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Quantum Information Research at NIST: Goals and Vision |
Security services are critical to modern telecommunications. For instance, they help ensure that the message received is the one sent, and that secrets remain secret. The most sensitive information, such as bank transfers, can be encrypted very effectively. But some widely used encryption systems could be defeated by quantum computers. And even if information is encrypted, an eavesdropper can still tap into a conventional communications channel and listen to or copy a transmission without being detected.
Quantum mechanics offers the potential for ultra-secure communications because a measurement of an unknown quantum system changes its state. As a consequence, accurate copying is impossible, and the changes caused by eavesdropping can be detected. Whereas today’s fiber-optic communication systems require bits made of tens of thousands of photons, quantum communication uses single photons to transmit 1s and 0s. This is the future of encryption technology. Very weak laser pulses are created so that a single pulse rarely has more than one photon. The photons are oriented for transmission through the air or cables, detected with special receivers, and interpreted by custom electronics and software. The idea is straightforward but the implementation is markedly less so. In particular, it is difficult to build fast, reliable, long-distance quantum channels, process the information generated at high speeds, and connect quantum links to networks.
One mode of quantum communications that is moving from the laboratory to early commercial production is quantum key distribution (QKD). (See “Making a Quantum Key”) QKD provides a secure method for distributing a secret key between two parties, who then can use the key to encrypt and decrypt a message. Secure distribution of such keys remains a difficult challenge. NIST is working to overcome these challenges by addressing hardware engineering issues, such as how to produce and detect single photons rapidly and reliably, and development of tools for faster and better processing of information. In addition, NIST is providing the metrology infrastructure for the development of quantum communications technologies as commodities on an industrial scale: the basic measurements needed to certify the quantum performance of components and integrated systems and to assure their conformance with standards.
Quantum Communications Testbed
NIST has built an open system for research, testing, calibrations, and technology development in a real-world, gigabit Ethernet telecommunications environment. At the testbed, infrared lasers generate single photons that are sent and received by telescopes over a wireless optical channel between two buildings 730 meters apart. A second system sends single photons through fiber. Development of the testbed and measurement infrastructure, including new rules for managing quantum systems and data, draws on and integrates multiple disciplines. The testbed also provides a well-characterized environment for testing new photon sources and detectors and analyzing the performance of different sets of rules.
High-Speed QKD System
Using the testbed, NIST has built a QKD system that transmits a stream of individual photons to generate a verifiably secret key at a rate of 1 million bits per second. This rate is about 100 times faster than previously reported systems of this type. High speed is essential for QKD to become commercially viable and usable for a variety of applications, such as a recent NIST demonstration of streaming encrypted video. Previous systems produced keys at rates that would fill no more than one CD-ROM in 2 months; the NIST system could fill a CD-ROM in less than 2 hours. QKD systems need to identify a photon from the sending laser amid many photons from other sources, such as the sun. To make this distinction, NIST scientists time-stamp the QKD photons, then look for them only when one is expected to arrive. They also select photons of a particular frequency, or color. NIST adapted techniques used in high-speed telecommunications to increase the rate at which the system can look for photons. The received photons are processed at high speed, in real time, by circuit boards designed at NIST, so that keys are produced automatically. NIST computer scientists also developed a high-speed approach to error correction adapted from telecommunications techniques. This makes it possible to correct bit errors rapidly without time-consuming discussions between sender and receiver and without wasting a great deal of the key by revealing it to a potential eavesdropper. Single-Photon Detectors
Many photon detectors do not efficiently detect single photons, cannot distinguish between one or more photons arriving at the same time, and produce high false (or dark) count rates due to random “noise.” They also tend to operate best with visible light instead of the near-infrared light needed for long-distance fiber-optic communications. NIST scientists have demonstrated single-photon detectors that operate with near-infrared light and count more than 100,000 photons per second while reducing false counts to virtually zero. The detection efficiency—the proportion of received photons that are actually detected—exceeds 85 percent. Single-Photon Sources Existing light sources, such as lasers, cannot produce single photons reliably. NIST is developing single-photon “turnstiles” that produce pulses of light containing exactly one photon. For instance, NIST has demonstrated efficient production of single photons from a “quantum dot,” 10 to 20 nanometers wide, made of semiconductor materials. Another method uses a special crystal that converts one photon to a pair of photons with lower energies. Because the photons are created in pairs, the detection of one indicates, with absolute certainty, the existence of the other. One photon is intercepted by a detector and the other is used for communications or for evaluating and calibrating other single-photon sources or detectors. NIST is working to increase the precision and reliability so the technique can be broadly applied.
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Date
created: 4-11-06
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