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Single-photon field trials secure peak performance

25 Oct 2002

Sophisticated laser equipment is probably the last thing one would expect to find on top of a snowy mountain in the German Alps. Oliver Graydon finds out why it's there.

From Opto & Laser Europe November 2002

Completely secure ground-to-satellite communication links may soon be a reality, thanks to recent quantum-cryptography experiments over record distances. It may not be long before we can send messages between any two points on the globe with guaranteed secrecy.

During the past 18 months, research groups in Europe and the US have been busy scaling mountains to perform their free-space field trials. In experiments they have sent a cryptographic key, comprised of a stream of single photons, more than 20 km through the air. Much greater distances are likely to be crossed soon.

"In a ground-to-ground setting I imagine that 50 km would be feasible in a high desert environment at night, while in daylight in a city the maximum distance is more likely to be a few kilometres," commented Richard Hughes, an expert in quantum cryptography at Los Alamos National Laboratory, US. "For space applications QKD [quantum-key distribution] with both low-earth-orbit and geosynchronous satellites would be feasible."

Long-distance QKD transmission has already been achieved over glass optical fibre. Research teams at British Telecom, UK, and the University of Geneva, Switzerland, have both sent keys over 50 km fibre links, but performing QKD through the air presents its own technical challenges.

Overcoming the obstacles "The main challenges in free-space QKD are the high background of photons in daylight, maintaining synchronization between the transmitted and received bit strings, and the atmospheric turbulence which affects the optical coupling into the receiver," explained Hughes. "However, we see no scientific or engineering 'showstoppers' to doing a satellite QKD experiment."

In the most recent test, a team from QinetiQ, UK, and the Ludwig Maximillians University in Munich, Germany, transmitted a cryptographic key over 23.4 km, equivalent to an optical loss of 20 dB. The line-of-sight experiment was performed high in the South German Alps to avoid problems with air turbulence, and took place at night to minimize the noise from background light.

Single infrared photons with a wavelength of 850 nm were sent between a transmitter - Alice - on the summit of Zugspitze (altitude 2950 m) and a receiver - Bob - on the summit of Karwendspitze (2244 m). The cryptographic key was encoded into the polarization state of the photons. The transmitter and receiver, specially designed by the Munich team to be as lightweight and compact as possible, were carried up the mountains by cable car and connected to tripod-mounted telescopes and computer equipment.

Minimizing the size and weight of the equipment is vital if it is ever going to be installed on a satellite. "Our system is now portable - we had to get it up the mountain," said John Rarity from QinetiQ. "Our Munich collaborators have developed miniature sources and detectors which we can hang on the back of simple telescopes."

Alice's telescope collimated the infrared pulses into a beam 5 cm in diameter, which was aimed at Bob using a visible laser beam as a guide. At Bob, the beam had expanded to a diameter of 1-2 m and a commercial 30 cm telescope with computer-controlled pointing focused the photons onto a sensitive silicon photodetector. Operating at night with 10 nm-wide spectral filters meant the key could be transferred at a rate of 1.5-2 kbit/s, with errors appearing in less than 5% of key bits. The system operated with a loss budget of up to 27 dB.

"Using slightly bigger telescopes, optimized filters and anti-reflection coatings, we expect to be able to build a system which is stable up to 34 dB of loss and capable of maximum ranges exceeding 1600 km, suitable for satellite-key upload," said Rarity. "The main problem now is not loss but pointing and tracking from the ground and from the satellite with sufficient accuracy."

The Anglo-German trial is the second long-distance experiment reported this year. This summer, Hughes's team from the Los Alamos National Laboratory reported the results of a free-space QKD experiment that operated over 10 km. The tests took place in the mountains of New Mexico near the lab at a wavelength of 772 nm.

Daytime success The Los Alamos system used spectral, spatial and temporal filtering to stop as many background photons as possible from striking the detector. As a result, the experiment worked during the daytime. Hughes says that they are now planning to use their ground-based system to test space-flight prototype equipment.

Both the European and US experiments represent a quantum leap forward in the range of free-space quantum cryptography. The first successful experiment, performed by Charles Bennett at IBM in Yorktown Heights, US, and Gilles Brassard from the University of Montreal, Canada, in 1984, operated over an air gap of just 30 cm. And until this year, the maximum free-space transmission distance was 1.9 km (which was reported by the QinetiQ team in 2001).

The recent boost in performance is largely down to improvements in single-photon sources and detectors, which have now become much more reliable and compact. Both the Anglo-German and US systems make use of specially designed laser diodes and high-performance avalanche photodetectors. In the past, multiple-photon emission and noisy detection have limited the bit rate and range of systems.

Start-ups race for success In fact, the technology has matured so quickly that several start-ups around the globe are now racing to commercialize it. A Swiss spin-off from the University of Geneva, idQuantique, has been founded to develop and sell compact QKD equipment for secure fibre-optic communication.

In New York, US, MagiQ technologies - a start-up founded by Hoi-Kwong Lo, a former QKD theorist at Hewlett-Packard - is busy constructing equipment for quantum information processing including cryptography applications. This level of commercial activity would suggest that the first "real-world" QKD systems may enter operation much sooner than first expected.

For more information New Journal of Physics vol. 4 www.njp.org

What is quantum cryptography? Quantum cryptography, or quantum-key distribution (QKD) as it is also called, uses a stream of single photons to transfer a secret key between two parties with complete confidence that it will not fall into enemy hands. The parties then use the key (a long random sequence of ones and zeros) to encode their messages before they send them over a public communications channel.

Using the polarization of a stream of single photons to transfer the key makes it completely secure. As single photons cannot be split or reliably cloned, any attempt to steal or copy the key is immediately apparent to the receiving party, who notices errors in the transmission.

One of the pioneers of the technology, Paul Townsend at Corning's research laboratory in Ipswich, UK, neatly sums up the benefits. "The central thing about quantum cryptography is that it's fundamentally different to what's used today. It's security based on the law of physics rather than complex mathematics."

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