New Quantum Cryptography Speed Record

By Roland Piquepaille

Physicists from the National Institute of Standards and Technology (NIST) have established a world's speed record for 'unbreakable' encryption with their cryptographic system based on the transmission of single photons. With this kind of method, messages cannot be intercepted without detection, meaning transmission is always safe. The NIST "quantum key distribution" (QKD) system was used between two buildings located 730 meters apart for transmitting a stream of photons at a rate of 1 million bits per second. This might not look very fast, but it's 100 times faster than with previous quantum distribution systems.

The demonstration, [which will be described in the May 3 issue of Optics Express,] is the first major reported result from a new NIST testbed built to demonstrate quantum communications technologies and cryptographic key distribution. The testbed provides a measurement and standards infrastructure for research, testing, calibrations and technology development. Scientists tested the QKD system by generating an encryption key that could be sent back and forth between two NIST buildings that are 730 meters apart. They are using the testbed to develop data-handling techniques associated with this type of encryption.

First, here is a little background on quantum cryptography.

Quantum systems -- exploiting the laws of quantum mechanics -- are expected to provide the next big advance in data encryption. The beauty of quantum key distribution is its sensitivity to measurements made by an eavesdropper. This sensitivity makes it possible to ensure the secrecy of the key and, hence, the encrypted message. The keys are generated by transmitting single photons that are polarized, or oriented, in one of four possible ways. An eavesdropper reading the transmission causes detectable changes at the receiver. When such changes are observed, the associated key is not used for encryption.

Now, why is the NIST system much faster than other systems?

Compared to previously described QKD systems, the major difference in the NIST system is the way it identifies a photon from the sender among a large number of photons from other sources, such as the sun. To make this distinction, scientists time-stamp the QKD photons, then look for them only when one is expected to arrive.

The NIST team has packaged data-handling electronics operating in the gigahertz (1 billion bits per second) range in a pair of programmable printed circuited boards that plug into standard PCs. Photon losses caused by imperfections in the photon sources and detectors, optics and procedures reduce the key generation rate. However, 1 million bps makes QKD practical for a variety of new applications, such as large network distributions or streaming encrypted video.

The system still needs to be improved before it can be widely used. In the mean time, here is a link to additional information on quantum key distribution.

"The sender, who cryptographers generally call Alice, randomly chooses both a mode and a digital value or orientation for each photon sent over the quantum channel. The receiver, generally called Bob, randomly chooses between the two modes when he tries to detect a photon. This can be visualized as choosing a mailbox slot that accepts only envelopes flying in certain orientations. If he chooses the same mode that Alice used for a particular photon, then Bob always measures the correct orientation, and hence, its digital value. But if he chooses a different mode, then he may get the wrong value for that photon." (Credits for cartoon and legend: NIST)

And for very detailed information about what quantum information theory is, you can read this long presentation by Carl J. Williams, from the NIST (March 23, 2004, PDF format, 77 pages, 2.6 MB).

Sources: National Institute of Standards and Technology (NIST) news release, May 3, 2004, via EurekAlert!; NIST website