Taking quantum leaps one at a time.

Taking quantum leaps one at a time

Niels Bohr made a revolution in physics by proposing that physical processes inside atoms do not go in the smooth, continuous way familiar in classical physics, but in sudden sharp, discontinuous acts that became known as quantum jumps. Bohr's proposal was accepted because it solved the difficulties encountered in trying to interpret data on atomic processes in a classical way, but until now the justification for it rested on the behavior of large aggregates of atoms. Nobody had been able to test the theory of quantum jumps on the level of the single atom.

Now there are two claims to the observation of single quantum jumps in single atoms, one from a group at the University of Washington in Seattle (Warren Nagourney, Jon Sandberg and Hans Dehmelt), the other from a group at the University of Hamburg (Thomas Sauter, Werner Neuhauser, Rainer Blatt and Peter E. Toschek).

As described last week in San Francisco at the International Quantum Electronics Conference '86, the two experiments are quite similar. Both used barium ions in electromagnetic traps. In such an apparatus, an electromagnetic field of a complicated shape holds the electrically charged ions in place. Red and green lasers irradiate them to excite a series of quantum transitions that yield fluorescent light. The amount of fluorescence recorded varies according to how many atoms are radiating at any moment, and the data the experimenters present show jumps that indicate whether one, two, three, etc. ions are in the trap and radiating. Thus, they say they have observed individual quantum jumps.

A questioner at the conference wanted to know whether the experimenters would try to see whether quantum jumps are truly instantaneous--another radical difference from classical physics, where changes and interactions always take time. Toschek said he had thought of doing it, but had not figured out a scheme.

Toschek also pointed out that this work opens to experiment a question that has so far been purely philosophical: "Do photons [for other subatomic particles] have an individuality?" Physicists have tended to assume that photons, other subatomic particles and atoms themselves were indistinguishable from others of their class. At higher levels of organization this is not so: One three is distinguishable from another, one person from another. Perhaps we shall have to start naming photons and barium atoms.