Physicists take first step towards practical teleportation.

By PHILIP BALL © Nature News Service / Macmillan Magazines Ltd 2001

Trillion-atom triumph

For the first time physicists have forged quantum entanglement between two large blobs of gas. The achievement brings closer the possibility of super-fast quantum computers and teleportation1.

Eugene Polzik and his co-workers at the University of Aarhus in Denmark have entangled about a million million caesium atoms. Four was the previous record.

"This work should pave the way for a new generation of experiments to teleport states of matter," says Ignacio Cirac, a quantum physicist at Austria's University of Innsbruck.

Teleportation will not involve the wholesale deconstruction and reconstruction of humans, Star Trek-style. It should allow the arrangement of one set of quantum particles to reproduce more or less instantly that of a similar collection of distant particles. In this way a message encoded in photons of light could be transmitted from one place to another without sending the photons across the intervening space.

Entanglement also underpins attempts to perform high-speed quantum computing. It is a property without any analogue in the everyday world.

Quantum particles such as atoms or photons can exist in distinct states, like the head or tail of a coin. Such particles can also exist in a superposition - in both states at once - comparable to a coin spinning in the air before it lands.

If we toss two coins at once, their outcomes are independent: if one is heads, the other could be heads or tails. Two entangled quantum particles, by contrast, have interdependent fates: if one is in a 'heads' state, for instance, the other must be in a 'tails' state.

Maintaining this kind of superposition is very difficult and for any practical applications, entanglement has to embrace thousands, or even millions, of particles. How can something so sensitive be sustained?

Polzik and colleagues forgo full entanglement, where the state of each particle depends on the state of every other particle. Instead, they generate two loosely entangled clouds of caesium gas, one with slightly more atoms in a 'heads' state and the other with slightly more in a 'tails' state. (These two states are actually defined by the directions of the atoms' magnetic fields.)

The interdependence of these clouds is more resilient to measurements or interactions that alter the quantum states of just a few of the constituent atoms.

It would be impossible to maintain full entanglement of this many atoms for longer than a million-billionth of a second. Polzik's team can keep their two clouds in a loose entanglement for half a millisecond. They hope to maintain it for longer in the future, and perhaps to achieve the same thing in the solid samples needed for making quantum computers.



If entangled quantum particles were coins, 'heads' or 'tails' would be interdependent.
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