Several years ago, physicists demonstrated in a lab experiment that an atom can be in two places at once. Now, they have shown why this and other strange atomic-scale behavior don't throw a kaleidoscope of weirdness into our daily lives.
Environmental effects won't let that happen.
Physicists have believed this to be true ever since quantum-mechanics theory, to explain the atomic-scale world, was developed early in the last century. But they have only recently been able to explore it in the laboratory. According to the theory, atoms and subatomic entities can exist in more than one state at a time. By "state," physicists mean such properties as spatial location, momentum, or spin. They often illustrate the theory by analogy with a marble in a shallow round-bottom bowl. The marble can be on the right side or the left side of the bowl - two distinct positions or "states."
As long as these states are not too widely separated, they are superposed on one another. "The marble can be simultaneously at opposite sides of the bowl, rolling from side to side and through itself at the center," according to the announcement of the new experiments at the National Institute of Standards and Technology (NIST) in Boulder, Colo.
The two states are sort of stacked one on top of the other. The scientists call this superposition. This principle of superposition is a key part of quantum theory. Superposition can only exist as long as the atom is part of a closed system isolated from outside influence.
Until now, physicists have not been able to study superposition directly because any disturbance, such as a measurement, causes the atom to snap instantly into one of the states. Then they learned how to use laser beams and electromagnetic forces to isolate individual atoms. These atom traps are as handy an experimental tool for physicists as test tubes are for chemists. They allow experimenters to study atoms in isolation.
In 1996, NIST scientists isolated atoms that, like the mythical marble, were in two positional states at once. They published follow-on studies last month in Nature in which they showed how outside disturbances destroy superposition. Instead of working with an atom's spatial location, they used a beryllium atom that was simultaneously in two different states as represented by the way one of its electrons was spinning. The spin axis could point up or down. NIST team member Christopher Monroe says, "It didn't take long" for a tiny disturbance to force the atom to unambiguously take up a single state. It had crossed the boundary between the world of quantum weirdness and the "real" world we see around us.
Now physicists can explore this process step by step. Dr. Monroe explains: "You need an experiment to cross over the boundary. This is the first experiment where we can do this systematically."
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