Quantum physics moves ahead near absolute zero

Hancock, N.H. - Researchers in Colorado are claiming an important breakthrough in the study of molecular quantum mechanics and superconductivity.

Their work, conducted jointly by the National Institute of Standards and Technology and the University of Colorado, Boulder, shows how to build molecules from atoms that are trapped in a container at near-absolute-zero temperature and that are all in their lowest energy state.

The insight came as the result of a recent experiment into a novel state of matter called a Bose-Einstein condensate. An earlier experiment, hinting at ways to produce BECs without complex laser-cooling systems, took place at Lawrence Berkeley National Laboratory.

The latest work, conducted by Deborah Jin and her colleagues at NIST's Jila lab, a joint institute with the University of Colorado at Boulder, used an isotope of potassium in which the nucleus had one extra neutron. After cooling the potassium isotopes to 150 nanokelvins using laser fields and magnetic traps-a standard BEC procedure-the researchers were able to move the atoms around until they loosely linked up into molecules.

In the original BEC experiment, a gas of rubidium atoms was cooled to near absolute zero, causing the atoms to merge into a single quantum state.

The atoms were manipulated with the same laser and magnetic fields used to cool the atoms. One surprise was the large number of atoms that could be linked up into molecules-about 50 percent of the potassium atoms ended up in molecular groups.

"This work could help us understand the basic physics behind superconductivity and especially high-temperature superconductivity," said Jin.

BECs are useful for probing quantum physics because a large number of atoms all behave as though they are one quantum particle, making it much easier to observe them. Also, an experiment at MIT conducted by Wolfgang Ketterle in 1997 showed that a beam of BEC atoms could be formed, creating a matter version of a laser beam. If that could be generated in a practical system, it could lead to a holographic type of material deposition that would allow complex nanostructures to be built without any photomasks.

Any such applications are a long way off, however, due to the extremely cold temperatures required to produce a BEC.

The first hint of collective quantum states was the discovery of superconductivity in mercury by Dutch physicist Kamerlingh Onnes in 1911. That phase change in the electron gas inside a conductor takes place at 4.2 Kelvins-very cold by room temperature standards but still about 100 million times warmer than the temperatures required for a BEC.

BECs also depend on the boson nature of the atoms in the condensate. All particles are classified as either fermions or bosons. The basic physical distinction is that bosons can all occupy the same quantum state, while fermions resist any attempt to push them into identical states. Photons are bosons and are therefore able to crowd together in high-bandwidth fibers, while electrons are fermions and repel one another. Atoms can be either type depending on the structure of their nucleus. In the JILA experiments, adding an extra neutron to the nucleus of the potassium atoms changed them from fermions to bosons. But when bosons are linked together in molecular configurations, the molecules become fermions.

When fermions are linked together in pairs, the reverse transformation happens: They become bosons. That is why work on BECs could shed light on superconductivity-the underlying mechanism that consists of electrons forming "Cooper pairs" results in a system of bosons in a condensed state.

Work at Lawrence Berkeley National Lab (LBNL) offers a tantalizing possibility of getting around the temperature barrier. While not completely confirmed at this point, physicist Daniel Chemla believes that he may have created a BEC inside a gallium arsenide superlattice.

In this case, the particles making up the BEC are excitons-an electron and hole linked together-which form a particle analogous to the hydrogen atom.

Exciton mass is similar to that of electrons and Chemla expects that a gas of excitons could condense into a BEC at a few degrees above absolute zero. This would be a big gain for BEC research, since the complex laser and magnetic-cooling systems would not be required.

Chemla has been using superlattice structures to coax excitons to recombine and condense into a stable, long-lived configuration.

Usually they decay in a few nanoseconds, but by causing them to form across an aluminum gallium arsenide layer with the electron and hole trapped in gallium arsenide layers, Chemla was able to extend their life.

Chemla's colleague Chih-Wei Lai reported the results at the recent Conference on Lasers and Electro-Optics in June.

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