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  • News FlashReally 'Cool' Lasers Bag Nobel Prize for Chu

Photonics Spectra
Nov 1997
More than 12 years after his first experiments to trap atoms optically, physicist Steven Chu has won a Nobel Prize for his use of lasers to cool atoms to nearly absolute zero.
Chu, a professor of physics at California's Stanford University, showed the world how beams of coherent photons, with nearly no mass of their own, could create an "optical molasses." Inside the laser beams, atoms slow to a crawl. From this point, two other researchers added their expertise to the laser cooling, creating a true optical trap that is capable of holding these slow, cool atoms indefinitely.

Investigations of an unusual form of matter called the Bose-Einstein condensate and atomic physics at MIT and other laboratories were made possible in large part by the laser theory developed by Steven Chu.
For their parts, Claude Cohen-Tannoudji, a professor at the Collège de France and Ecole Normale Supérieure in Paris, and William D. Phillips of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., will share in the $1 million cash award from the Royal Swedish Academy of Sciences.
Together these scientists have enabled a new field of physics. As a result, scientists can trap certain types of atoms and study their internal workings at their leisure.
Along this growing scientific horizon, lasers perform both old and new roles. They interact directly with molecules through absorption and emission, while their electric fields manipulate atomic streams similar to the way lenses and mirrors direct a laser beam.

An idea realized
As part of Chu's work, atoms traveling through precisely controlled laser beams lose speed and energy. This effect is called cooling the atom. As a result of laser cooling, atoms in a vacuum slow from approximately 1000 m/s to a couple of centimeters per second. It is necessary to perform the experiment in a vacuum to keep the atoms from condensing into a liquid or solid, which would complicate single atom observations.
The process uses a conjunction of up to six laser beams. At the central focus, the illuminated space becomes an "optical molasses." In this molasses, atoms act like salmon swimming upstream against a river of laser light.
Taking a clue left by Albert Einstein and theoretical physicist Satyendra Nath Bose, Chu and a group of researchers demonstrated the first optical trap at Bell Laboratories of Holmdel, N.J., in 1985.
In its modern iteration, the experiment slows a beam of sodium atoms by trapping them among six orthogonal beams from an argon-ion-pumped dye laser. (Although the researchers still use one of the first argon-ion lasers constructed by Spectra-Physics Lasers Inc. of Mountain View, Calif., to pump a tunable dye laser, most experiments employ a Ti:sapphire laser pumped by an argon-ion laser, both from Coherent of Santa Clara, Calif.
Circularly polarized, counterpropagating beams form X, Y and Z axes. At the center of the beams, sodium readily absorbs photons from 588 to 589 nm, while cesium collects energy at 852 nm. Although photons have no intrinsic mass themselves, their energy represents a certain momentum. Assuming that the wavelength of the light is an exact match to the atom's absorption frequency, the photon transfers this momentum to the atom, which absorbs and then re-emits the photon. This process, repeated millions of times, slows down the atom.
Controlling the beam is crucial to slowing the atoms. Because of this, Chu's group uses a single source whenever possible, splitting the beam into one, two or six parts and controlling the polarization with quarter-wave plates. Multiple beams would require exact phase and frequency matching.
High-speed electro- and acousto-optic modulators from companies such as Newport Corp. of Irvine, Calif., CVI Laser Corp. of Albuquerque, N.M., and Special Optics Manufacturing and Design of Wharton, N.J., provide fine-frequency tuning of the beam. According to Chu's research assistant Achim Peters, they must tune the beams to within hundreds of megahertz, or a
few hundredths of a nanometer.
By tweaking the frequency of individual laser beams, Chu's group can control the momentum and position of the incoming atom stream. Once an atom reaches the junction of the six beams, small frequency adjustments allow scientists to virtually pop individual atoms into the air like baseballs. At the top of the curve, the atoms come to a standstill before gravity gently pulls them back to Earth.

A fountain of knowledge
This process of first slowing atoms and then popping them into the air shows the first practical application of Chu's work: the atomic fountain.
When atoms slow to within a couple of millikelvin of absolute zero, they tend to hang around longer, giving scientists more time to bounce microwaves off them and study their internal workings.
Today's atomic clocks lose about one second every 32 million years. They measure an atom's vibrational frequency or Raman transition as it moves through the microwaves. Existing clocks take measurements while the atoms zip past at about 300 m/s. With Chu's fountain, however, the atoms fly by at a rate of a couple of centimeters per second, which should improve the clocks' accuracy to within one second for every 3 billion years. v "People are developing atomic clock prototypes using this system at NIST, another in Paris, and another at the US Naval Observatory," Peters said.

A new type of interferometer
Peters and Chu have adapted the fountain to other applications, such as the study of gravity. In an upcoming paper, Chu and his colleagues will discuss how the fountain, trapping lasers and Raman transitions combine to form an atomic interferometer that is capable of measuring gravitational pulls to within a few hundred parts per trillion.
The interferometer starts with the laser cooling trap. Beams emitted by a pair of ex-ternal-cavity diode lasers from SDL Inc. of San Jose, Calif., act as benchmarks for the atom interferometer, marking the speed, energy and distance state of the cesium atoms.
The group used the diode lasers because they needed
to be separate from the cooling light sources, and buying another Ti:sapphire was out of the laboratory's budget. "[The diodes] are built to narrow bandwidth and deliver about 25 mW through fiber optics, but we'd like more power, like about 0.5 or 1 W," Peters said.
Peters said he looks forward to the development of UV diode lasers to replace the argon-ion beams. "The argons are usually the ones that break down. They need a lot more cleaning of the windows, too. We could use a doubled Nd:YAG, but it's too expensive."

New frontiers
Besides the potential impact on satellite and space navigation, Peters said the team hopes to develop a turnkey interferometer solution that would help geophysicists search for new caches of natural resources or monitor long-term changes in the global environment.
In the past, gravity interferometers have attempted to determine exactly where large bodies have struck the Earth. One such experiment searched for the exact impact point of the meteor that scientists suspect might have led to the extinction of the dinosaurs.
The benefits to society gained by Chu's work do not stop with his lab. Work at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., and the University of Colorado at Boulder seeks to understand energy and matter by observing pea-size nodules of trapped atoms.
The atoms, once collected and cooled, form a kind of "super atom" that was first described by Einstein and Bose, hence the name Bose-Einstein condensate.
"It started with the dipole trap," said Ananth Chikkapura of MIT, "and of course Steve Chu was a big part of that."
These two groups use magnetic fields to trap the unusual atomic masses in a vacuum.
"The shortcut is a magnetic field," said Peters. "It's well-defined and clean, but it's really hard to turn them off fast." This is an important element because gravity scatters the atoms in about one second.
"Optical systems can be turned off in a instant," he added. Other experiments in Chu's laboratory will employ the dipole lattice effect in an atomic trap to study time-inversion invariance, a theory that details the relationship between matter and antimatter.

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