Erwin Schroedinger
In 1935 the Austrian physicist Erwin Schroedinger proposed a thought experiment that has titillated philosophers and appalled cat lovers ever since. Now four researchers at the National Institute of Standards and Technology (NIST) in Boulder, Colo., have carried out the notorious "Schroedinger's cat" test using not a living feline but a positively charged atom- -yes, a "cation" in scientific parlance--of beryllium.

Although somewhat smaller than a cat, the cation is still much larger than the electrons, photons and other particles that commonly exhibit bizarre quantum behavior. In a report in the May 24 issue of Science, the NIST team expresses the hope that their "mesoscopic system may provide insight into the fuzzy boundary between the classical and quantum worlds."

Schroedinger devised the cat experiment to illustrate just how radically the quantum realm differs from the macroscopic, everyday world that we inhabit. He himself had shown that a particle such as an electron exists in a number of possible states, the probability of each of which is incorporated into an equation known as the wave function. In the case of an atom of radioactive material, for example, the atom has a certain probability of decaying over a given period of time.

Based onour "classical" intuition, we would assume that there are only two possibilities: either the atom has decayed, or it has not. According to quantum physics, however, the atom inhabits both states simultaneously. It is only when an observer actually tries to determine the state of the atom by measuring it that the wave function "collapses," and the atom assumes just one of its possible states: decayed or undecayed.

Schroedinger reasoned that such probabilistic behavior could exist in the macroscopic world as well, even if we are rarely aware of it. He imagined a box containing an atom having a 50 percent likelihood of decaying in an hour, a radiation detector, a flask containing poison gas and a cat. When or if the atom decays, the Geiger counter will trigger a switch that causes a hammer to smash the flask, releasing the gas and killing the cat. When the experimenter opens the lid of the box and peers inside after an hour has passed, he or she will find the atom either intact or decayed and the cat either alive or dead. But according to quantum mechanics, during the period before the lid is opened, the cat exists in two superposed states: both dead and alive.

During the 1980s, the late theorist John Bell suggested a more palatable version of Schroedinger's experiment, one in which the decay of the atom causes a bottle of milk to spill onto the floor; the superposed cat is thus hungry or full rather than alive or dead. But either version seems weirdly nonsensical: the outcome seems logical from a quantum physics viewpoint, but common sense tells us that a cat cannot be alive and dead (or hungry and full) at the same time.

The paradox of Schroedinger's cat has provoked a great deal of debate among theoretical physicists and philosophers. Although some thinkers have argued that the cat actually does exist in two superposed states, most contend that superposition only occurs when a quantum system is isolated from the rest of its environment. Various explanations have been advanced to account for this paradox--including the idea that the cat, or simply the animal's physical environment, can act as an observer.

The question is, at what point, or scale, do the probabilistic rules of the quantum realm give way to the deterministic laws that govern the macroscopic world? This question has been brought into vivid relief by the recent work done by the NIST team, which includes Christopher Monroe, Dawn Meekhof, Brian King and Dave Wineland. The group confined a charged beryllium atom in a tiny electromagnetic cage and then cooled it with a laser to its lowest energy state. In this state the position of the atom and its "spin" (a quantum property that is only metaphorically analogous to spin in the ordinary sense) could be ascertained to within a very high degree of accuracy, limited by Heisenberg's uncertainty principle.

The workers then stimulated the atom with a laser just enough to change its wave function; according to the new wave function of the atom, it now had a 50 percent probability of being in a "spin-up" state in its initial position and an equal probability of being in a "spin-down" state in a position as much as 80 nanometers away, a vast distance indeed for the atomic realm. In effect, the atom was in two different places, as well as two different spin states, at the same time--an atomic analog of a cat both living and dead.

(The clinching evidence that the NIST researchers had achieved their goal came from their observation of an interference pattern; that phenomenon is a telltale sign that a single beryllium atom produced two distinct wave functions that interfered with each other.)

The NIST investigators are planning experiments that will probe more deeply into the process by which superposed systems "decohere," losing their schizoid qualities and giving way to more mundane classical behavior. Monroe says he and his colleagues hope both to coax single ions into a superposition of three or more states and to manipulate two or more ions in the same trap.

If the past is any guide, these experiments will not resolve the mystery of quantum mechanics but will only underscore just how deeply it seems to contradict our usual expectations. As Niels Bohr was fond of saying of quantum mechanics, "If you think you understand it, that only shows you don't know the first thing about it."