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Quantum communication sets new distance record
09 June 2007
From New Scientist Print Edition.
Saswato Das

A new distance record for transmitting entangled photons over free space has brought the prospect of using quantum cryptography for secure satellite communication a step closer.

Entanglement, one of the strange phenomena of quantum physics, has tempting uses in communications. Entangled particles are linked to each other, and remain so even when separated by great distances. Quantum cryptography keeps messages safe by sharing entangled photons between two sites to establish encryption keys for coding and decoding a message. Measure a property of an entangled photon at one site and you instantly know the corresponding property of its distant partner.

Using entangled photons to keep a message secret should guarantee privacy because, in theory, any eavesdropper can always be detected.

But in practice sending entangled photons long distances through the air while preserving their fragile quantum properties has proved challenging. In 2003, Anton Zeilinger of the University of Vienna, Austria, and his colleagues transmitted such photons 600 metres across the river Danube (New Scientist, 28 June 2003, p 15). Then, in 2005, a Chinese group led by Pan Jian-Wai of the University of Science and Technology of China in Hefei extended the record to 7 kilometres (New Scientist, 30 April 2005, p 11).

Now, Zeilinger and his co-workers have gone a step further using ultraviolet photons, which were then passed through a crystal designed to convert them into entangled pairs. The physicists placed their laser source atop a mountain on La Palma, one of the Canary Islands, and beamed one of a pair of entangled photons to a 1-metre telescope at the Optical Ground Station of the European Space Agency on Tenerife, 144 kilometres away. The other entangled photon was kept on La Palma. The set-up allowed observers at the two sites to exchange quantum keys (Nature Physics, DOI: 10.1038/nphys629).

The key to setting the new record was having a fast picosecond laser and a telescope to collect the entangled photon, Zeilinger says. The result has impressed Dick Slusher of the Georgia Institute of Technology in Atlanta, who says both the experiment and the techniques employed were "first class".

Raymond Laflamme, director of the Institute for Quantum Computing at the University of Waterloo in Ontario, Canada, says that the experiment is "a major milestone towards a satellite-based system of completely secure global communication".

Meanwhile, Zeilinger says he and his colleagues "are working to establish a quantum cryptography network in the city of Vienna".

From issue 2607 of New Scientist magazine, 09 June 2007, page 14

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