A DEVICE that cost only a few hundred dollars is yielding discoveries about the fundamental nature of matter that normally turn up only in giant particle accelerators with billion-dollar budgets.

Experiments conducted with the device at the Joint Institute for Laboratory Astrophysics in Boulder, Colo., have confirmed some key predictions of a major theory of the structure of matter, and forthcoming experiments will investigate the possibility of subtle deviations from the theory that would reveal previously undetected relationships among the basic forces of nature. And, while operating their table-top apparatus, scientists at the institute have achieved a temperature only one-millionth of a degree Celsius above absolute zero, a world record for extreme cold.

The astrophysics institute is operated jointly by the University of Colorado and the National Institute of Standards and Technology. Its director, Dr. Carl E. Wieman, said that modifications in the apparatus would soon bring its operating temperature to substantially less than one-millionth of a degree above absolute zero. The absolute zero, calculated using equations of the science of thermodynamics, is a temperature of approximately minus 273.16degrees Celsius, or minus 459.69degrees Fahrenheit.

It is impossible to reach any temperature lower than the absolute zero, and even the sparse atoms and molecules drifting in intergalactic space have much higher temperatures.

"We hope this summer to reach a temperature so close to the absolute zero," Dr. Wieman said, "that we will observe for the first time a weird condensation of matter," predicted by a theory of Albert Einstein and Satyendranath Bose, an Indian mathematician and theoretical physicist. By reducing the temperature of a substance to essentially absolute zero, the theory predicts, atoms can be made to lose their individual identities and merge as a kind of homogeneous atomic soup.

The key to experiments at the astrophysics institute and similar investigations at Stanford University and the Ecole Normale Superieure in Paris is a technique called laser cooling. As the institute group has reported in a series of papers in Physical Review Letters and other major physics journals, carefully tuned and focused laser beams are used to halt flying atoms in their tracks, thereby reducing their temperatures to virtually nothing. Not an Ordinary Swirl

A new technique for producing extreme cold was devised two decades ago, when physicists began to reach temperatures within 0.8degrees Celsius of the absolute zero by diluting ordinary liquid helium (He-4) with a liquefied isotope of helium, He-3.

At such temperatures, the startling behavior of ultrasmall particles obeying the rules of quantum mechanics becomes visible in the full-size world. When an ordinary liquid is made to swirl around in a jar, for example, it forms a single vortex that deepens as the swirling speed increases. But a jar of ultracold liquid helium behaves differently because it is "quantized" -- that is, it obeys the laws of quantum physics rather than the laws that rule the everyday world.

As swirling is increased, the central vortex does not deepen, but with increases in the speed of swirling, the liquid helium undergoes quantum jumps. First, the liquid abruptly forms two vortices (never seen in ordinary liquids), then three and more with each quantum leap.

Such odd effects have given physicists a full-size window through which the microscopic quantum world of subatomic particles can be directly observed.

Taking this a step further, the experiments at the Joint Institute for Laboratory Astrophysics have opened another window -- one through which the Standard Model theory of matter can be explored. The device complements the experiments that huge particle accelerators can perform.

The heart of the institute's apparatus is a glass chamber about the size of a fist, which is filled with a gas consisting of atoms of the metal cesium. A pump removes most of these atoms until a very high vacuum is established in the chamber. Despite the vacuum, some 100 million cesium atoms remain in the vessel, randomly colliding with each other and the walls of the container, and endlessly recoiling, as in any ordinary gas. This constant motion is better known as heat. The Effect Is Cumulative

But by shining six infrared lasers (similar to lasers used in compact-disk players) through the chamber from different angles, the team has succeeded in slowing down and virtually halting every atom -- that is, in reducing each atom's temperature to almost zero. The lasers must be perfectly tuned to the resonances of the atoms, and must be positioned perfectly to avoid imparting momentum to them.

Dr. Wieman explained that this method of slowing down flying atoms was achieved by hitting the atoms with photons of light from various directions, in such a way that each atom is robbed of its momentum and slowed down.

"Each head-on impact of a photon with a cesium atom slows the atom down by about one-third of a centimeter per second," he said. "The overall effect is cumulative -- something like slowing down a bowling bowl by bombarding it with ping-pong balls."