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Bose-Einstein Condensate: A New Form of Matter
Capturing the "Holy Grail"
Eric A. Cornell of the National Institute of Standards and Technology and Carl E. Wieman of the University of Colorado at Boulder led a team of physicists at JILA, a joint institute of NIST and CU-Boulder, in a research effort that culminated in 1995 with the creation of the world's first Bose-Einstein condensate—a new form of matter. The work earned them, along with Wolfgang Ketterle, a researcher at the Massachusetts Institute of Technology who did early studies on the properties of the BEC, the 2001 Nobel Prize in physics.
Predicted in 1924 by Albert Einstein, who built on the work of Satyendra Nath Bose, the condensation occurs when individual atoms meld into a "superatom" behaving as a single entity at just a few hundred billionths of a degree above absolute zero. The
The BEC allows scientists to study the strange and extremely small world of quantum physics as if they are looking through a giant magnifying glass. Its creation established a new branch of atomic physics that has provided a treasure-trove of scientific discoveries.
The condensation was first achieved at 10:54 a.m. on June 5, 1995, in a laboratory at JILA. The apparatus that made it is now at the Smithsonian Institution.
The atoms within the condensate obey the laws of quantum physics and are as close to absolute zero—minus 273.15 Celsius or minus 459.67 degrees Fahrenheit—as the laws of physics will allow. The physicists likened it to an ice crystal forming in cold water.
"It really is a new form of matter," Wieman said. "It behaves completely differently from any other material."
The team led by Cornell and Wieman used laser and magnetic traps to create the BEC, a tiny ball of rubidium atoms that are as stationary as the laws of quantum mechanics permit. The condensate was formed inside a carrot-sized glass cell. Made visible by a video camera, the condensate looks like the pit in a cherry except that it measures only about 20 microns in diameter or about one-fifth the thickness of a sheet of paper.
Wieman started searching for the BEC in about 1990 with a combination laser and magnetic cooling apparatus he designed. He pioneered the use of $200 diode lasers—the same type used in compact disc players—showing they could replace the $150,000 lasers others were using. Cornell joined the effort about a year later.
Wieman's tactics in pursuing the condensation initially were met with skepticism in the scientific community. But as his and Cornell's methods began to show the goal was achievable, several other teams of physicists joined the chase.
Beginning with atoms of rubidium gas at room temperature, the JILA team first slowed the rubidium and captured it in a trap created by light from the lasers. The infrared beams were aligned so that the atoms are bombarded by a steady stream of photons from all directions—front, back, left, right, up and down. The wavelength of the photons was chosen so that they would interact only with atoms that moved toward the photons.
For the atoms, "It's like running in a hail storm so that no matter what direction you run the hail is always hitting you in the face," Wieman said. "So you stop."
Where No Temperature Has Gone Before
Wieman's technique cooled the atoms to about 10 millionths of a degree above absolute zero, still far too hot to produce Bose-Einstein condensation. About 10 million of these cold atoms were captured in the light trap. Once the atoms were trapped, the researchers turned off the laser and kept the atoms in place by a magnetic field. Most atoms act like tiny magnets because they contain spinning charged particles like electrons. The atoms can be trapped, or held in place, if a magnetic field is properly arranged around them.
The atoms were further cooled in the magnetic trap by selecting the hottest atoms and kicking them out of the trap. This works in a way similar to the evaporative cooling process that cools a hot cup of coffee—the hottest atoms leap out of the cup as steam.
The trickiest part was trapping a high enough density of atoms at a cold enough temperature. Cornell came up with an improvement to the standard magnetic trap—called a time-averaged orbiting potential trap—that was the final breakthrough which allowed them to form the condensate.
"It's like playing keep-away with the atoms because the hole kept circulating faster than the atoms could respond," Cornell said.
The result was a Bose-Einstein condensate of about 2,000 rubidium atoms that lasted for 15 to 20 seconds. New machines can now make condensates of much greater numbers of atoms that last up to three minutes.
Working with Cornell and Wieman on the initial BEC were postdoctoral researcher Michael Anderson and CU-Boulder graduate students Jason Ensher and Michael Matthews. Over the six years preceding the discovery, the experiment involved eight graduate and three undergraduate students at CU-Boulder.
Further Journeys into the Supercold
As of September 2001, some three dozen laboratories worldwide had replicated the BEC discovery and were conducting a wide variety of experiments.
In 1997, researchers under 2001 Nobel Physics Laureate Ketterle at MIT developed an atom laser based on the Colorado discovery that was able to drip single atoms downward from a micro-spout. In March 1999, scientists at the NIST facility in Gaithersburg, Md., under 1997 Nobel Physics Laureate William Phillips created a device that shoots out streams of atoms in any direction, just as a laser shoots out streams of light.
Made possible by nudging super-cold atoms into a beam, the breakthrough could lead to a new technique for making extremely small computer chips. Eventually, such a device might be able to construct nanodevices one atom at a time.
In February 1999, a team of researchers from Harvard University led by Lene Vestergaard Hau used the BEC to slow light—which normally travels at 186,000 miles per second—to just 38 miles per hour by shining a laser light through the condensate. In 2001, Hau's team announced that it had briefly brought a light beam to a complete stop.
On June 18, 1999, JILA researcher Deborah Jin of NIST and CU-Boulder graduate student Brian DeMarco used the technique in achieving the first Fermi degenerate gas of atoms, a state of matter in which atoms behave like waves. While the Bose-Einstein experiments used one class of quantum particles known as bosons, Jin and DeMarco cooled atoms that are fermions, the other class of quantum particles found in nature. This was important to physicists because the basic building blocks of matter—electrons, protons and neutrons -- are all fermions.
BEC pioneers Cornell and Wieman are continuing to explore the properties of their discovery. In 1999, they were the leaders of a group that created the first vortices ever seen in the condensates. They also have been doing extensive studies of two-component condensates.
In July 2001, Cornell and Wieman were part of a CU-Boulder/JILA team that was able to make a BEC shrink—an event which was followed by a tiny explosion. The team said the phenomenon was similar in some ways to a microscopic supernova explosion and dubbed it a "Bosenova." About half of the original atoms appear to vanish during the process.
"We have gotten down to the nitty-gritty science and have been able to study the behavior of a new material by manipulating it in new and different ways," Wieman said. In doing so, the JILA team cooled the matter to 3 billionths of a degree above absolute zero, currently the lowest temperature ever achieved.