Physics team studies atomic life at ‘absolute zero’

(Editor’s note: This is the first of a series of stories about the everyday – yet extraordinary – process of discovery at UW–Madison.)

With a lab full of lasers to corral and chill atoms, physicist Thad Walker is plunging into the frigid domain of “absolute zero.” It’s not just cold there. It’s weird.

From a behavior-modification perspective, controlling the behavior of atoms is the physics equivalent of transforming a heavy-metal mosh pit into a military parade.

At 460 below zero, give or take a millionth of a degree, things start to behave freakishly. Atoms, normally darting around at 1,000 mph, begin to move as if suspended in molasses. Their once-chaotic movement becomes almost choreographed in smooth little waves.

This is the chilly climate in which physics professor Walker and his team of “atom trainers” work to ultimately control the behavior of atoms. From a behavior-modification perspective, this is the physics equivalent of transforming a heavy-metal mosh pit into a military parade.

As a science, atom trapping and cooling was thought to be an impossible dream 15 years ago. Today, there are hundreds of laboratories capable of exploring super-cooled atoms, and it’s one of the most full-bore explorations in physics.

Physicist Thad Walker, left, and senior Nathan Harrison, right, adjust infrared laser equipment used to trap and cool atoms to near absolute zero. By using a special infrared lens, one can see a tiny green ball of light, smaller than a marble, that constitutes the trapped atoms scattering laser light. It can take more than a year to set up a functional atom trap. Photo: Jeff Miller

A few labs have even reached the ultimate in cold, a new state of matter called a Bose-Einstein Condensate. It’s the point where all movement ceases, and single atoms seem to morph together into a uniform, beautifully arced wave.

Walker is one of the early pioneers in this work. In the late 1980s, he built only the second atom trap in existence while at the University of Colorado. Scientists from Stanford and Paris who did similar work went on to win a Nobel Prize in 1997.

“One thing I really try to do with the research is emphasize it’s curiosity-driven,” Walker says. “We don’t know where it’s going.”

There are, however, some wild-eyed technology predictions. The ability to control atomic movement could produce extremely precise atomic clocks, small enough to fit in a glove box. Such a clock could revolutionize navigation systems, and make global positioning systems, for example, accurate to within a blade of grass. They could also be used to create a laser that sends out a uniform beam of atoms, which could etch integrated circuits with staggering detail.

But gadgets are not the driving force for Walker. His atom traps, which can take years to correctly calibrate, are after fundamental questions.

This computer graphic illustrates a theory behind Professor Thad Walker’s new atom trap, which uses a hologram to trap atoms inside a cylinder of light. Each cone in this light field would hold an individual atom precisely in place, like an atomic egg carton. Image courtesy Thad Walker

“I have an opportunity to study matter under circumstances that it’s never been studied in before,” he says.

Inside Walker’s lab, all outside light is extinguished with foil coverings on the windows. The dominant feature is a large steel-plated table rigged with peg-holes. Hundreds of optical tools are screwed into the table to intercept beams of laser light. The beams get focused through lenses and scopes, deflected by mirrors and parted by beam-splitters. Ultimately, a series of five to six precisely manicured beams are all trained on a tiny center point inside a vacuum chamber. By using a special infrared lens, one can see the tiny green ball of light, smaller than a marble, that constitutes the trapped atoms scattering laser light.

Thus, the trap is set. But the cooling is trickier. The laser light emits photons that constantly pound against atoms moving toward the laser. It’s like throwing ping-pong balls at a billiard ball, causing it to gradually lose momentum. The slower an object moves, the colder it becomes. Stopping atoms dead in their tracks constitutes zero degrees Kelvin, or minus-459.6 below zero. These experiments all achieve temperatures in the range of one thousandth to one billionth of a degree from absolute zero.

“If you can precisely control these laser beams, you can con the atom into doing what you want,” he says. Visually this process is a challenge, since infrared light is not visible to the naked eye. But Walker uses tiny surveillance cameras to keep the hazy ball of light glowing on a TV monitor.

Walker is perfecting a stripped-down, portable atom trap that he can set up for lectures and public talks. When he first used it a year ago in a lecture hall, the ball of light popped on the screen and the crowd cheered in approval. The sound waves promptly threw his machine into chaos, and the beam was lost. Once he got the crowd quieted down, the ball of light reappeared.

Thad Walker

Today, Walker continues to refine the tools for this work. He’s also developed a specialized research area into how these atoms interact with light and interact with each other. By using lasers to cool atoms, they are also bathing atoms in light. The atoms are constantly absorbing and shedding photons from the laser light, which produces a force between atoms a thousand times higher than normal. Walker’s goal is to minimize the effects of light on the atoms, and to produce atom traps that do not scatter light.

And he’s added a new tool to the arsenal to achieve that goal. He uses a hologram to split a single beam five ways, trapping atoms inside the cylinder of light. In theory it will do amazing things, like corral individual atoms into “atomic egg cartons,” holding individual atoms inside a dimpled light field that comprises the interior of the cylinder. For the first time, he will be able to increase the density of his trapped atoms by more than a factor of one thousand, at a temperature a few millionths of a degree above absolute zero. No other place in the world is using such a device. “If what happens is what we think will happen, it will be fantastic,” he says. “If it doesn’t work, it will likely be because of some new phenomena we don’t yet know about, which will also be great.” “Where this is all headed, I don’t know,” Walker adds. “But 30 years ago, people were saying the laser was an invention without an application.”

“It’s kind of my job to be pie in the sky.”