'Spooky action at a distance'
One of the most famous felines of the 20th century is slowly opening the door to discoveries that could revolutionize communications, and computers - and spawn ever more rigorous tests of the very foundations of physics itself.
The cat is the potentially tragic hero(ine) in a quantum-physics paradox penned by Austrian physicist Erwin Schrödinger in 1935. The cat finds itself enclosed in a box with an atom. When the atom decays, the cat will die.
In quantum physics, the cat and atom exist in both states - alive and dead, decayed and undecayed - until someone opens the box and checks. Regardless of what the observer sees inside the box, the states of the cat and the atom are inextricably interlinked.
Among the notions Schrödinger tried to illumine with his paradox is a property he dubbed entanglement. In effect, the state of the cat "knows" the state of the atom - even at a distance. Thus, if an experimenter measures the state of one, he or she will know the state of the other without making the additional measurement - the relationship between their two states remains constant.
Entanglement forms the basis for key elements in the burgeoning field of quantum computing and communication. Whether quantum computers will ever be built remains an open question, some researchers say. But if such computers are built, achieving and maintaining entanglement will be critical for everything from processing data to transmitting it.
Hence the excitement over a report this week that physicists in Denmark have entangled two large clusters of atoms in neighboring containers. The feat, the team says, represents the first demonstration of entanglement between separated, large clusters of atoms, at room temperature, and for relatively long periods of time.
The Danish team's effort is not the first time scientists have entangled atoms, notes Eugene Polzik, who led the team at the University of Aahrus in Aarhus, Denmark.
Last year, for example, researchers at the National Institute of Standards and Technology in Boulder, Colo., reported that they had achieved entanglement with four atoms. The entangled atoms in the NIST experiment established their relationships through close-up interactions.
"That was a milestone for quantum computing," says Dr. Polzik. "But it's not so good for quantum communications, where you need to have entangled particles miles apart."
His team advanced that prospect by using a laser to entangle atoms in two containers a few millimeters apart. His team gathered cesium atoms and confined them in a pair of glass containers. Each held a trillion atoms.
The researchers treated each sample with a laser to give each cluster's overall magnetic "spin" its own orientation. Then the team sent a single laser beam through the samples to entangle the disparate clouds. A similar laser shot half a millisecond later showed that while the orientation of each cloud's spin had shifted somewhat, the original relationship between the two clouds' orientations remained the same.
"This is a real step forward," says William Wootters, a physicist at Williams College in Williams, Mass., who studies quantum interactions and did not take part in the experiment.
The team's use of a laser to entangle the disconnected clouds of atoms holds the promise for longer-distance quantum communication, which requires a set of entangled particles at each end of the quantum "connection."
Like kids at an egg-toss contest, the team plans to continue to widen the gap between samples to see how far they can separate the clouds and still trigger entanglement.
Entanglement has an embattled history in physics, Dr. Wootters says.
Back when Herr Schrödinger was writing about stuffing cats and atoms into boxes, he also held that entanglement was the one feature of quantum theory that distinguished it from "classical" physics, in which cause and effect could be distinguished and one object is forbidden from influencing another object at a distance instantaneously.
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.
