In a tour de force of ultracold physics, scientists have coaxed a microscopic mist of atoms into a new form of matter--often predicted but never before seen--in which atoms quit obeying normal rules and start following the weird laws of quantum mechanics.
The feat by two Colorado researchers "represents an experimental breakthrough," said William D. Phillips of the National Institute of Standards and Technology (NIST) in Gaithersburg, who won the 1997 Nobel Prize in physics for his work in trapping atoms at extremely low temperatures. And it may eventually lead to vastly improved understanding of superconductivity--the condition in which materials lose all resistance to electric current--and the creation of new electronic materials, experts said.
Many scientists around the world have been pursuing the eerie state, which goes by the forbidding technical name of "Fermi degenerate gas." But Deborah S. Jin and Brian DeMarco, who reported their findings in the Sept. 10 issue of the journal Science, are the first to overcome a host of daunting physical obstacles and chill a tiny cluster of potassium atoms to 290 billionths of a degree above absolute zero (about minus 460 F) in a vacuum.
In such hyperfrigid, near-motionless circumstances, atoms don't just sit there. They do one of two surprising, creepy and quite opposite things: Either they glom together into a sort of amalgamated low-energy "super-atom"; or they stubbornly refuse to condense and instead spread out into a number of different energy levels.
Which path they take depends on whether they belong to one or the other of two categories--bosons and fermions--that together make up everything in the cosmos.
Both kinds of particles have a host of physical properties, such as mass, energy content and so forth. One of those properties is called "spin" because the particles behave as if they were spinning on an axis.
Bosons--such as photons, the smallest quantities of light--have whole-number units (0,1,2) of this property called spin. Fermions, such as electrons or protons, have half-integer spin (1/2, 3/2, 5/2 and so forth). The spin can "point" up or down.
Atoms are either bosons or fermions depending on whether the spins of their constituent particles add up to whole or half spin units. In the macroscopic world we live in, the difference has no practical importance. But in quantum mechanics, which describes the character of matter and energy on the very smallest scale, it makes a critical difference.
Any number of bosons can comfortably share exactly the same quantum characteristics such as energy, magnetic properties, position and spin. Make a bunch of bosonic atoms cold enough, and they will overlap and coalesce into a single conglomerate in which all the atoms coexist at the same minimal energy.
That state, called a Bose-Einstein condensate for the physicists who predicted its existence 70 years ago, made international headlines when it was finally achieved in 1995 by scientists at the Joint Institute for Laboratory Astrophysics (JILA), a lab in Boulder, Colo., run by NIST and the University of Colorado.
But a formidable physical law prevents fermions from doing the same thing. Known as the Pauli exclusion principle (for physicist Wolfgang Pauli), it decrees that no two fermions, such as the electrons that orbit atomic nuclei, can exist in exactly the same quantum mechanical state.
It is this principle that is chiefly responsible for the fact that substances are divided into the discrete elements that populate the periodic table. At first glance, it seems to contradict common sense. Ordinarily, everything in nature tends toward the lowest energy condition it can attain, which is why when you knock over a glass, it falls to the floor rather than suddenly leaping upward. So it seems natural that all the electrons circling an atomic nucleus would subside into the lowest-energy orbit.
But the exclusion principle forbids that. Only two electrons (one with spin pointing up, the other down) can occupy the lowest-energy orbit. Others must take the next-lowest orbit, and when that is filled, the next, and so on.
It's the equivalent of open seating at a concert: As each of the front rows is filled, newcomers can occupy seats only in rows farther back. That principle accounts for the distinctive differences in the shell-like electron configuration of separate elements, and thus their differing chemical properties.
The same principle, however, would ordinarily prevent what Jin and DeMarco, both of JILA, started trying to do in their Boulder lab a couple of years ago.
They wanted to cool their fermion gas--a few million potassium atoms--to within a fraction of a millionth of a degree above absolute zero. Typically physicists do that by trapping atoms in a magnetic vacuum device designed so that the most energetic, or hottest, atoms can boil off. As this happens, the remainder gets colder and colder, just as a bowl of hot soup gradually cools by shedding its most energetic molecules as vapor into the air.
But in order for the hottest atoms to get bounced out of a soup bowl or an atom trap, they have to collide with their neighbors. And two fermions are prohibited from being in the same physical conditions, which makes it very hard to bang into each other. That would appear to make evaporative cooling impossible for fermion gases.
Jin and DeMarco, however, devised an ingenious method. They used magnetic fields and beams of radiation to split their tiny gas sample--occupying about one-millionth of a cubic centimeter--into two halves, each of which had a different spin state.
Although the exclusion principle prevented the atoms within each half from colliding with each other, it permitted them to collide with atoms in the other half. So each half of the sample was able to cool the other.
Finally the researchers removed one half. The remaining 780,000 atoms quit evaporative cooling and displayed the distribution of energy levels demanded by quantum mechanics and the exclusion principle, rather than the classical gas laws that govern large collections of atoms.
The result is "an interesting new chapter in quantum fluids," said Daniel Kleppner of the Massachusetts Institute of Technology, one of the world's leading experts in the field. Now, he said, "one can start to probe this new state, and one expects that there are very interesting phenomena to be discovered--one of which is the gas equivalent of superconductivity in a metal."
What's the next chapter? Under the right superfrigid conditions, theory suggests, two fermion atoms with half-integer spin might pair up to make an integer-spin composite that acts like a boson, creating a never-before-seen fermion condensate.
The JILA gas is "the precursor to the condensate," said Jin, who is continuing to investigate the phenomenon. If a condensate were observed, it would be an extremely important finding. A related experiment--which demonstrated atom pairing in liquid helium--earned the 1996 Nobel Prize in physics.
Such research could "help us understand in a much more fundamental way what is going on in superconductors," said Phillips of NIST. Superconductivity involves pairing of electrons, which are fermions. "So understanding the physics of fermion systems," he said, "could give us a handle on how we can change some of the fundamental" properties of electronic materials "rather than just take what nature gives us."
Shown above are images of two tiny clouds of potassium-40 atoms chilled to ultracold temperatures by researchers at JILA. The gas on the left contains about 2.5 million atoms at a temperature of 2.4 millionths of a degree above absolute zero. (Density is denoted by color: White is highest; blue and black are lowest.) The gas cloud on the right -- about 1/100th the width of a human hair -- contains 780,000 atoms at 290 billionths of a degree above absolute zero. The white circles, labeled "Fermi state" for physicist Enrico Fermi, show where the atoms theoretically would lie at absolute zero (-460 F).
Bosons Condense, Fermions Stack
All the particles in the known universe are either bosons, named for physicist Satyendra Nath Bose (1894-1974), or fermions, named for Enrico Fermi (1901-1954). Make either of them so cold that they are nearly motionless, and they start to behave in two different, but equally weird, ways. Bosons collapse into a conglomerate called a Bose-Einstein condensate in which all the atoms occupy the same physical state. But an inviolable law of nature prevents two fermions from being in the same state. So instead of clustering, they arrange themselves into layers of successively higher energies.
SOURCE: National Institute of Standards and Technology