a third example embodiment of HAP-based QKD system wherein HAP  300  includes both ALICE and BOB QKD stations; 
           [0013]      FIG. 6  is a schematic diagram of a fourth example embodiment of a HAP-based QKD system that employs entangled photons to perform QKD; and 
           [0014]      FIG. 7  is a schematic diagram of a fifth example embodiment of a HAP-based QKD system that involves a spacecraft that includes a QKD station BOB, wherein the QKD system allows for a common key to be provided to a ground based QKD station and a space-based QKD station. 
       
    
    
       [0015]    The various elements depicted in the drawing are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawing is intended to illustrate an example embodiment of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The present invention is directed to QKD systems and methods that employ high-altitude platforms (HAPs). An example embodiment of QKD stations ALICE and BOB suitable for use with HAPs is first described, follow by several different example embodiments of HAP-based QKD systems that employ ALICE and BOB. 
       ALICE 
       [0017]      FIG. 1  is a schematic diagram of an example embodiment of a QKD station ALICE. ALICE includes quantum optics communication layer  4 A, and a classical optics communication layer  6 A and a radio-frequency (RF) communications layer  8 A, all operably coupled to a controller CA. 
         [0018]    Quantum optics layer  4 A includes a laser unit  12  optically coupled to a beam splitter  30  that has an input port (face)  31  and output faces  32  and  33 . Beam splitter  30  is optically coupled to laser unit  12  via an optical fiber section F 2  optically coupled to input face  31 . Beam splitter  30  in turn is optically coupled to another beam splitter  40  that has two input faces  41  and  42  and two output faces  43  and  44 . Beam splitter  40  is optically coupled to beam splitter  30  via an optical fiber section F 3  optically coupled to output face  33  and input face  41 . Optical fiber section F 3  includes a delay line DL. 
         [0019]    Beam splitters  30  and  40  are also optically coupled to one another via an optical fiber section F 4  that is optically coupled to output face  32  and input face  43 . A phase modulator MA is arranged in optical fiber section F 4 . A detector DA is optically coupled to beamsplitter output face  43  via an optical fiber section F 5 . An optical telescope  50 A is optically coupled to output face  43  of beamsplitter  40  via an optical fiber section F 6 . Laser unit  12  and phase modulator MA are operably coupled to controller CA. 
         [0020]    Classical optics communications layer  6 A includes an optical synchronization unit  110 A and a beacon unit  120 A each operably coupled to controller CA and that include respective telescopes  114 A and  124 A. 
         [0021]    RF communication layer  8 A includes an RF transceiver  130 A operably coupled to controller CA. RF transceiver  130  includes an antenna  132 A. 
         [0022]    Controller CA includes a key bank  180  that stores classical and/or quantum keys that are either generated by ALICE and BOB, and/or that are pre-loaded. 
       BOB 
       [0023]      FIG. 2  is a schematic diagram of an example embodiment of a QKD station BOB suitable for use in combination with QKD station ALICE described immediately above and shown in  FIG. 1 . BOB also includes a quantum optics communication layer  4 B, a classical optics communication layer  6 B, and a RF communications layer  8 B, all operably coupled to a controller CB and to the corresponding layers at ALICE, as described below. Quantum optics communication layers  4 A and  4 B constitute a quantum channel. Classical optics communication layers  6 A and  6 B constitute a synchronization channel and a beacon channel. RF communication layers  8 A and  8 B constitute an RF channel. 
         [0024]    Quantum optics communications layer  4 B includes a 50/50 beamsplitter  200  that has an input face  201  and two output faces  202  and  203 . Quantum optics layer also includes two polarizing beamsplitter  206  and  210 . Beamsplitter  206  has an input face  207  and two output faces  208  and  209 . Beamsplitter  210  has an input face  211  and two output faces  212  and  213 . 
         [0025]    Beamsplitter  200  is optically coupled to beamsplitter  206  via an optical fiber section F 7  optically coupled to output face  202  and input face  207 . Beamsplitter  200  is also optically coupled to beamsplitter  210  via an optical fiber section F 8  optically coupled to output face  203  and input face  211 . A half-wave plate  20 B is arranged in optical fiber section F 8 . An optical telescope  50 B is optically coupled to input face  201  of beamsplitter  200  via an optical fiber section F 9 . 
         [0026]    Quantum optics communications layer  4 B also includes a first set of SPDs DB 1  and DB 2  optically coupled to beamsplitter  206  at output faces  208  and  209 , respectively. Likewise, a second set of SPDs DB 3  and DB 4  are optically coupled to beamsplitter  210  at output faces  212  and  213 , respectively. Each SPD is operably coupled to controller CB. 
         [0027]    Classical optics communications layer  6 B includes an optical synchronization unit  110 B and a beacon unit  120 B each operably coupled to controller CB and that include respective telescopes  114 B and  124 B. 
         [0028]    RF communication layer  8 B includes an RF transceiver  130 B operably coupled to controller CB. RF transceiver  130  includes an antenna  132 B. 
         [0029]    A QKD system formed from ALICE and BOB of  FIGS. 1 and 2  communicates over free-space. Quantum optics communication layers  4 A and  4 B are in optical communication via telescopes  50 A and  50 B. In an example embodiment, adaptive optics are employed in combination with telescopes  50 A and/or  50 B or other type of optical system to correct wavefront errors in the optical signals (pulses) that arise due to atmospheric distortion. Classical optics communication layers  6 A and  6 B are in optical communication via synchronization-unit telescopes  114 A and  114 B, and via beacon-unit telescopes  124 A and  124 B. The RF communications layers  8 A and  8 B are in RF communication via RF signals  33 A transmitted by antenna  32 A and received by antenna  32 B, and via RF signals  33 B transmitted by antenna  33 B and received by antenna  32 A. 
       General QKD System Operation 
       [0030]    In operation, the quantum optics communication layer  4 A at ALICE operates by controller CA sending a signal S 0  to laser  12  to cause the laser to emit a polarized optical pulse P 0  that travels over optical fiber F 2  to beamsplitter  30 . Polarized optical pulse P 0  enters beamsplitter  30  at input face  31 . Beamsplitter  30  splits this pulse into two orthogonally polarized (say, horizontal (H) polarized and vertical (V) polarized) optical pulses P 1  and P 2  that exit the beamsplitter at output faces  32  and  33  and travel over optical fiber sections F 4  and F 3 , respectively. Optical pulse P 2  enters beamsplitter  40  at input face  42  and because of its polarization, is directed out of output face  44  and over to SPD DA 1  via optical fiber section F 5 . The detection of optical pulse P 2  is used for stabilizing ALICE. 
         [0031]    Meantime, optical pulse P 1  travels over optical fiber section F 4  to input face  42  of beamsplitter  40 . Controller CA sends a modulation signal SA to modulator MA that causes the modulator to impart to optical pulse P 1  a modulation randomly selected from a set of basis phase modulations as the optical pulse passes through the modulator. This forms a modulated optical pulse P 1 ′. Because of its polarization, modulated optical pulse P 1 ′ is outputted from beamsplitter  40  at output face  42  and travels over optical fiber section F 6  to telescope  50 A. Optical pulse P 1 ′ constitutes the quantum signal. 
         [0032]    BOB&#39;s telescope  50 B is in optical communication with telescope  50 A and receives optical pulse P 1 ′. Optical pulse P 1 ′ is communicated to input face  201  of beamsplitter  200  via optical fiber section F 9 . Beamsplitter  200  splits modulated optical pulse P 1 ′ into two optical pulses P 1 ′- 1  and P 1 ′ 1 - 2 , which exit the beamsplitter at respective output faces  202  and  203  and travel over respective optical fiber sections F 7  and F 8  to respective beamsplitters  206  and  210 . 
         [0033]    The role of classical optics layers  6 A and  6 B and RF communication layers  8 A and  8 B are discussed below in connection with the different example embodiments of the HAP-based QKD systems of the present invention. 
         [0034]    Note that neither ALICE nor BOB include a Pockels cell. This is because it is undesirable to use a Pockels cell for quantum signal modulation due to the EMI it emits during modulation. 
       HAP QKD System 
       [0035]    In an example embodiment of the present invention, Alice and Bob are used to form a HAP-based QKD system. Several different example embodiments of a HAP-based QKD system are set forth below. 
       FIRST EXAMPLE EMBODIMENT 
       [0036]      FIG. 3  is a schematic diagram of an example embodiment of a HAP-based QKD system  200  that employs a HAP  300 , such as a zeppelin, as shown. HAP  300  includes a HAP controller  310  operably coupled to ALICE and that controls the position, speed and general operation of HAP  300 . ALICE is carried by HAP  300 , and BOB is ground-based. With reference to  FIGS. 1 through 3 , quantum optics layers  4 A and  4 B are in free-space optical communication via telescopes  50 A and  50 B at ALICE and BOB, respectively, as discussed above. In an example embodiment, the quantum channel is transmitted at ˜1550 nanometers. Although the task of single photon detection is challenging at 1550 nm, this wavelength region is attractive due to lower background flux from diffused sunlight and compatibility with standard telecommunications equipment. 
         [0037]    The classical optics communication layers  6 A and  6 B are in optical communication via sync-unit telescopes  114 A and  114 B and beacon-unit telescopes  124 A and  124 B. In an example embodiment, the beacon and/or classical/synchronization channels have a wavelength in the range from 700 nm to 850 nm. Sync units  110 A and  110 B and their corresponding telescopes  114 A and  114 B provide synchronization between ALICE and BOB via optical synchronization signals PS so that the SPDs can be gated to the expected arrival time of the quantum signals P 1 ′. 
         [0038]    The distance between ALICE and BOB varies due to movement of HAP  300 . Further, changes in the optical path can occur due to temperature and pressure variations in the atmosphere, which affect the index of refraction profile of the optical path OP between ALICE and BOB. Optical path variations change the expected arrival time of the quantum signals. Accordingly, optical beacon signals PB sent between beacon units  120 A and  120 B via their respective telescopes  124 A and  124 B are used to establish the optical path distance OPD AB  for the optical path OP between ALICE and BOB, e.g., for each quantum signal P 1 ′ transmitted. Information about the optical path distance OPD AB  is provided to controller CA, which makes the corresponding adjustment to sync signals SO, SA, SD 1 , SD 2 , SD 3 , and SD 4  to account for changes in the optical path distance OPD AB . 
         [0039]    In an example embodiment, sync units  110 A and  110 B and their corresponding telescopes  114 A and  114 B also serve as a classical communication channel between HAP and ground stations, e.g. for error correction and privacy amplification, via classical signals PC. 
         [0040]    In an example embodiment, either ALICE or BOB can initiate a request for a QKD session. The request is sent through via the classical optics communication layers  6 A and  6 B, or via RF communication layers  8 A and  8 B. Once the request for QKD connection is confirmed by both stations, Alice starts a standard QKD process. 
         [0041]    ALICE and BOB are in RF communication via RF communication layers  8 A and  8 B and RF signals  33 A and  33 B. The RF communication between ALICE and BOB allow for controllers CA and CB to communicate non-optically. This ability is particularly useful when the optical path OP between ALICE and BOB is obscured (e.g., by clouds) so that optical communication via the quantum optics layers  4 A and  4 B and/or via the classical optics communication layers  6 A and  6 B is difficult or impossible. RF communication via RF signals  33 A and  33 B is used, for example, to send instructions to the HAP controller  310  to change the position of HAP  300  so that optical communication can take place over optical path OP. In an example embodiment, RF signals  33 A and  33 B are also used to place ALICE and BOB in “standby” mode while communication over the classical and/or quantum optical communication channels is not available. 
         [0042]    In an example embodiment, QKD system  200  runs the BB84 protocol with polarization encoding. ALICE has a delay line DL in optical fiber section F 3  that makes the arms of her Mach-Zehnder interferometer equal. Phase modulator MA provides the needed polarization rotation. This design avoids the use of Pockels cells, which as mentioned above, are not secure because of a pronounced EMI signature during modulation. 
       SECOND EXAMPLE EMBODIMENT 
       [0043]      FIG. 4  is a schematic diagram illustrating a second example embodiment of a HAP-based QKD system  200  used to transfer quantum keys between QKD stations ALICE, BOB- 1  and BOB- 2 , wherein BOB- 1  and BOB- 2  at different locations L 1  and L 2  and ALICE resides in HAP  300 . 
         [0044]    In one example, ALICE first establishes contact with BOB- 1  and then ALICE and BOB- 1  exchange quantum signals P 1 ′ to establish a first quantum key. ALICE and BOB- 2  then establish contact and exchange quantum signals to establish a second quantum key. ALICE then uses the second quantum key to encode and transmit the first quantum key to BOB- 2  so that BOB- 1  and BOB- 2  now share the same first quantum key. BOB- 1  and BOB- 2  can then transmit encoded messages using such commonly shared quantum keys. 
         [0045]    In another example, ALICE has quantum keys stored in key bank  180  and uses the first and second quantum keys to encode and transmit stored keys to BOB- 1  and BOB- 2 , respectively. Again, this allows BOB- 1  and BOB- 2  to have a common set of secure keys. 
         [0046]    Note that in the example embodiment of  FIG. 4 , HAP  300  need not be stationary. Thus, if BOB- 1  and BOB- 2  are too far apart for HAP  300  to communicate directly with them both at the same time, HAP  300  moves from the vicinity of location L 1  to the vicinity of location L 2  and then established contact and communicates with BOB- 2 . 
       THIRD EXAMPLE EMBODIMENT 
       [0047]      FIG. 5  is a schematic diagram of a third example embodiment of HAP-based QKD system  200  wherein HAP  300  includes both ALICE and BOB QKD stations. This allows for HAP  300  to serve as a platform for an ALICE-BOB relay station that provides for cascaded key transmission from BOB- 1  at location L 1  to BOB- 2  at location L 2 . The ALICE-BOB QKD stations can also communication with an ALICE ground station, as shown. 
       FOURTH EXAMPLE EMBODIMENT 
       [0048]      FIG. 6  is a schematic diagram of a fourth example embodiment of a HAP-based QKD system  200  that employs entangled photons to perform QKD. HAP  300  includes a source  300  of entangled photons, and ALICE and BOB have receivers suitable for entangled-photon QKD, such as disclosed, for example, in U.S. Pat. No. 6,028,935 to Rarity, which patent is incorporated by reference herein. 
       FIFTH EXAMPLE EMBODIMENT 
       [0049]      FIG. 7  is a schematic diagram of a fifth example embodiment of a HAP-based QKD system  200  that involves a spacecraft  340  that includes a QKD station BOB, as indicated by B. The QKD system  200  of  FIG. 7  allows for a common key to be provided to a ground based QKD station and a space-based QKD station. 
         [0050]    In this example embodiment, HAP  300  supports an ALICE, as indicated by A. A ground-based BOB- 1  establishes contact with ALICE A in HAP  300  and establishes a first quantum key between them, as described above. ALICE A then uses this key to encrypt and send BOB- 1  a key stored in key bank  180 . Next, QKD station A exchanges quantum signals and establishes a second quantum key with QKD station B in spacecraft  340 . QKD station A then uses this second key to encrypt and send B the same key provided to QKD station BOB- 1 . Thus, BOB- 1  and space-based QKD station B share a common key for secure communication. 
         [0051]    This example embodiment is particularly useful because HAP  300  can be located at an altitude that allows for an unobstructed optical path to spacecraft  340 . 
       Advantages 
       [0052]    A HAP-based\QKD system offers several key advantages over other QKD systems, and particularly QKD systems that rely on space-based platforms such a spacecraft and satellites. HAPs are much less expense to deploy and maintain than spacecraft and satellites. If there is a doubt about the security of a HAP, it can be brought back to the ground, inspected and redeployed in short order. HAPs are also quite mobile and can be steered or piloted to different locations as needed. HAPs can also fly or be positioned at different altitudes and so can stay below clouds or and otherwise avoid obstructions in the free-space optical path that cause optical signal attenuation. HAPs also offer a broad signal coverage area. For example, a HAP positioned in the stratosphere at an altitude of ˜20 km ensures nearly a 1000 km simultaneous link distance between ground-based QKD stations, which provides secure data communication coverage over an area about the size of New England. Further, the mobility of the HAP extends this signal coverage area.