Quantum cryptography (QC) can deliver utterly secure data transmission through the harnessing of the laws of physics, photon quantum states, and Heisenberg’s uncertainty principle. BBN Technologies devised a fully operational, multi-node QC system that has been running for over two years, connecting a trio of Boston-area institutions through a 12-mile loop of unused dark optical fiber. The system, which was developed under a 2002 Defense Advanced Research Projects Agency grant, was based on the random polarization of photons, and subsequent selective polarization filtering and polarization-direction detection. A QC system can be employed for either one-time pad or key-exchange cryptography. The generation of a single photon with known quantum states involves the stimulation of a nonlinear crystal by a laser pump, which consequently creates twin photons with identical quantum states, also known as “entangled” photons. Completing a quantum-encrypted link between sender and receiver requires a setup that includes all-optical, electronic, and electro-optical components, including sources, delay lines, phase shifters, couplers, splitters, and optical fibers that incorporate elements that both do and do not maintain polarization. Although very sophisticated, the system can operate by itself with autocalibration, start-up mode, and self-test mode. Continuous data throughput is also supported. BBN Technologies’ Chip Elliott says the next step is to make the systems smaller, cheaper, and more hardware-based.
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