Quantum key distribution (QKD) involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (i.e., 1 photon or less, on average, and typically 0.1 photon on average) optical signals or “qubits” transmitted over a “quantum channel.” Rather than relying on computational impracticality, the security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. Thus, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
The above mentioned references by Bennett each describe a QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent) discloses a so-called “two-way” QKD system that employs an autocompensating interferometer of the type invented by Dr. Joachim Meier of Germany and published in 1995 (in German) as “Stabile Interferometrie des nichtlinearen Brechzahl-Koeffizienten von Quarzglasfasern der optischen Nachrichtentechnik,” Joachim Meier. —Als Ms. gedr.—Düsseldorf: VDI-Verl., Nr. 443, 1995 (ISBN 3-18-344308-2). Because the Meier interferometer is autocompensated, the two-way QKD system based thereon is generally less susceptible to environmental effects than a one-way system.
In a typical QKD system, Alice generates a quantum signal and randomly modulates this signal based on a select number of possible basis modulations. This process is referred to herein as “selective random modulation.” The once-modulated quantum signal is then sent to Bob, who receives this signal and selectively randomly modulates it to form a twice-modulated quantum signal. The twice-modulated quantum signal is then detected at Bob at one of two single-photon detectors (SPDs). Bob is arranged so that an overall modulation of one value (e.g., an overall phase modulation of 0) is detected at one of SPD, while an overall modulation of another value (e.g., an overall phase modulation of π/2) is detected at the other SPD. This quantum signal exchange process is repeated for a large number of photons (e.g., 104 photons), and known QKD protocols and procedures (e.g., sifting, error correction, privacy amplification, etc., as described in the above-cited reference by Bouwmeester et al.) are then followed to establish a secure “quantum key” between Alice and Bob.
The operation of the QKD system relies on the synchronized operation of its key active elements—namely, the light source, the modulators and the SPDs. The activation of the key elements is based on the expected arrival times of the quantum signals (photons). While it may be fairly straightforward to operate a QKD system in a laboratory environment with rudimentary synchronization of the aforementioned active elements, a commercially viable QKD system needs to have a simple and robust synchronization system that can be adjusted to maintain the stability of the QKD system over time, as well as provide for ongoing efficient (e.g., optimized) system performance. This is true even for the so-called autocompensated system, because the “autocompensation” applies to the quantum signals and not to the synchronization signals used to coordinate system operation.