Quantum crypto reaches 150 km (March 2004)

[R. A. Hettinga]

<http://optics.org/articles/news/10/3/11/1>
optics.org - News -  

 Quantum crypto reaches 150 km
 12 March 2004

A single photon is sent over a 150 km optical link beating the previous
transmission record by 50 km.

Scientists at NEC in Japan claim to have smashed the transmission distance
record for quantum cryptography. The team says it successfully sent a
single photon over a 150-km-long optical fiber link. This significantly
extends the previous record of 100 km, which was announced in June 2003.

Quantum cryptography uses a stream of single photons to transfer a secret
key between a transmitter and a receiver. Each transmitted bit of the
cryptographic key is encoded upon a single photon. Any attempt to intercept
the key changes the quantum state of the photons, which reveals the
presence of a hacker.

NEC's record-breaking system relies on planar lightwave circuit (PLC)
technology and a low-noise photon receiver. The system containing was
developed by a collaboration of researchers from NEC, the
Telecommunications Advancement Organization of Japan and the Japan Science
and Technology Agency.

According to NEC, its system has two distinct benefits. Firstly, it offers
stable one-way photon transmission. This reduces the noise of backscattered
photons from the optical fiber to below one-tenth that of conventional
round-trip systems.

The system's second plus-point is an alleged ten-fold increase in
signal-to-noise ratio compared with current systems. This is largely thanks
to the receiver's increased sensitivity to photons that have been broadened
by dispersion in the long fiber-optic link.

"Due to wide-area coverage, this system can realize quantum cryptography
transmissions in optical network in metropolitan areas and is expected to
contribute to the realization of an optical fiber network system requiring
advanced safety levels to prevent code-breaking in the future," said NEC in
its press release announcing the breakthrough.

Author
 Jacqueline Hewett is technology editor on Optics.org and Opto & Laser
Europe magazine.

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R. A. Hettinga <mailto: rah at ibuc.com>
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