It is a quantum leap of sorts: wireless quantum encryption over very long distances just got a step closer.
Quantum communication relies on sending entangled photons, each carrying a bit of information. It is secure because it will be obvious if anyone intercepts the photons, since doing so will disrupt the entanglement. Safely received bits are then used as a key to encrypt further communications.
The furthest a quantum key has been sent through free space is 144 kilometres (New 杏吧原创, 9 June 2007, p 14). Achieving greater distances is tough because the atmosphere disturbs the photons鈥� delicate quantum states.
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Now a group of Italian researchers, led by Cesare Barbieri of the University of Padova, Italy, has teamed up with Anton Zeilinger and his colleagues at the Institute for Quantum Optics and Quantum Information in Vienna, Austria. Their idea? Send the photons via satellite. Because the atmosphere thins with altitude, travelling from a satellite thousands of kilometres up is equivalent to travelling just 8 kilometres at the Earth鈥檚 surface.
To prove that single photons sent from an orbiting satellite can be detected on Earth, the team sent ordinary laser pulses from a telescope at the Matera Laser Ranging Observatory in Italy to the Ajisai satellite, some 1485 km above the Earth. A precisely positioned on-board mirror bounced single photons back to the observatory (). The work will appear in the New Journal of Physics.
鈥淭hese experiments show that quantum communication from space is possible,鈥� says Zeilinger, who is now proposing building a satellite that can produce entangled photons.
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