Putting Quantum Theory to a Nuclear Test

Article excerpt

Putting quantum theory to a nuclear test

Quantum mechanics, the preeminent theory of matter and its interaction with radiation, has a remarkable track record, allowing scientists to predict accurately such properties as the energy levels of a hydrogen atom. Indeed, the theory agrees so well with experiment that it's difficult to imagine alternatives to the present formulation of quantum mechanics. And that has made it hard for scientists to think of experiments to effectively test the theory's completeness. However, the recent development of techniques that allow atomic measurements of unprecedented precision, combined with a novel generalization of quantum mechanics, now makes it possible to set stringent limits on conceivable corrections to standard quantum mechanics.

To provide a plausible framework for testing quantum mechanics, physicist Steven Weinberg of the University of Texas at Austin reworked ordinary quantum mechanics to include an extra, small "nonlinear" term in the equations expressing the theory. His generalized theory predicts that the frequency of radiation used to drive any atomic system from one energy level to another would depend on the amplitude of the radiation, an effect normally ruled out.

Weinberg's original proposal, published in the Jan. 30 PHYSICAL REVIEW LETTERS, prompted four experiments. Three concentrate on nuclear interactions, where the effect seems likely to be largest. The fourth focuses on hydrogenatom energy transitions.

John J. Bollinger and his colleagues at the National Institute of Standards and Technology in Boulder, Colo., reported their findings first. Bollinger's team studied the behavior of the nuclei of beryllium ions held for long periods of time in a magnetic trap.

An atomic nucleus, which can be pictured as a tiny magnet, has a characteristic spin. When that spinning nucleus is tipped relative to an external magnetic field, the nucleus precesses at a particular frequency. The effect of a nonlinear correction to quantum mechanics would be to make the precession frequency depend on the angle between the spin axis and magnetic field direction.

To detect such a small effect, Bollinger and his colleagues cooled 5,000 to 10,000 beryllium ions in a magnetic trap to temperatures of less than 1 kelvin. They then measured the precession frequency for different tipping angles, looking for deviations as small as 5 microhertz in a 303-megahertz signal.

"We didn't see any effect," says David J. Wineland, a member of the Boulder team. The result, reported in the Sept. 4 PHYSICAL REVIEW LETTERS, sets a limit of 4 X 10-27 on the fraction of the binding energy per proton and neutron in a beryllium nucleus that could be due to nonlinear corrections to quantum mechanics. …