Keeping a quantum kettle from boiling.

Keeping a quantum kettle from boiling

The adage that a watched pot never boils may have some truth in it after all -- at least in the quantum realm. A team of researchers has demonstrated that making frequent measurements of the state of a quantum system inhibits transitions from one state, or energy level, to another. In other words, the act of observing an atom to determine its state can interfere with quantum jumps between atomic energy levels.

"Our experiment demonstrates the effect clearly and simply," says Wayne M. Itano of the National Institute of Standards and Technology in Boulder, Colo. Itano and his colleagues describe their experiment in paper recently submitted for publication. The research touches on a number of questions concerning the nature of quantum measurements.

The team used radio waves of a particular frequency to drive laser-cooled beryllium ions held in an electromagnetic trap from one energy level to another (from level 1 to level 2 in the diagram). While an ion was going through this quantum jump, the researchers sent in short pulses of light to determine the ion's state.

If the measurement happened to force the ion back into state 1, the light pulse could then shift the ion into energy level 3. The ion would immediately reemit that energy, and the researchers would see scattered light. If the ion were to end up in state 2, no transition to level 3 could occur, and the observers would see no scattered light.

According to quantum theory, the more frequently one tries to observe a system's state as the system is going through a quantum jump, the more likely the system will show up in its initial state. Thus, observing the system's state should interfere with the transition that ought to take place between level 1 and level 2.

That's exactly what Itano and his colleagues found. They detected scattered light, indicating the ions tended to end up in state 1 despite the influence of the radio waves.

Such effects may play subtle but important roles in quantum measurements. "A lot of quantum mechanics and a lot of the things we observe in nature are under conditions where frequent measurements are being made," says physicist Richard J. Cook of the frank J. Seiler Research Laboratory in Colorado Springs. For instance, looking at a particle means observing the photons of light scattered from the particle. "Every time a photon is scattered off and enters your eye, that's a measurement of the position of the particle," he says.

One intriguing possibility is that making appropriate measurements or observations quicly enough could slow or even stop the spontaneous decay of an unstable particle such as a radioactive isotope. But no one is certain whether such a scheme could ever be practical.