Quantum crypto gets a speed boost

R. A. Hettinga rah at shipwright.com
Thu May 6 22:47:32 EDT 2004



Quantum crypto gets a speed boost

6 May 2004

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

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).

NIST test bed

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.

Crypto components

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

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

R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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