THE Great Wall of China is poised to play its part in pushing back the boundaries of quantum cryptography. Later this year a Chinese team, which has just broken the record for transmitting entangled particles, will test the feasibility of satellite-based quantum communication using the wall.

The Great Wall’s new role was revealed after Jian-Wei Pan of the University of Science and Technology of China in Hefei and his colleagues successfully transmitted “entangled” photons through more than 7 kilometres of the Earth’s turbulent lower atmosphere without losing the photons’ fragile quantum properties.

Quantum entanglement allows two particles to behave as one even if they are very far apart. Measure the property of one particle and you instantly know the property of the other. Entanglement allows you to transmit secure encryption keys over a public channel, but until now the furthest anyone had transmitted entangled particles through air was about 600 metres. This was achieved by researchers at the University of Vienna, who sent entangled photons across the river Danube (New Scientist, 28 June 2003, p 15).

Depending on atmospheric conditions, the amount of air between a base station and a satellite in low Earth orbit is equivalent to 5 to 10 kilometres of air at ground level, so the Chinese experiment brings satellite-based quantum communication within reach.

The team achieved their record distance by improving on the optics used by the Viennese team. Instead of a using a 1-millimetre-diameter laser beam to transmit photons, the Chinese team used large telescopes to transmit laser beams 12 centimetres across. These were beamed to two receivers, one 7.7 kilometres away in one direction and another 5.3 kilometres away in another (Physical Review Letters, vol 94, p 150501). The wide beam ensured clear reception even though one of the beams passed through the polluted air of downtown Hefei, says Pan.

Other physicists in the field are impressed. “I congratulate the Chinese group on their success,” says John Rarity, a researcher in quantum communications systems at the University of Bristol, UK. “It shows space-based quantum key distribution is technically possible.”

But further tests over longer distances are needed to verify this, which where the wall comes in. “In the autumn we will test transmission of entangled photons between receivers 20 kilometres apart on the Great Wall,” he says. “We used to use light from fires on the Great Wall to signal invasions, now we are going to signal the future.”

by Paul Marks

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