Ultracold Molecules

Atoms have been combined for the first time into tightly bound molecules in large numbers at temperatures close to absolute zero. This is good news for scientists who hope to have greater control over basic chemical reactions and for those who want to build a new kind of computer, one based on mysterious quantum behavior. Atoms are the basic constituents of ordinary matter, but in our everyday life most things---plastic, water, air, even our own bodies---are made of molecules, combinations of two or more atoms, so it's important to study them too.

Performing tests at a high level of detail often involves holding particles in place in a tiny enclosure, the better to look at them, and chilling them to very low temperatures, the better to measure their properties accurately. This is relatively easy to do for atoms, hard to do for molecules. That's because molecules, with several internal parts, squirm around in complex combinations of rotation and vibration. They can also break apart.

In several new experiments, molecules, each consisting of two atoms, hold together longer than in previous experiments and at a higher density, allowing the molecules to be studied in more detail. The trick here is to first make single atoms cold and then convert them into equally cold molecules, bonding the atoms using both laser pulses and magnetic forces. The molecules are so cold that they are nearly at rest and reside in the lowest possible energy condition, one in which they neither rotate nor vibrate. In previous experiments cold molecules were also achieved, but the process produced only widely-spaced, weakly-bound molecules which quickly broke up. Furthermore, the new research converts atoms into molecules more efficiently, with success rates as high as 90 percent.

Scientists resort to ultracold and ultra-slow molecules since a molecule nearly at rest is more likely to behave according to the kind of quantum rules used in today's fastest electronic products. Measurements on a molecule at rest will be less blurry than for a molecule wobbling around. The temperature used in an experiment at the JILA consortium of the National Institute of Standards and Technology (NIST) and the University of Colorado in Boulder and one at the University of Innsbruck in Austria was only a few hundred billionths of a degree above absolute zero (the lowest possible temperature allowed by physics) or about minus 450 degrees Fahrenheit. This is just about the coldest temperature in the solar system. Even the space between the planets is warmer than in the trapping device at the heart of these labs.

Atoms have a simple spherical shape, whereas two-atom molecules look more like footballs or dumbbells. Molecules with additional atoms are even more complicated. Because of this complexity, molecules can be harder to study---they're hard to catch one at a time. But this complexity, scientists hope, can be exploited in making new kinds of materials with novel properties, or in producing new forms of computing or communication.

The molecules used in the Boulder experiment are made of a potassium atom tied to a rubidium atom. Even though the molecule as a whole is electrically neutral, a tiny bit of negative charge tends to congregate at one end of the molecule and a bit of positive charge at the other end. This arrangement is called an electric dipole. Common examples of such "polar" molecules include water, which is made of two hydrogen atoms and one oxygen atom. Polar molecules, possessing their miniature separation of charge, can be controlled by tiny nearby electrodes. For this reason polar molecules show up in components for things such as displays for wristwatches and televisions.

In the Innsbruck experiment, the atoms being chilled don't merely move about haphazardly, as in a gas, but are stuck into assigned locations as if they were pieces on a microscopic three-dimensional chessboard. This special trapping, carried out with laser beams that gently restrain the atoms from moving, creates an array of atoms floating in mid air, a sort of dilute artificial material called an optical lattice. Actually, the criss-crossing laser beams are arranged so that two atoms (of the element rubidium) sit at each square of the "chessboard." Another way of visualizing the atoms in space is to picture them as sitting in the pouches of an egg carton (see the accompanying figure at http://www.aip.org/png/2008/306.htm ). Then, through the application of additional magnetic fields, the pair of atoms is hitched to become molecules.

Innsbruck physicist Johannes Denschlag likes to think of each spot in the lattice as a "nano-testube," a tiny zone less than a millionth of a meter (micro-meter) across, where chemical reactions can take place with just a few atoms at a time. Not only that, but the reaction can be completely controlled and the strength of the interaction can be adjusted. In the Innsbruck experiment the molecules aren't polar. Instead they behave like tiny magnets. And this gives researchers still another method (with the use of tiny nearby magnets) for controlling chemistry at the atomic level. In the Boulder experiment the ultracold molecules are produced in a ground state, the lowest and most stable possible energy state.

The molecules are packed into this state with a record-setting high density, more than a billion per cubic centimeter. Because these molecules are polar, scientists will be able to control them with electrodes and maybe even encode information into the molecules. And since the molecules are so close together, an important goal will be to perform microprocessing activities by letting the molecules interact with each other in a controllable way. This, in turn, would help to make possible a quantum-based computer at the nanoscopic level, able to execute certain calculations, such as searching large data bases or factoring large numbers into component parts, much faster than conventional digital computers. One of the Boulder scientists, Jun Ye, says that unprecedented control over molecules might also allow the development of even better timekeeping than is possible with today's atomic clocks. Earlier this year Ye participated in the establishment of one of the most accurate clock ever produced.

The NIST results were published in a recent issue of Science magazine, while the Innsbruck results appeared in the journal
Physical Review Letters, 26 September (journalists can view the PRL article at www.aip.org/phynews/select ) (Phillip F. Schewe)