Physicists in Boulder, Colo., have achieved a temperature far lower than has ever been produced before and created an entirely new state of matter predicted decades ago by Albert Einstein and Indian physicist Satyendra Nath Bose.
Cooling rubidium atoms to less than 170 billionths of a degree above absolute zero caused the individual atoms to condense into a "superatom" behaving as a single entity, said Eric Cornell and Carl Wieman of JILA, a joint program of the Commerce Department's National Institute of Standards and Technology and the University of Colorado at Boulder.
Results of the experiment are published in the July 14 issue of the journal Science.
Before photographing the superatom, the physicists cooled the atoms to 20 billionths of a degree above absolute zero, the lowest temperature ever achieved.
"It really is a new state of matter," Wieman said. "It has completely different properties from any other kind of matter."
"This state could never have existed naturally anywhere in the universe," added Cornell. "So the sample in our lab is the only chunk of this stuff in the universe, unless it is in a lab in some other solar system."
The Bose-Einstein condensation was first achieved at 10:54 a.m. on June 5. Scientists have sought to create the effect for more than 15 years.
Working with Cornell and Wieman, postdoctoral researcher Michael Anderson played a key role in the discovery. Anderson was assisted by CU-Boulder graduate students Jason Ensher and Michael Matthews.
The team used laser and magnetic traps to create the Bose-Einstein condensate, a tiny ball of rubidium atoms which are as stationary as the laws of quantum mechanics permit. This ball is surrounded by a diffuse cloud of normal rubidium atoms. 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.
The condensate was formed inside a small glass cell surrounded by a tabletop array of magnets, lasers and computers in a JILA laboratory. Within the atom trap are about 2,000 atoms of rubidium squeezed into a very small space at a very low temperature.
The atoms within the condensate obey the laws of quantum physics and are fundamentally different from the normal atoms in the much less dense cloud surrounding it. The physicists likened it to an ice crystal forming in cold water.
"If it weren't for quantum mechanics these atoms would have no energy at all," Wieman said. "They are as close to absolute zero as the laws of science will allow."
Absolute zero, minus 459.67 degrees Fahrenheit or minus 273.15 Celsius, is the hypothetical point at which a substance would have no motion and no heat. But that temperature can never be reached due to the laws of thermodynamics, they explained.
The JILA team cooled the atoms to a temperature 300 times lower than has ever been achieved in other scientific laboratories. Even the most remote regions of interstellar space are a billion times warmer, due to background radiation left over from the Big Bang.
"Atoms in a room temperature gas normally move about 1,000 miles per hour and slow down as temperatures drop," Cornell said. "The normal atoms at these low temperatures move about 3 feet per hour. The Bose-Einstein condensate atoms move a lot slower, too slow for us to measure yet."
Wieman started searching for the Bose-Einstein condensation about six years ago with a combination of laser and magnetic cooling apparatus that he designed. Cornell joined the effort about a year later. Over the past six years, the experiment has involved eight graduate and three undergraduate students at CU-Boulder. 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 that the goal was achievable, several other teams of physicists joined the chase.
Beginning with a gas of room-temperature atoms, the JILA team first slowed the rubidium and captured it in a trap created by the light produced by diode lasers similar to ones used in compact disk players. The infrared lasers are 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 is chosen so that they will interact only with atoms that are moving 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."
This cools the atoms to about 10 millionths of a degree above absolute zero, still far too hot to produce Bose-Einstein condensation, and about 10 million of these cold atoms are captured in the light trap. Once the atoms are trapped, the lasers are turned off and the atoms are kept in place by a magnetic field. All atoms have a tiny magnet attached to them caused by the spin of the electron.
The atoms can be trapped, or held in place, if a magnetic field is properly arranged around them, the researchers said. The atoms are further cooled in the magnetic trap by selecting the hottest atoms and kicking them out of the trap. It 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, according to the team. 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 reach the record-setting temperature.
Because the coldest atoms had a tendency to fall out of the center of the standard atom trap like marbles dropping through a funnel, Cornell designed a technique to move the funnel around. "It's like playing keep-away with the atoms because the hole kept circulating faster than the atoms could respond," Cornell said.
The 2,000 rubidium atoms in the condensate are in a strange condition, existing in a kind of smeared-out, overlapping stew, most of the properties of which are still a big unknown. The condensation is like an atomic counterpart to the laser because it puts a large number of atoms into the same quantum mechanical state, the scientists said. Lasers cause a large number of photons to have identical energy and direction. Anderson put it this way: "The condensate is to ordinary matter as laser light is to the light from a light bulb."
Wieman said he was surprised that the condensation appeared so dramatically. "It was almost too good to be true," he said. "This is always the picture I had in mind of the best possible thing we could have observed." He added that the apparatus to conduct the experiment is not particularly exotic or expensive, so the results should be fairly easy to reproduce and confirm in other laboratories.
"I would expect a large number of people to replicate this work," he said. "It will provide physicists with a new way of studying quantum effects on a large scale, similar to the threshold effects observed in superconductivity and superfluidity."
In addition to being a fellow of JILA, Cornell is a senior scientist at NIST and an adjoint assistant professor of physics at CU-Boulder. Wieman is a fellow and former chairman of JILA, a professor of physics at CU-Boulder and a member of the National Academy of Sciences.
Both physicists are actively involved in teaching undergraduate and graduate students at CU-Boulder.