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
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
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
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)