'Spooky action at a distance'

By contrast, according to quantum mechanics, an experimenter could entangle a pair of particles, separate them by vast distances, then instantaneously change the state of one by changing the state of the other - even at distances of millions of light years.

This "spooky action at a distance," according to Albert Einstein and two colleagues, was a direct result of quantum mechanics if it failed to have more-classical underpinnings. It so defied common sense that they refused to accept quantum mechanics as a complete explanation for how physics really worked at the level of the very small.

The debate remained in the realm of "thought experiments" until 1964, when Irish physicist John Bell, working at the European Center for High Energy Physics in Geneva, described a way to test the idea.

Moreover, he concluded that if one followed the details of Einstein's argument to their logical conclusion, quantum mechanics was more than incomplete, it was wrong. This triggered an initial wave of experiments that demonstrated entanglement in the 1970s and '80s.

In 1997, a team at the University of Geneva conducted a particularly dramatic demonstration by entangling packets of light called photons, then sending them in opposite directions down fiber-optic lines to detectors nearly seven miles away.

When they measured properties of one photon, it had an instantaneous effect on the other. If the interaction behaved in a classical way, a measurable amount of time would have passed between measurement of one and the effect on the other.

Polzik's team is riding what Dr. Wootters calls a "new wave" of entanglement experiments, which has emerged only in the mid-1990s and is driven by the quest to design and build quantum computers.

Cal Tech physicist Richard Feynman is credited with being the first to propose the use of quantum computing, particularly for studying quantum phenomena.

But the idea got its biggest boost in 1993, researchers say, when Peter Shor at AT&T Laboratories in Florham Park, N.J., showed that a quantum computer could solve several types of problems much faster than they could be solved on a conventional computer.

Such problems range from factoring large prime numbers, the key to breaking data-encryption codes, to the "traveling salesman" problem, which tries to find the most efficient path for people to take if they need to visit several customers in a given amount of time.

Quantum computers, Wootters notes, require large assemblages of entangled particles to achieve the data-crunching power required to solve these problems. Entanglement also holds the key to quantum communication and quantum teleportation - ways of transferring quantum information within and among quantum computers.

The possibility of quantum teleportation was first posited in 1993 by IBM researcher Charles Bennett and colleagues.

"Teleportation is a really unfortunate term," says University of Michigan physicist Christopher Monroe. "It implies moving people from point A to point B," when in fact it refers to "creating a quantum state in one place that used to exist somewhere else" with no intervening connection.

In order to instantly teleport those states, he continues, the sender and receiver must share entangled resources, such as Polzik's atomic clouds.

In what Dr. Monroe calls the most notable teleportation experiment yet, three years ago a team of researchers at the California Institute of Technology in Pasadena used quantum teleportation to transfer photons over a three-foot distance.

Unlike other experiments that destroyed the transported photons as part of the process that confirmed their arrival, the Cal Tech group devised a system to verify the photons' arrival without destroying them.

The next step, Monroe continues, will be to teleport states of atoms or other particles of matter - a feat he estimates is still 20 years away.

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