Quantum Pileup: Ultracold molecules meld into oneness

Two independent teams of physicists have coaxed molecules into an extraordinary state of ultracold matter previously demonstrated only with atoms.

ALL FOR ONE. Released from a trap, an ultracold cloud of potassium molecules (right) barely disperses–a sign that nearly motionless molecules have condensed into a new form of matter. Slightly warmer cloud (left), which hasn’t condensed, spreads after release. M. Greiner

In each of the new experiments, the researchers created minuscule gas clouds with an amazing property. All of the constituent two-atom molecules meld to form a single supermolecule, says Deborah S. Jin of JILA, a joint institute of the National Institute of Standards and Technology and the University of Colorado, both in Boulder.

In an upcoming Nature, she and her colleagues report magnetically influencing ultracold potassium atoms to make about 200,000 weakly bound pairs. Of those, about a tenth coalesce into a supermolecule.

Taking a different ap-proach, Rudolf Grimm and his colleagues at the University of Innsbruck in Austria cooled 100,000 two-atom lithium molecules into a supermolecule. The team reports its success in a future issue of Science.

Blending the identities of atoms, and now of simple molecules, relies on a quantum-mechanical process made possible by wave-like characteristics of the particles. That merger is the hallmark of so-called Bose-Einstein condensates (BECs).

Satyendra Nath Bose and Albert Einstein independently predicted BECs in 1924, and the first was made in 1995 from rubidium atoms (SN: 7/15/95, p. 36). Since then, many laboratories have created BECs from different elements and have studied the condensates’ properties (SN: 8/12/00, p. 102: Available to subscribers at Attractive atoms pick up repulsive habits).

To take BEC science further, researchers have been striving to achieve quantum condensations of simple molecules, such as the potassium or lithium pairs.

The new condensates are an “important milestone,” comments physicist Christophe Salomon of the École Normale Supérieure in Paris.

Both teams created long, thin condensate clouds that measured tens of micrometers in diameter. The Innsbruck group claims to have made a relatively long-lived condensate, which lasted more than 20 seconds. The Boulder condensate stuck around for only about 10 milliseconds.

Both groups say that by creating molecular condensates, they have devised a new means to investigate fundamental aspects of physics and chemistry. Potential topics include how electric charge is distributed within individual electrons and the ways in which chemical bonds form and break.

The new accomplishments may also lead to deeper understanding of superfluidity (SN: 10/25/03, p. 262: Available to subscribers at Super Spinner: Seven-atom speck acts like superfluid), which is the flow of fluids without friction, and of superconductivity (SN: 10/11/03, p .229: Available to subscribers at Nobel prizes go to scientists harnessing odd phenomena), the resistancefree flow of electrons, comments Wolfgang Ketterle of the Massachusetts Institute of Technology. Ketterle shared the 2001 Nobel Prize in Physics for his pioneering work on BECs.

Physicists have long recognized that elementary particles of ordinary matter fall into two broad classes, bosons and fermions. Chummy by nature, nearby bosons readily occupy the same quantum state. For instance, photons will share a particular energy level in a laser. In contrast, standoffish fermions won’t share a quantum state with even one other fermion.

Although electrons, protons, and neutrons, the building blocks of atoms, are all fermions, some atoms are bosons and some are fermions. It’s easy to tell which is which: If an atom’s total number of building blocks is even, the atom is a boson. If the total number is odd, it’s a fermion.

To make BECs, scientists trap and cool bosons to temperatures just above absolute zero. Attempts to do the same with molecules made of bosonic atoms failed, however, because collisions shattered those molecules before condensation could take place.

Earlier this year, the BEC scientific community discovered that molecules composed of just two fermionic atoms are far less vulnerable to such disintegration. Conveniently, such molecules–because they contain an even number of fermions–are actually bosons.

The discovery has proved critical in the race for molecular BECs, Grimm says. Turning to molecules made of fermionic atoms, the Innsbruck and Boulder groups both achieved the coveted goal.


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