TEMPERATURE COMPENSATION FOR QKD SYSTEMS
FIELD OF THE INVENTION
The present invention relates to quantum cryptography, and in particular relates to systems and methods for temperature compensation for a quantum key distribution (QKD) system.
Laboratory and prototype QKD systems can be adjusted to account for system drifts can under very controlled and artificial conditions. However, making the same kinds of adjustments for a commercial QKD system in the field is a far more daunting endeavor. And, unlike with a laboratory or prototype QKD system, end-users of commercial QKD systems have an expectation that their QKD system will automatically run in an optimal state with minimal or no operator intervention.
BACKGROUND OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
Quantum key distribution involves establishing a key between a sender ("Alice") and a receiver ("Bob") by using weak (e.g., 0.1 photon on average) optical signals transmit- 15 ted over a "quantum channel." The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper ("Eve") that attempts to intercept or otherwise measure the 20 quantum signal will 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," 25 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 publication by C. H. Bennett entitled "Quantum Cryp- 30 tography Using Any Two Non-Orthogonal States", Phys. Rev. Lett. 68 3121 (1992). The general process forperforming 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. 35
The above-mentioned references describe a so-called "one-way" QKD system wherein Alice randomly encodes the polarization or phase of single photons, and Bob randomly measures the polarization or phase of the photons. The one-way system described in the Bennett 1992 paper 40 and incorporated by reference herein is based on a shared interferometric system. Respective parts of the interferometric system 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 45 different paths. As a consequence, the interferometers need to be actively stabilized to within a few tens of nanoseconds during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so- 50 called "two-way" QKD system that is autocompensated for polarization and thermal variations. Thus, the two-way QKD system of the '234 patent is less susceptible to environmental effects than a one-way system.
When operating a QKD system in practice (e.g., in a 55 commercial setting), multiple variables need to be aligned in time and then maintained aligned for optimal system performance. For example, in a commercial QKD system one or more single-photon detectors (SPDs) are gated with one or more corresponding detector gating signals from a con- 60 trailer to synchronize the detection of optical pulses with expected pulse arrival times. However, once the system is set up, the timing drifts due to various systemic and environmental factors (e.g., temperature) and the photon count can drop. This leads to a reduction in the transmission rate 65 of the system, and also to an increase in the bit—error rate—i.e., to diminished system performance.
FIG. 1 is a schematic diagram of a two-way QKD system;
FIG. 2A is a close-up schematic diagram of the controller for the QKD system of FIG. 1, illustrating the elements that provide thermal compensation of the detector gating signal for the SPD;
FIG. 2B is a close-up schematic diagram of the SPD control board of Bob of FIG. 2A;
FIG. 3A is a plot of the SPD photon count versus the timing of the detector gating signal illustrating the optimum detector gating signal timing t^^as indicated by the maximum photon count number N^;
FIG. 3B is a timing diagram of detector gating signals as a function of temperature;
FIG. 4 is a schematic diagram of a one-way QKD system; and
FIG. 5 is a close-up schematic diagram of the controller for the QKD system of FIG. 4, illustrating the elements that provide thermal compensation of the detector gating signal for the two SPDs in the SPD unit.
The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.
SUMMARY OF THE INVENTION
A first aspect of the invention is a method of providing temperature compensation for the timing of a gating signal for a single-photon detector (SPD) in a QKD station of a QKD system. The method includes determining a reference detector gating signal timing value corresponding to an ambient reference temperature of the QKD station, e.g., at or near an SPD electronics control board. The method also includes incrementally varying and measuring a temperature of the QKD station over a range of temperature values, and determining a change in timing from the reference detector gating signal timing value for each measured temperature value. The method further includes storing the measured temperature value and the corresponding change in timing of the detector gating signal in a look-up table, operating the QKD station at an operating temperature that varies over time within the range of temperature values, and then adjusting the timing of an operational detector gating signal by an amount associated with the operating temperature as defined in the look-up table.
A second aspect of the invention is a method of providing temperature compensation for the timing of a gating signal for a single-photon detector (SPD) in a QKD station of a QKD system. The method includes incrementally varying and measuring the temperature of the QKD station over a range of temperature values, determining an optimum detector gating signal timing value for each measured temperature