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Quantum crypto gets a speed boost

06 May 2004

NIST scientists transfer a quantum key made of single photons at a rate of 1Mbps.

A team of US scientists from the National Institute of Standards and Technology (NIST) in Colorado and Acadia Optronics, Maryland, claims to have built the world’s fastest quantum cryptography system (Optics Express 12 9).

Its 730 m free-space link, which uses a stream of single photons to transfer a secret encryption key, offers a key transfer rate of 1Mbps -- about 100 times faster than previously demonstrations. NIST says that the increase in speed could potentially make quantum cryptography practical for applications such as streaming encrypted video or communications across large networks.

Quantum key distribution (QKD) has recently emerged as an attractive technique to create completely secure communication links between banks and military bases and the first commercial systems are now starting to appear.

Although the transmission distances have steadily improved over the past few years, the current records are 150 km in fiber and 23 km in free space, the transfer rate of the key has remained painfully slow, typically 1 kbps or so.

The NIST-Acadia team has boosted this transfer rate to 1 Mbps by employing a clock synchronization scheme typically found in high-speed optical communications.

The innovation is to operate a classical (unsecure) link at 1.5 microns in parallel with an 845 nm QKD link over a 730 m span between two NIST buildings. The classical link, at a clock rate of 1.25 Gbps, is used to synchronize the QKD receiver and tell it when to look for the key’s photons.

This synchronized detection helps distinguish the QKD photons generated by a pair of 845 nm VCSELs from stray light such as photons from the Sun and thus raise the key transmission rate.

Although in theory it should be possible to achieve key transmission at up to the clock-rate, the team has found that the 350 ps timing resolution of its silicon avalanche photodetectors currently limits performance to 1 Mbps. The team says with better detectors the key rate could be raised further.

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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