Two Americans and a German were awarded the 2005 Nobel Prize in physics yesterday for theoretical research explaining how lasers work and for practical developments using lasers to explore the fine structures of atoms.

Roy Glauber of Harvard University will receive half the $1.3 million prize for applying quantum theory to the light emitted by lasers, a feat that reconciled the dual nature of light, which can behave like both a particle and a wave.

"You don't need Glauber's theory to invent the laser, but you do need it to understand its properties," said physicist Daniel Kleppner of the Massachusetts Institute of Technology.

John Hall of the JILA Institute at the University of Colorado, Boulder, and Theodore Hansch of the Ludwig-Maximilians-Universitat in Munich, Germany, will share the other half equally for their development of techniques to precisely control the frequency of lasers. The techniques allow measurement of the physical properties of atoms — and also of space and time — with unprecedented accuracy.

Such precision should enhance the accuracy of clocks, increase the accuracy of global positioning systems, improve the navigation of long space flights and help in the pointing of space telescopes, among other things, according to the Royal Swedish Academy of Sciences, which selected the winners.

Before Glauber published his seminal paper in 1963, researchers used classical optics theory from the 19th century to explain the behavior of light. Many researchers believed that quantum theory, which had already proved successful in describing the behavior of matter, could not even be applied to light.

But the development of the laser showed that the 19th-century theory was fraying around the edges and could not be used to make accurate predictions for new applications.

"It occurred to me ... that one had better develop the quantum theory to the fullest extent possible," Glauber said yesterday.

His work "laid the foundations for future developments" in optics, the Nobel citation said.

The development of lasers operating at single frequencies made possible advances in the study of atoms and molecules. But those studies were limited, Hall said, by the ability to lock a laser onto a specific frequency.

His and Hansch's achievement, he said, was "learning how to stabilize a laser so its frequency doesn't change."

In addition to improved spectroscopy and analysis of chemicals, the development "has for the first time given us a practical way to measure the frequency of light," MIT's Kleppner said.

That, in turn, makes it possible to use newly developed atomic clocks that are accurate to 15 digits, compared with previous clocks accurate to 10 digits.

The prize is the second Nobel to be announced this week. On Monday, Drs. Barry Marshall and Robin Warren, both Australians, won the Nobel Prize in medicine for proving that bacteria, not stress, were the main cause of ulcers.

The awards for chemistry, peace and literature will be announced through the end of the week, with the economics prize to be awarded Monday.

Material from The Associated Press is included in this report.