Research May Lead to Better Understanding of Nature’s Building Blocks
Researchers at JILA, a joint research institute of the Commerce Department’s National Institute of Standards and Technology and the University of Colorado at Boulder, report in tomorrow’s edition of Science that they have achieved the first Fermi degenerate gas of atoms, a state of matter in which atoms behave like waves. Study of this new system could lead to a greater understanding of the basic building blocks of matter.
Similar to experiments in 1995 that first created a new state of matter—the Bose-Einstein condensation—the NIST-led group at JILA cooled a gas of potassium atoms to ultralow temperatures where the quantum (most basic) nature of gas is dominant (reaching a state known as "quantum degeneracy"). However, while the Bose-Einstein experiments used one class of quantum particles known as bosons, the JILA group cooled atoms that are fermions, the other class of quantum particles found in nature. Using laser cooling and magnetic confinement, they cooled about a million potassium atoms to temperatures near 300 nanokelvin (less than one-third of a millionth of a degree above absolute zero).
"The creation of a Fermi degenerate gas is a major scientific achievement and a lot of scientists have been trying to make it ever since we created the Bose-Einstein condensate," said Carl Wieman, CU-Boulder physics professor and co-creator of the first Bose-Einstein condensate with NIST’s Eric Cornell. "It will probably be at the top of the list of important physics news for this year."
When gas is cooled to near absolute zero, each atom stops behaving as a point-like particle and instead behaves like a wave, with the wavelength of each atom overlapping those of neighboring atoms. When bosonic atoms reach this regime, they all fall in step with each other, resulting in a Bose-Einstein condensate or "super-atom." As the JILA group cooled fermionic atoms to quantum degeneracy, they found instead—as predicted—that the atoms began to avoid each other, resulting in an "excess" energy in the gas.
Study of the Fermi degenerate gas will increase our knowledge about and understanding of fermions, which are important throughout physics since the basic building blocks of matter—electrons, protons and neutrons—are all fermions. In addition, the unique properties of the fermionic atoms could be exploited for improving atomic clock technology. Future work also will explore the possibility of achieving a fermionic superfluid state in the gas, which ultimately could shed light on the physics of superconductivity.
Authors of the Science paper are Deborah Jin and Brian DeMarco. Jin, a physicist in the NIST Physics Laboratory, is an associate fellow of JILA and an adjoint assistant professor of physics at CU-Boulder. DeMarco is a graduate student in the CU-Boulder department of physics. Support for this work comes from NIST, the National Science Foundation and the Office of Naval Research.
JILA, formerly known as the Joint Institute for Laboratory Astrophysics, is an interdisciplinary institute for research and graduate education in the physical sciences located on the main campus of the University of Colorado at Boulder.
As a non-regulatory agency of the U.S. Department of Commerce’s Technology Administration, NIST strengthens the U.S. economy and improves the quality of life by working with industry to develop and apply technology, measurements and standards through four partnerships: the Measurement and Standards Laboratories,the Advanced Technology Program, the Manufacturing Extension Partnership and the Baldrige National Quality Program.
The University of Colorado at Boulder was founded in 1876 and has an enrollment of about 25,000 undergraduate and graduate students. The department of physics is part of the College of Arts and Sciences.
Fermions—the family of atomic particles that includes electrons, neutrons, and protons—behave very differently from the other group of quantum (basic) particles called bosons. Fermions have a "spin" state (a state of quantum mechanics) that is different from that found in bosons.
When bosons are cooled to near absolute zero, they all fall into step with each other and form a kind of "super-atom" called the Bose-Einstein condensate. Extremely cold fermions, however, begin to avoid each other, as they obey the Pauli exclusion principle that forbids two identical fermions (with the same spin) from occupying the same quantum mechanical state. This means that as they are cooled, some of them are forced to "stack up" into higher and higher energy states.
These effects can be observed only by cooling to below the degeneracy temperature (which is dependent on the density of the gas) that is typically below one millionth of a Kelvin above absolute zero (minus 273.15 degrees Celsius).
The catch is, the same Pauli principle prevents them from colliding with each other at low temperatures, and collisions are necessary for cooling the gas uniformly.
NIST researcher Deborah Jin and University of Colorado graduate student Brian DeMarco solved the problem by preparing the atoms in two different spin states, which gets around the Pauli exclusion.
Atoms with different spins can collide, because they are not in the same state. Often after a collision, one of the atoms will have a higher speed (or temperature) than the other, and the "hotter" one is allowed to escape from the trap. This leaves the remaining trapped atoms with a slightly lower temperature than before the collision.
As more and more of the hotter atoms "evaporate" in this way from the trap, the temperature of the remaining atoms falls below the degeneracy temperature, and the atoms become Fermi degenerate.
The researchers deduced they had achieved Fermi degeneracy because the energy of the gas at about 300 nanokelvins (300 billionths of a Kelvin above absolute zero) stopped decreasing linearly with lower temperature, implying a non-zero energy even at absolute zero. Along with this "excess" energy, they found that the gas atoms behaved as if there had been stacking of the atoms in energy levels—the predicted characteristic of a degenerate gas state.
"Debbie’s and Brian’s work not only can but surely will lead to a new understanding of fermions, and as such it is one of this year’s most exciting scientific breakthroughs." – James Faller, Chief, Quantum Physics Division, NIST Physics Laboratory
"Creating a gas of degenerate fermions has been the primary goal of many research groups around the world for some years now. It is a considerably more difficult task than making a Bose-Einstein condensate, and Jin’s and DeMarco’s success says a lot about the level of laboratory excellence they have achieved. From the point of view of both immediate scientific goals and longer-term technological goals, the characteristics of a degenerate Fermi gas are very much complementary to those of a degenerate boson gas. This is really a significant advance in cold-atom research." – Eric Cornell, NIST physicist, JILA Fellow, and co-creator of the Bose-Einstein condensate
"This latest fundamental breakthrough by a NIST scientist at JILA may lead to a new generation of atomic timekeeping devices and improved superconductivity-based technologies. It continues a long JILA tradition of working at the foundations of physics, while remaining mindful of NIST’s mission ‘... to develop and apply technology, measurements and standards.’ " – David Norcross, Director, NIST Boulder Laboratories
"These talented young people have made an important extension to the new field of ultralow temperature physics in trapped gases, a field opened up in 1995 by the world’s first observation at JILA of a new state of matter, the Bose-Einstein condensate. Because a Fermi degenerate gas should be understandable from first principles, it can provide new insights into the physics of electronic devices and superconductivity. It should also provide a new understanding of the fundamental differences between bosons and fermions. We are proud to have Debbie on the NIST staff and delighted that Brian is working with her." – Katharine Gebbie, Director, NIST Physics Laboratory