CRYPTOGRAPHERS claim that a phenomenon called quantum entanglement could be used to build a secure communication channel. But entanglement is a delicate state that is easily destroyed, severely limiting the channel鈥檚 length. Now there might just be a way to preserve it over long distances, making quantum cryptography a real possibility.
Entanglement is a quantum property that allows two particles to behave as one no matter how far apart they are. It鈥檚 critical to quantum cryptography because it lets two parties 鈥� each of whom has one half of the entangled pair 鈥� exchange keys for encryption and decryption and be sure that no one has intercepted their message.
The problem is that simply sending an entangled photon down an optical fibre, for example, will destroy the entanglement after as little as 20 kilometres. To preserve entangled states over longer distances, Peter Zoller of the University of Innsbruck in Austria and colleagues devised a scheme that used 鈥渜uantum repeaters鈥�.
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To demonstrate the principle, Alex Kuzmich and Dzmitry Matsukevich from the Georgia Institute of Technology in Atlanta used a cloud of about 100,000 rubidium atoms, contained by strong magnetic fields and chilled to within a whisker of absolute zero. In this special state, one of the atoms spontaneously emits a photon that is entangled with the whole cloud.
The researchers took the photons from two such clouds of atoms and mixed them in a device called a beam splitter. This entangled the photons and hence the two clouds of atoms.
Next, the team fired a laser at one cloud in an entangled pair, making it emit another photon. This transferred the quantum state of the cloud into the photon (Science, vol 306, p 663).
To build a repeater, the team plans to take two such photons from two pairs of atomic clouds and pass them through a beam splitter. This will entangle the atomic clouds at the far end of each pair (see Graphic). The process can be repeated until the entangled atomic clouds are as far apart as necessary. 鈥淥ne goal is to build a long-distance quantum connection between, say, Washington DC and New York city,鈥� says Kuzmich.
