In Quantum Feat, Atom Is Seen in 2 Places at Once
By GEORGE JOHNSON

Appalled by the weird implications of quantum mechanics, the rules that explain the workings of the tiny particles that make up the universe, Albert Einstein used to stroll the streets of Princeton wondering why the moon wasn't smeared all over the sky. 

After all, the moon is made of these particles, and quantum theory holds that until it is observed, a particle doesn't have a definite position. It remains suspended in a mathematical limbo: a state of pure potentiality consisting of all the possible positions it could conceivably occupy. 

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Or, as Einstein put it, the moon exists even if no one is watching it. 

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Do you really believe, Einstein once asked a younger colleague, that the moon exists only when you look at it? 

Einstein had come face to face with the fundamental paradox of quantum mechanics: why the laws that apply so precisely to the subatomic realm do not appear to carry over into the domain of everyday things. 

"According to quantum mechanics, a bottle of Coke should be able to exist in a superposition of two locations, both here and there," said Dr. Wojciech H. Zurek, a theorist at Los Alamos National Laboratories in New Mexico. "We do not see such states. Ever. Bottles are either here or there. What eradicates quantum weirdness?" And could the solidity of reality really be dependent on the presence of observers? 

Some physicists believe they are coming closer to an answer with a phenomenon called decoherence, in which the particles themselves constantly "observe" one another, eliminating the quantum fuzziness and yielding the familiar world of solid objects. 

In recent weeks this theoretical notion received what might be its strongest support yet. Experimenters at the National Institute of Standards and Technology in Boulder, Colo., observed decoherence in action. They used laser beams to gently manipulate an atom, essentially putting it in two places at once. Then they measured the breakdown of this perplexing state of existence more methodically than ever before. 

Dr. Seth Lloyd, an associate professor in the department of mechanical engineering at the Massachusetts Institute of Technology and an expert in quantum theory, called the work an "experimental triumph." 

"Certainly no experiments have been done previously that so carefully and thoroughly examine the decoherence process," he said. "The results confirm to a high degree of precision the theoretical predictions of the last 20 years." 

Though other groups have measured decoherence in the past, the Boulder experiment is the first to systematically observe how quickly quantum ambiguity is resolved when a particle is exposed to different kinds of environments. 

"This is indeed a significant piece of work," said Dr. Gerard J. Milburn, head of the physics department at the University of Queensland in Australia. Dr. Milburn's calculations about how quickly quantum fuzziness gives way to tangible objects were borne out by the demonstration. "This is the most definitive experimental validation of these predictions to date," he said. 

In pure isolation, sealed off from the influence of its surroundings, a particle is represented by a mathematical device called a quantum wave function: all of the particle's possible states (its position or momentum, for example) cling together in a condition known as quantum superposition. In traditional interpretations of quantum mechanics, it is the act of observation that causes this wave function to "collapse," forcing the particle to choose one state or another. 

Physicists have tried to discourage mystics by emphasizing that the observer need not be a conscious being: an electronic detector or a photographic plate will do. It is the collision with the rock-solid world that resolves the particle's ambiguous existence. 

But many physicists find this explanation dissatisfying. They would like quantum mechanics to be a completely self-contained theory, with no need to invoke any kind of outside measurer. 

After all, if quantum mechanics is taken to its logical extreme, the universe itself can be described by a wave function, all its possible histories hovering together in superposition. 

By definition, there can be nothing outside the universe, no external observer or measurer to conjure up this particular universe from the plentitude of possibilities. 

Maybe all it takes to collapse the wave function, Dr. Zurek and his colleagues propose, is for a particle to undergo some kind of tiny disturbance, to come into contact with other particles. The delicately balanced superposition in which all the possibilities stick together, or "cohere," would come unglued. It would "decohere." Then the particle could assume a particular position. 

Or, as Dr. Zurek has described it, "the watchful eye of the environment" -- the particles and waves that pervade creation -- is constantly making measurements, banishing quantum ambiguity and conjuring up hard-edged reality, the familiar world dominated by the commonsensical laws of classical physics. 

There would be no need for a curious observer or even a measuring instrument to solidify Einstein's moon or to put Dr. Zurek's Coke bottle on one side of the table or another. The decoherence caused by the jiggling of an object's own atoms and the particles around it would be enough. 

To dramatize the problems of applying quantum theory to the classical world, the Austrian physicist Erwin Schrodinger devised his famous thought experiment in which the fate of a cat is tied to that of a single subatomic particle. In one version, a photon (a particle of light) is fired at a half-silvered mirror, giving it a 50-50 chance of reflecting back or sailing through. If the photon passes through the mirror, it strikes a photoelectric detector, activating a circuit that breaks a vial of poison and kills the cat. If the photon is reflected away from the detector, the cat is spared. 

Schrodinger argued that until the box was opened and the outcome of the experiment was registered, the photon would linger in a superposition of the two possible paths it could take, leaving the cat in the uncomfortable position of being simultaneously dead and alive. 

Decoherence suggests why this is not worth worrying about. Each atom in the cat is tied into a complex environment of other atoms that constantly interact, spiriting away the quantum effects. 

More specifically, the theory predicts that the speed at which quantum superpositions collapse, giving rise to what theorists call "classicality," depends on how far apart the alternate possibilities are in something called Hilbert space. Put more colloquially, it depends on how different they are. A Coke bottle on one side of a table is far removed from a Coke bottle on the other side, and a live cat is very different from a dead one. Hence, the superpositions -- the bottle both here and there, the cat both dead and alive -- almost immediately go away. 

To observe decoherence in action, the researchers in Boulder trained their sights on the "mesoscopic" realm -- between the submicroscopic world, where particles can hover indefinitely in superposition, and the macroscopic world of objects, where superpositions disappear too soon to be noticed. 

In the experiment, reported Jan. 20 in Nature, Dr. David Wineland and his colleagues trapped a charged atom inside an electromagnetic field. Using laser pulses, the researchers coaxed it into a "cat state" in which its outer electron simultaneously had two opposite "spins." It was as though a tiny sphere were rotating clockwise and counterclockwise at the same time. Laser beams were then used to nudge apart the two states separating them by about 100 billionths of a meter. 

The experimenters had created weird superpositions like this before. This time, though, they went on to see what would happen if the "cat state" was exposed to various disturbances in the form of electrical fields. As predicted by decoherence theory, this interaction with an environment rapidly forced the superposition to come undone and the atom took its place in the world. How fast this happened depended, as theory predicted, on how far the two superpositions were separated. 

"We hope these kinds of experiments may shed some light on the inconsistencies between what quantum mechanics predicts and our everyday experience," Dr. Wineland said. He also noted that a deeper understanding of decoherence could help scientists build experimental quantum computers. 

In these theoretical devices, all the calculations needed to solve a problem would be performed simultaneously in quantum superposition. The result could be extremely powerful machines that crack problems now considered impenetrable. But first scientists would have to learn how to control decoherence, keeping the superpositions from collapsing before a calculation is done. 

Decoherence has also been measured in other laboratories under entirely different conditions. 

In 1996, Dr. Serge Haroche and his colleagues at the École Normale Supérieure in Paris created cat states by putting an electromagnetic field into a superposition in which its waves were simultaneously in different phases -- reaching their crests and troughs at different times. Then they upset the quantum balancing act by sending an interloping atom -- a "quantum mouse" -- through the field. They too found that the larger the separation between the alternate quantum states, the faster decoherence came into play. 

"In all of the cases studied, decoherence behaves as predicted by theory," Dr. Zurek said. 

"What is best, they confirm the prejudice of theorists (like yours truly) that quantum theory can explain this emergence of classicality without any modifications (which have been invoked by other, more desperate but no less reputable theorists)." 

The University of Oxford physicist Roger Penrose, for example, has proposed that the mystery of how classicality arises cannot be completely understood without radically overhauling quantum theory and uniting it with Einstein's general theory of relativity -- one of the biggest challenges facing physics. 

Even if decoherence succeeds in solving the problem of Einstein's moon, a central mystery will remain: the theory may explain why people do not see the weird quantum state in which the Coke bottle is on both sides of the table. But nothing explains why, when the superposition collapses, the bottle ends up on, say, the left side rather than the right. 

As some physicists see it, decoherence must cause the universe to somehow split in two, spawning this world, where the bottle is on one side, and another, parallel world where it is on the other. According to this "many worlds" interpretation, all the different ways history might have unfolded coexist in superposition. 

Some physicists, like Dr. David Deutsch at Oxford, insist that these parallel universes are as solid and real as our own. Others, like Dr. Lloyd, believe they should merely be thought of as abstract possibilities -- things that did not occur. 

"The criterion for things being real ought to be that we're able to get information about them," Dr. Lloyd said. "The alternatives in the other worlds are inaccessible and therefore unreal. I really got up this morning and fried an egg for my daughter and myself. There is another world where my daughter and I had cereal. 

"The cereal world is in the wave function of the universe, but it's not real in the sense that any information I'm going to get will falsify the hypothesis. All the information says we had eggs. Look at my cholesterol level!" 

Is there another universe where he is picking a milk-sogged Cheerio from the end of his spoon and wondering how anyone could possibly think he had eaten eggs? As far as this Dr. Lloyd is concerned, it does not really matter.