Schrodinger's cat: two atoms in one?

Underlying the peculiar world of quantum mechanics is the notion that, under certain circumstances, matter can exist in more than one state or position at the same time.

Long held as an oddity of quantum mechanical theory, this property of matter has defied easy experimental realization. Even more exotic is the idea that the act of observation or measurement somehow determines which state exists at any given moment.

To illustrate quantum mechanics' strange nature, Austrian physicist Erwin Schrodinger proposed in 1926 a puzzling thought experiment. If a cat is placed in a sealed box and its fate-to live or die-is correlated with whether or not an atom radioactively decays, then the presence of the atom's decayed and undecayed quantum states translates into a cat that is simultaneously dead and alive-a highly counterintuitive idea.

Now, a team of physicists has managed to create a "Schrodinger-cat-like state of matter" in a single atom. By supercooling a beryllium atom with a laser, then prodding it with a rapid sequence of laser pulses, the physicists have managed to get the atom to oscillate in such a way that it exists "in the bizarre state of being in two well-separated positions at once." "This situation defies our sense of reality," say Christopher Monroe, a physicist at the National Institute of Standards and Technology in Boulder, Colo., and his colleagues in the May 24 Science. "Schrodinger's cat paradox is a classic illustration of the conflict between the existence of quantum superpositions and our real-world experience of observation and measurement."

When we observe cats, they say, we don't expect our observations to influence whether the felines die or stay alive-or to see one cat both dead and alive. In the recent atomic experiment, the researchers make one cold beryllium atom vibrate harmonically, producing what they call a superposition of two "coherent-state wave packets." The atom's electrons oscillate in a way that creates a dual presence, as if two atoms existed in distinct locations at the same time.

"Imagine a marble in a bowl, rolling back and forth," says Monroe. "At one point, the one marble appears as two marbles rolling back and forth in opposite directions, passing through each other and appearing simultaneously at each edge of the bowl."

The atom's two states are separated by 80 nanometers. "For a brief period, the atom appears to exist in two places," Monroe says. "This is a marvelous experiment," says Wojciech H. Zurek, a physicist at Los Alamos (N.M.) National Laboratory. "They're putting an ion into a trap and separating it into a very weird superposition. It's very clever."

Although this experiment involves only a single atom rather than a visible object, such as a cat, Zurek says it will open the door to deeper experimental probes into "the boundary between classical physics and quantum mechanics." While this experiment ignores macroscopic effects on objects such as Schrodinger's cat, it may at least have demonstrated, says Zurek, "the paradox of Schrodinger's kitten."