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Einstein thought quantum entanglement was too strange to be real. But today, physicists can see it in action. Image: Department of Energy.

Small is beautiful. But very small -- quantum small -- is weird. Even Einstein thought so. The laws governing teeny particles, he said, are so bizarre they couldn't possibly be a complete explanation of reality.

In Einstein's day, those laws -- collectively known as quantum theory -- were predicted by mathematics but couldn't yet be tested. But in recent years, quantum theory has charged into the laboratory with stunning success. So far, reality fits the forecast.

Indeed, tiny particles of light and matter behave in some very strange ways. But that hasn't stopped physicists from fooling around with them. And for good cause: Quantum physics holds unmatched promise in computing and code making.

That promise could be harnessed soon, a team of Austrian researchers recently reported in the journal Science.

It takes two to tangle
A subatomic particle can spin clockwise or counterclockwise. Or, if it is cut off from the environment, a particle can stay in limbo -- going clockwise and counterclockwise at once. Only when the particle is measured or observed does it pick a direction and stick with it.

Like we said, it's weird.

University of Wisconsin-Madison physicist Mark Saffman explains with this analogy: Imagine a dime sliced down the middle, into a heads half and a tails half. One half goes to a person who then flies to the moon, and you keep the other half here on Earth. When you look at your half, and see that it is heads, there is no need for you to talk to your moon-bound friend to know she has tails.

But here's the startling difference between two halves of a dime and two quantum particles. In the moment before you look at a quantum particle, it is neither heads nor tails. Rather, it is both at once. When you look, it becomes one or the other. Your mere curiosity has changed its condition.

Weirder still, a quantum particle's quirkiest talent may be its ability to be intimately linked, or entangled, with another. Even when two entangled particles are far apart, a change to one always affects the other.

Two entangled particles can be coupled so that they must swivel in opposite directions. Forcing one to spin clockwise will set the other spinning counterclockwise, no matter how far they are separated in space. They are fatefully entwined.

Einstein called this phenomenon "spooky action at a distance," but we Why Filers think it's romantic.

For those of you who aren't satisfied with analogies, here's a formula that describes how quantum particles get entangled.

Entanglement by moonlight
So what better setting for quantum fireworks than the picturesque Danube River?

Because quantum states (the speed and direction of spin, for example) are fragile, entanglement doesn't last long and can be difficult to arrange. Even small disturbances -- light, moisture, hills, and the like -- can disrupt the bond. So until recently, scientists were only able to entangle particles under pristine laboratory conditions or by connecting distant particles with fiber optic cables.

But since real-world applications like quantum cryptography require distant entanglement, physicists have puzzled over how to overcome these obstacles. The Austrian team has done just that, in an experiment showing the entanglement of particles on opposite banks of the Danube in Vienna.

Star-crossed particles can maintain a magical connection, even across the Danube River. Image: Istanbul News.

Even from two buildings nearly 600 meters apart, during a night of 50 km/h winds, across trees and power lines, the particles stayed entangled. In effect, one particle could "communicate" with the other in an instant, without any visible connection bridging the two.

Such entanglement might someday lead to flawless cryptography. Like many traditional codes, a quantum code would consist of a series of ones and zeros (clockwise or counterclockwise). But since observing a message will change it, any intruders would be caught in the act.

The researchers envision a global network of satellites that would allow entangled particles to send instant messages far and wide. "It is now clear that ... experiments with entangled photons involving satellites can actually be done. This will eventually allow a new era of long-distance ... quantum communciations experiments such as quantum cryptography," says lead author Markus Aspelmeyer.

Such a system would be a breakthrough for spies and bankers -- or anyone else hoping to send a secure message in unbreakable code.

"Using entanglement appears to be absolutely safe" for quantum cryptography, Saffman says. "If anyone tried to steal the data, you'd be able to detect that."

Entangled particles may also transform the way we do computing. Single atoms or particles could be used as counters, like super-small abacus beads. So-called "quantum computers," physicists say, could solve problems now considered impossible. The quantum world may, it turns out, stretch possibilities as far as it stretches minds.

-- Sarah Goforth

Bibliography
"Long-Distance Free-Space Distribution of Quantum Entanglement," Markus Aspelmeyer et al, Science, 19 June 2003.

"The Mystery of the Quantum Cakes," P.G. Kwiat and L. Hardy, American Journal of Physics, January 2000.

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