Code for ‘Unbreakable’ Quantum Encryption

Gaithersburg, Md.—Raw code for “unbreakable” encryption, based on the principles of quantum physics, has been generated at record speed over optical fiber at the Commerce Department’s National Institute of Standards and Technology (NIST). The work, reported today at the SPIE Defense & Security Symposium in Orlando, Fla.,* is a step toward using conventional high-speed networks such as broadband Internet and local-area networks to transmit ultra-secure video for applications such as surveillance.

The NIST quantum key distribution (QKD) system uses single photons, the smallest particles of light, in different orientations to produce a continuous binary code, or "key," for encrypting information. The rules of quantum mechanics ensure that anyone intercepting the key is detected, thus providing highly secure key exchange. The laboratory system produced this “raw” key at a rate of more than 4 million bits per second (4 million bps) over 1 kilometer (km) of optical fiber, twice the speed of NIST’s previous record, reported just last month.** The system also worked successfully, although more slowly, over 4 km of fiber.

The record speed was achieved with an error rate of only 3.6 percent, considered very low. The next step will be to process the raw key, using NIST-developed methods for correcting errors and increasing privacy, to generate "secret" key at about half the original speed, or about 2 million bps.

NIST has previously encrypted, transmitted and decrypted Web quality streaming video using secret keys generated at 1 million bps in a 1-km fiber QKD system using a slightly different quantum encoding method.*** Using the same methods for correcting errors and improving privacy with the key generated twice as fast or faster should allow real-time encryption and decryption of video signals at a resolution higher than Web quality, according to NIST physicist Xiao Tang, lead author of the paper.

“This is all part of our effort to build a prototype high-speed quantum network in our lab,” says Tang. “When it is completed, we will be able to view QKD-secured video signals sent by two cameras at different locations. Such a system becomes a QKD-secured surveillance network."
image from animation showing how photons are sent and detected by the NIST QKD system

View an animation that shows how single photons are sent and detected by the NIST QKD system.
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Animation credit: Bill Pietsch, Astronaut 3 Media Group Inc.
Applications for high-speed QKD might include distribution of sensitive remote video, such as satellite imagery, or commercially valuable material such as intellectual property, or confidential healthcare and financial data. In addition, high-volume secure communications are needed for military operations to service large numbers of users simultaneously and provide multimedia capabilities as well as database access.

NIST is among a number of laboratories and companies around the world developing QKD systems, which are expected to provide the next generation of data security. Conventional encryption is typically based on mathematical complexity and may be broken given sufficiently powerful computers and enough time. In contrast, QKD produces encryption codes based on the quantum states of individual photons and is considered “verifiably secure.” Under the principles of quantum physics, measuring a photon's quantum state destroys that state. QKD systems are specifically designed so that eavesdropping causes detectable changes in the system.

NIST systems are much faster, although operating over shorter distances, than previously reported QKD systems developed by other organizations. High-speed transmission is necessary for widespread practical use of quantum encryption over broadband networks. The NIST fiber QKD system was designed by physicists, computer scientists and mathematicians and is part of a testbed for demonstrating and measuring the performance of quantum communication technologies. NIST has used the testbed to demonstrate QKD in both a fiber-based system and an optical wireless system operating between two NIST buildings.

The NIST fiber QKD system has two channels operating over optical fibers that are wrapped around a spool between two personal computers in a laboratory. The photons are sent in different quantum states, or orientations of their electric field, representing 0 and 1. The system compensates for temperature changes and vibration, which could affect performance, with a NIST-designed module that automatically adjusts photon orientation on a time schedule. More extreme environmental changes are likely to occur in fibers buried or suspended outdoors as in telephone networks; the researchers plan to test a fiber QKD system in the field in the future.

After raw key is generated and processed, the secret key is used to encrypt and decrypt video signals transmitted over the Internet between two computers in the same laboratory. The high speed of the system enables use of the most secure cipher known for ensuring the privacy of a communications channel, in which one secret key bit, known only to the communicating parties, is used only once to encrypt one video bit (or pixel). Compressed video has been encrypted, transmitted and decrypted at a rate of 30 frames per second, sufficient for smooth streaming images, in Web-quality resolution, 320 by 240 pixels per frame.