* Astronomy

If you want to really see quantum mechanics in action, you've got to turn the temperature down so low that even atoms stop moving. Physicists have come close to achieving this "absolute zero" state by using precision-tuned lasers, but the technique has only allowed researchers to freeze small groups of atoms at a time. Now members of an international team say they have managed to cool a dime-sized mirror to within one degree of absolute zero, the lowest laser-induced freeze yet achieved with a visible object.
One of the greatest enigmas in physics is how matter can be governed by the four basic forces of nature--electromagnetism, which governs light, heat and electricity; the strong and weak nuclear forces, which bind atoms together; and gravity--and still follow the rules of quantum mechanics, which operate only at the subatomic level. In other words, scientists want to know how solid objects keep from flying apart when their atoms are also influenced by the chaotic nature of quantum physics. The major research obstacle has been that natural forces overwhelm quantum effects. The only way to cancel those forces entirely is to cool an atom down to absolute zero (-237 degrees Celsius), where quantum forces apply exclusively.

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The atomic constituents of matter are never still, even at absolute zero (-273.15 degrees Celsius). This consequence of quantum mechanics can result in continuous transition between different material states. Physicists at the Max Planck Institute for Chemical Physics of Solids have studied this phenomenon using ytterbium, rhodium and silicon at very low temperatures under the varying influence of a magnetic field. Until now, it has been assumed that the properties of a transition of this nature can be described completely with the fluctuations of one parameter, in this case, magnetic order. However, the experiments that have now been published reveal, completely unexpectedly, an additional change to the electronic properties of the transition. It confirms again that quantum effects can result in phenomena that are inconceivable in classical physics. On the one hand, the results extend the general understanding of phase transitions and, on the other, are also relevant to complex systems, such as high-temperature superconductors (Science, February 2007

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-- Edited by Blobrana at 07:08, 2007-03-10

For the first time, MIT scientists have directly observed the transition of a gas to a superfluid, a form of matter closely related to the superconductors that allow electrical currents to travel without resistance.

Observations of superfluids may help solve lingering questions about high-temperature superconductivity, which has widespread applications for magnets, sensors and energy-efficient transport of electricity.
The superfluid gas created at MIT can also serve as an easily controlled model system to study properties of neutron stars or the quark-gluon plasma that existed in the early universe.
The work, reported in the July 6 issue of Nature and in the July 18 issue of Physical Review Letters, was led by Nobel laureate Wolfgang Ketterle, the John D. MacArthur Professor of Physics and a principal investigator in MIT's Research Laboratory of Electronics.
The team observed the transition to superfluidity of a gas of so-called fermionic atoms. Fermionic atoms are atoms with an odd number of neutrons, protons and electrons. They can become superfluid only if they form pairs. These pairs then have an even number of basic constituents and can form a kind of Bose-Einstein condensate, a type of matter where all pairs act as a giant matter wave, "march in lockstep" and flow without friction.
For several years, research groups around the world have seen the transition of a Fermi gas to superfluidity only indirectly because this transition was not accompanied by any change in the appearance of the gas cloud.
The new trick used by the MIT group was to have an unequal number of two kinds of fermions, sometimes labelled as spin up and spin down. In this situation, not all the atoms can find a partner to form a pair, and the difference between the paired superfluid and the gas of unpaired atoms is clearly visible.
One may regard the two kinds of fermions as women and men on the dance floor who have to pair up to perform a superfluid dance. At first, it was not clear what would happen if the men outnumbered the women. Would the single men take part in the dance, would they stay at the side of the dance floor or would their presence cause everyone to stop dancing?
The dance ends when the ratio of men to women exceeds six to one -- this breakdown of superfluidity was observed by the MIT researchers in 2005.

For a smaller ratio, the superfluid dance continues.
In the current work, the MIT team has found that for the superfluid dance to go on, the single men (excess atoms) must be expelled from the dance floor. This expulsion is directly observed as the shell of excess atoms surrounding the superfluid core. When the atoms are cooled down, the appearance of superfluidity is accompanied by a sudden change in the shape of the cloud.

"To see directly how the superfluid core forms in the center is quite amazing. Our results challenge state-of-the-art theory" - Martin Zwierlein, physics graduate student.

"The features we have observed are very difficult to reproduce in calculations"Yong-Il Shin, Postdoctoral associate.

Ketterle's team members, in addition to Zwierlein and Shin, were MIT physics graduate students André Schirotzek and Christian Schunck. All are members of MIT's Center for Ultracold Atoms.
The team observed fermionic superfluidity by cooling the gas close to about 50 billionths of 1 Kelvin, very close to absolute zero (-273 degrees C or -459 degrees F). By using phase-contrast imaging -- a standard microscopy technique -- in a novel way, they could directly observe the superfluid core and the shell of excess atoms around it.
Properties of superfluid ultracold fermions are also being studied by teams at the University of Colorado at Boulder, the University of Innsbruck in Austria, the École Normale Supérieure in Paris, Duke University and Rice University. The Rice group has also studied imbalanced Fermi mixtures.

The MIT research was supported by the National Science Foundation, the Office of Naval Research and NASA.

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In quantum physics, every tiny spec of matter has something called "spin" -- an intrinsic trait that cannot be changed and which dictates, very specifically, what other bits of matter the spec can share quantum space with. When fermions, the most antisocial type of quantum particle, do get together, they pair up in a wondrous dance that enables such things as superconductivity.

For the first time, researchers at Rice University have succeeded in creating and observing an elusive and long-sought quantum state - a superfluid of fermions with mismatched numbers of dance partners.
Despite more than 40 years of theoretical musings about what would occur in such a case, the result - a cluster of matched pairs surrounded by a cloud of would-be dance partners - was largely unexpected.
The research, which appears online this week, is slated to appear in an upcoming issue of the journal Science, together with a paper from MIT reporting related results. The experiments offer physicists a new window into two of the least understood and most intriguing phenomena in physics - superconductivity and superfluidity.
Both phenomena result from a change in the phase of matter. Anyone who has seen ice melt has seen matter change phases, and when electrons, atoms and other specs of matter change quantum phases, they behave just as differently as do ice and water in a glass.
Superconducting and superfluid phases of matter occur in fermions - the antisocial particles that can't share quantum space - only when quantum forces become dominant. Because thermodynamic forces are typically so powerful that they overwhelm quantum interactions - like loud music overwhelms the whisper of someone nearby - superconductivity and superfluidity only occur in extreme cold.


This is a 3-dimensional projection of an image of a phase separated atomic cloud. The tall central (semi-transparent) region consists of paired fermionic 6 Li atoms, and is believed to be a superfluid. The shorter (opaque) peaks on either side, as well as the faint ring around the bottom, are unpaired atoms which have been expelled from the paired central region. The light in the background is a representation of the probe laser beam used to image this cloud.
Image credit: Rice University.

In the Rice experiment, when temperatures drop to within a few billionths of a degree of absolute zero, fermions with equal but opposite spin become attracted to one another and behave, in some respects, like one particle. Like a couple on the dance floor, they don't technically share space, but they move in unison. In superconductors, these dancing pairs allow electrical current to flow through the material without any resistance at all, a property that engineers have long dreamed of harnessing to eliminate "leakage" in power cables, something that costs billions of dollars per year in the U.S. alone.

The superconducting and superfluid phases are analogous except that superconductivity happens with particles carrying an electrical charge and superfluidity occurs in electrically neutral particles. In superfluids, fermionic pairing leads to a complete absence of viscosity - like a wave rippling back and forth in a swimming pool without ever diminishing.

"Conventional theory tells us superconductivity or superfluidity occurs only in the presence of an equal number of spin-up and spin-down particles. Physicists have speculated for almost 50 years about what would happen if this condition were not met. Because of the pristine and controlled nature of our ultracold atoms, we're able to offer definitive evidence of what happens with mismatched numbers of spin-up and spin-down particles" - lead researcher Randy Hulet, Fayez Sarofim Professor of Physics and Astronomy.

Ultracold experiments at temperatures just a few billionths of a degree above absolute zero are Hulet's specialty. It's only been technically possible to chill atoms to these temperatures for the past 10 years, but in that time, this ability has proved remarkably useful for testing the predictions of quantum mechanics and for exploring the properties of what physicists call "many-body phenomena," including superconductivity and superfluidity.
In the latest experiments, Hulet's team - which includes graduate students Guthrie Partridge, Ramsey Kamar and Yean-an Liao and postdoctoral researcher Wenhui Li - cooled a mixture of fermionic lithium-6 atoms to about 30-billionths of a degree above absolute zero. That's far colder than any temperature in nature - even in deepest interstellar space - and it's sufficient to quell virtually all thermodynamic interaction in the atoms, leaving them subject to superfluid quantum pairing.
Using radio waves, Hulet's team altered the ratio of spin-up and spin-down atoms in the cooled atoms with great precision. They found that the superfluid was able to tolerate an excess of up to 10 percent unpaired fermions with no detrimental effects.
Hulet's team found that increasing the ratio of spin-up to spin-down atoms eventually caused a phase change. When unpaired spin-up atoms rose above 10 percent of the total sample, the unpaired loners were suddenly expelled, leaving a core of superfluid pairs surrounded by a shell of excess spin-up atoms.
It is the unbalanced yet seemingly unaffected superfluid, however, that is capturing most of the scientific attention at the moment.

"The gas behaves as if it is still perfectly paired, which is quite remarkable given the excess of spin-up atoms. This was unexpected, and it could signal a new, exotic form of superfluidity that may be akin to the electron pairings in unconventional superconductors or to the quark soup that's predicted to exist at the heart of the densest neutron stars" - Randy Hulet.

In the largest neutron stars - known as "quark stars" - a mass about five times greater than the sun is pressed into a space smaller than the island of Manhattan. Some physics theorists believe gravity is so strong at the heart of these stars that it creates something called a "strange matter," a dense superfluid of up quarks, down quarks and strange quarks.

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