Nov 292007
 
Authors: Beth Malmskog

Instead of the usual routine of shooting laser beams at Rubidium atoms in his lab, CSU physics professor Dr. Jacob Roberts found himself in Washington D.C. during the first week of November, accepting the Presidential Early Career Award in Science and Engineering from President Bush.

Roberts, as well as Dr. Amy J. Pruden-Bagchi of CSU’s Department of Civil and Environmental Engineering, were among 56 researchers to receive this year’s award, the highest honor that the U.S. government presents to scientists and engineers whose work has shown incredible potential early in their career.

Roberts was recognized for his work’s potential in finding new ways to cool atom gasses. Cold atoms are helpful in measuring precise time, acceleration and gravity. Such cold atoms are used in the atomic clock at the National Institute of Standards in Boulder.

Through the use of lasers, Roberts and his research team have effectively slowed the speed of atoms, thus pulling substances to colder temperatures.

Matt Hamilton, a physics graduate student who works in Roberts’ lab at CSU, said the idea was unusual.

“We don’t expect lasers to be able to cool something,” Hamilton said. “You usually think of a laser as something that burns or heats stuff up, but here we’re using lasers to make something really, really cold. It’s kind of weird.”

Deep space is approximately a million times warmer than the atoms cooled by Roberts and his team. In comparison, the center of the sun is only around 40,000 times hotter than a normal day on earth.

The light used in the laser is important, as only certain colors will slow atoms. Light comes in different frequencies, which a human eye sees as colors. Blue light has a higher frequency than red light. Higher frequency equals higher energy.

Atoms only take in or emit energy in specific amounts, known as quanta. The sizes of these chunks of energy correspond to particular frequencies of light that can affect the atoms.

In Roberts’ lab in the basement of the Engineering Building, he and his team carefully tune the lasers to produce light at frequencies just below what affects the atoms the most. The lasers they use are actually the same type as the ones that CD and DVD players use to read discs.

When a fast-moving atom moves toward the light source, the frequency of the light as the atom receives it is raised. This makes the light “bounce off” the atom, in the process slowing it down a little.

The collisions only make very small changes in the atom’s speed, but they add up, Roberts said.

“You have a bowling ball rolling down the lane, and a ping-pong ball gun,” Roberts said. “Shoot a bunch of ping-pong balls at a bowling ball, well eventually you can actually slow it down and stop it.”

Perhaps even stranger than the cooling process is what happens when the atoms get cold enough. These ultra-cold atoms act like nothing else on earth.

Under normal conditions on Earth, matter exists in one of three states: solid, liquid, or gas. However, scientists have found that this is not the end of the story.

Matter can act differently under extreme conditions. The sun is so hot that the electrons and protons that comprise its atoms separate, creating a sea of swarming ions and electrons called plasma.

On the other end of the spectrum, extremely cold atoms can become what is known as a Bose-Einstein condensate (BEC). In this state, matter shows itself to be a wave. The atoms blend together instead of acting like discrete packets.

To understand why, one needs to invoke the Heisenberg uncertainty principle. The uncertainty principle states that it is impossible to know simultaneously both how an atom is moving and where it is at a given moment.

The key is that when atoms get very cold, they move very slowly.

“If you slow an atom down, you really know what its speed is. That means you start to not know where it is,” Roberts said. “You take a gas of atoms and make them go so slow that the indeterminacy of their position is bigger than the spacing between atoms.”

In this state, the atoms are no longer really distinct.

“You know that you have an atom and an atom, but they overlap each other,” Roberts said.

These BECs behave as waves. If one is broken in half, and the two halves run into each other, they create interference patterns in matter like those made by waves on the surface of water.

BECs were first created in 1995 by a group including Dr. Carl Wieman of the CU-Boulder and Dr. Eric Cornell of the National Institute of Standards. Wieman, Roberts’ Ph.D. advisor, was among several scientists who won the Nobel Prize in 2001 for creating the world’s first BEC.

Roberts’ cooling work could be used to make a BEC at CSU, though that is not his highest priority. There are many interesting physics questions to be answered about laser cooling itself, Roberts said.

Roberts doesn’t spend as much time in the lab as he once did. While he has the option of pursuing a career strictly in research, he chose to work in an academic setting so he could also teach. Roberts said though that he spends some time in the lab each day, it is now his graduate students that log more lab hours.

Anthony Gorges, a graduate student who said he’s been on the project “since lab book one,” spends most of his days in the basement lab.

“It ruins your pale if you go outside,” Gorges said.

However, Gorges retains enthusiasm for science.

“You run into a lot of brick walls but eventually you get there. You figure it out. It’s a lot of fun,” Gorges said. “In the end, it’s just like a giant puzzle.”

Staff Writer Beth Malmskog can be reached at news@collegian.com.

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