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Entanglement is the strange, ghostly phenomenon in which quantum particles share the same existence (actually, the same wave function). So a measurement on one instantaneously influences the other, no matter how far apart they might be.

So-called action-at-a-distance lies at the heart of many of modern physic’s most dramatic new technologies: quantum cryptography, quantum teleportation and quantum computation all rely on it.
That makes entanglement important stuff.

“Stuff” is the way many physicists are beginning to think of entanglement: as a resource, rather like water or energy, to be called upon when needed in the new quantum world. These physicists want to be able to create entanglement, use it and store it whenever they need to.

The first two of these–creating and using entanglement–has been the subject of intense research for the last 30 or 40 years. But the ability to store entanglement in a useful way has eluded physicists. Until now.

Today, Christoph Clausen and buddies at the University of Geneva demonstrate not only how to store entanglement but how to release it again in fully working order.

Their device consists of a load of neodymium atoms buried in a crystal of ytterbium silicate, which when cooled, can absorb and store photons. The question that Clausen and co attempt to answer is whether this device can store entanglement too.

So they created a pair of entangled photons, sent one into the crystal and waited until it was emitted again. They were then left with this new photon and the original member of the pair. They then carried out a standard experiment, known as a Bell test, and proved that the pair were still entangled.

That’s impressive for several reasons. For a start, for the entanglement to be preserved, the entire crystal has to be involved. This crystal is about a centimetre in size and the idea that entanglement can be exchanged between a photon and an object of this size is amazing.

Next is the ability to transfer entanglement form a flying qubit–the photon–to a stationary one, the crystal. And to do it with photons with a wavelength of 1338nm, the so-called telecommunications wavelength that can pass easily through fibre optic cables. Any other wavelengths are interesting but practically useless for communications.

But the most exciting aspect of all this is that the entanglement survives the process of storage and release at all. Notoriously fragile, entanglement leaks into the environment like water through a sieve. Being able to store and release it is the enabling technology that could make devices such as quantum repeaters work.

There’s not shortage of uses for this kind of ability. The quantum internet, to name just one, will require the ability to store and send on entangled photons. At one time, it looked more or less impossibile to do this. Entanglement was just too fragile. Now it looks merely a matter of time before we’ll have it on tap.

Ref: arxiv.org/abs/1009.0489: Quantum Storage of Photonic Entanglement in a Crystal

Update 13 September 2010:
Erhan Saglamyurek points out this paper about a memory device for entangled photons published on the arXiv on the same day as the above item

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