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Schroedinger's Cat in an Atomic Cage

They say, "You can't have your cake and eat it, too."

They say, "You can't be in two places at one time."

"They" may be wrong, however, since scientists at the Commerce Department's National Institute of Standards and Technology have just disproved the latter by preparing a beryllium atom that is simultaneously located in two widely separated places.

At the NIST laboratories in Boulder, Colo., Christopher Monroe, Dawn Meekhof, Brian King and David Wineland isolated a single beryllium ion (an atom with one of its two outer electrons stripped away) in an electromagnetic trap and cooled it nearly to absolute zero with precisely tuned laser beams. This confined it to a tiny region of space less than a millionth of a centimeter across, where it rested almost without motion.

The ion's single remaining outer electron can be in two internal quantum states: "up" and "down." These states correspond to different orientations of the spin of the electron. The laws of quantum mechanics also allow the electron to be placed in a "superposition" of the two states, in which the two states both exist and are, in a sense, sort of stacked upon one another. Until the particle is disturbed by an outside agent, there is an equal probability that it is in either state, and thus it is considered to be in both states.

Next, additional pulses of laser radiation were delicately applied, producing a tiny force in a manner that pushed one way on the "up" electron state, and the opposite way on the "down" state. This force, in effect, gently shoved the two states apart without collapsing them to a single entity, so that the states that were superimposed on each other in the original ion became two physically separated states. The separation was more than 80 nanometers, or 11 times the size of the original ion.

This bizarre state, of being in two well-separated places at once, can be visualized by imagining a large, shallow, round-bottomed bowl with a marble simultaneously at opposite sides of the bowl, rolling from side to side and through itself at the center. The experiment provides a glimpse of quantum superposition states at a scale never seen before.

As detailed in the May 24 issue of Science, this experiment has connections to the works of Albert Einstein and Erwin Schroedinger, both of whom in 1935 described hypothetical scenarios allowed by quantum mechanics which seemed to defy reality. Schroedinger, for example, considered the possibility that a cat could be made to be both dead and alive at the same time. "Schroedinger's cat" soon became a shorthand way to refer to a whole class of superposed states, and quantum particles in microscopic superposed states have been observed for many years. Until now, however, no one has ever prepared a particle where the superposition was transformed into a physical separation on so large a scale and under such controlled conditions.

Schroedinger cat states are extremely fragile. Any interaction with the surroundings will destroy the superposition and the ion collapses into a single entity (becomes "decoherent"). As the separation is made larger, the cat state becomes more fragile; this is interpreted as the reason such states are never seen in the larger world of common experience.

Given that fragility, how did the scientists detect that the Schroedinger cat state really existed? If observing the ion collapses it, how did they know?

Their technique was to repeat the experiment many times while slowly varying the direction of the force that shoved the states apart, and to look at the end result after each trial. They found that when the direction was such that the two states began to physically overlap, they produced an interference pattern, and a narrower pattern demonstrated a greater amount of original separation of the superposed states.

The NIST experiment is an outgrowth of advanced research on trapped atoms aimed at developing new kinds of atomic clocks. NIST operates the U.S. primary standard of time and frequency, and is charged with providing the country with a standard that meets the ever-advancing demands of industry and technology. Trapped atoms are one of the mechanisms being studied for their potential to provide time and frequency thousands of times more precisely than today's standards.

The experiment also may provide a route to the first controlled studies of quantum decoherence, a topic that has received much interest lately for its importance in the fields of quantum computers and quantum cryptography.

However, this work probably won't offer any hope to greedy birthday celebrants or overscheduled modern folks. As Monroe says, "I don't think this is likely to lead to our being able to attend a son's piano recital and a daughter's softball game at the same time, or eat a cake and store its superposed double in the freezer. The quantum states of birthday cakes and people are just too complicated to separate like we can a single atom."

An agency of the Commerce Department's Technology Administration, NIST promotes economic growth by working with industry to develop and apply technology, measurements and standards.

Released May 23, 1996, Updated November 27, 2017