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Two crystals entangled, quantum mechanically

Mon, 03/05/2012 - 7:20am
For almost 15 years, Professor Nicolas Gisin and his physicist collaborators at the University of Geneva, Switzerland, have been entangling photons. If this exercise seems to them perhaps henceforth trivial, it continues to elude many of us. The laws that govern the quantum world are so strange that they often make little sense when confronted with the laws of the macroscopic world. This apparent difference in nature between the infinitesimally small and our world poses the question of what link exists between the two.

However these two worlds do interact. To realize this, one must fol- low the latest experiment of the Group of Applied Physics (GAP). Gisin, researcher Mikael Afzelius, and their team have actually produced the entanglement of two macroscopic crystals, visible to the naked eye, thanks to a quantum particle, a photon, otherwise known as a particle of light.

To achieve this exploit, the physicists developed a complex device to which they hold the key. After a first system that allows them to verify that they've actually managed to release one, and only one, photon, a condition essential to the success of the experiment, a second de- vice "slices" this particle in two. This splitting allows the researchers to obtain two entangled photon halves. In other words, even though they are not in the same location, the two halves continue to behave as if they were one.

Wait for the photons to exit

The two halves are then each sent through a separate crystal where they will interact with the neodymium atoms present in its atomic structure. At that moment, because they are excited by these entangled photons, the neodymium lattices in each crystal likewise become entangled. But how can we be certain that they've actually reacted to the two photon halves?

They just have to wait for the two particles to exit the crystals—since they exit after a rather brief period of about 33 nanosecs—and to verify that it really is the entangled pair.

"That's exactly what we found since the two photons that we captured exiting the crystals showed all the properties of two quantum particles behaving as one, characterised by their simultaneity in spite of their separation," says Félix Bussières, one of the authors of the article.

In addition to its fundamental aspect, this experiment carries with it potential applications. Actually, for the specialists in quantum entanglement, this phenomenon has the unpleasant habit of fading when the two entangled quantum objects are too far from one another. This is problematic when one envisions impregnable quantum cryptography networks which could link two distant speakers separated by several hundreds or even thousands of kilometers.

"Thanks to the entanglement of crystals, we can now imagine inventing quantum repeaters," Nicolas Gisin explains. “In other words, the sorts of terminals that would allow us to relay entanglement over large distances. We could then also create memory for quantum computers."

Entanglement still has many surprises in store for us.

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