University Park. There he has developed a
way to trap cesium atoms in two regions,
each with an oppositely oriented electric
field. Applying an external electric field to
the entire thing should cause electrons in
both regions to react equally. And because
the fields had opposite orientations to
start, any problems that might arise as
an artifact of the test should be obvious.
Weiss’ team is building equipment now
and hopes to start putting atoms into it
soon. From there, though, analysis could
take years. “The real question is how well
you can ultimately control
for systematic errors,” Weiss
says. “You have to be sure
that it’s right.”
Material of choice
Since those days, though, breakthroughs
in trapping and cooling atoms using
lasers have made atomic studies much
more sensitive. One promising atom-
based search today is in the lab of David
Weiss at Pennsylvania State University in
Las Vegas gamblers would do well to
put their chips on ytterbium fluoride
as the molecule most likely to yield a
new limit on the electron electric dipole
moment. A team led by physicist Edward
Hinds, now at Imperial College London,
has been using YbF in the hunt since
1993, and has submitted a paper describ-
ing its latest limit for publication.
Unlike atoms that can be trapped in
one spot for a while, heavy molecules can
be studied only in flight: A research team
makes a beam of them and looks for the
electric dipole moment sig-
nal as they fly by. Hinds’ group
can currently detect only
about one in every 100 YbF
molecules that zip past, but
is working on a new source
that sends 10 times more
molecules past and sends
them at one-third the speed.
Because the experiment’s
sensitivity is proportional
to how long researchers can study the
molecule, the next generation should be
10 times better at spotting the electric
dipole moment, Hinds says.
Even if one
team does
manage
to detect the
electric dipole
moment, the
work could be
far from over.
Some scientists are taking
a different tack by looking for
the electron electric dipole
moment in molecules. Polar
molecules, which have one
end with a slightly positive
charge and the other with a
slightly negative charge, look particularly
promising. In polar molecules with one
heavy atom and one light atom, electrons
zoom around the heavy end quickly, like
comets zipping into the solar system
and past the sun. This gets the electrons
going at nearly the speed of light, which
naturally enhances the way the electron
responds to an applied electric field,
ramping up any dipole moment signal.
The group plans to have the new
source up and running soon and, within
the next few years, to lower the limit to
10–29, where the electron electric dipole
moment might be detected at last. “It’s
conceivable that it’s just not there,”
Hinds says. “But there should be a dipole
moment unless there is some extraordinary accident.”
Gerald Gabrielse of Harvard University (shown) and colleagues are looking for
signs of an electron’s electric dipole moment in molecules of thorium monoxide.
Hot on Hinds’ heels is another molecular experiment. A team led by DeMille
along with Gerald Gabrielse and John
Doyle of Harvard chose the thorium
monoxide molecule because it would
naturally enhance the electric dipole
moment signal by quite a lot. The team
first vaporizes some thorium dioxide
with a pulse of laser light, then lines
up the resulting thorium monoxide
molecules in a beam line so they are all
spinning in the same direction. Then,
applying an electric field, the researchers try to figure out if the electron spin
shifted within the molecule as it would
if it had an electric dipole moment.
“These are incredibly tiny signals
that we’re looking for,” says DeMille.
“It’s not hard to imagine that effects can
mimic the tiny thing you’re looking for
Kris snibbe/harvard univ.
electrons produce a magnetic field), it’s
easy to accidentally change the magnetic
field when the external electric field is
applied. If this happens, the electron’s
spin changes in unwanted ways that
mimic how it should respond if it had an
electric dipole moment.
Scientists have thus developed a bag
of tricks to maximize their chances of
detecting an electric dipole moment — by
watching the electron for as long as possible, by enhancing its reaction and by
removing as many sources of outside
error as possible. The work is finicky and
frustrating. At Amherst, Hunter spent
years fine-tuning an experiment with
cesium atoms and published a limit in
1989, only to be overtaken the next year by
Commins. That work, at Berkeley, looked
for the electric dipole moment in thallium
atoms in the wee hours of the morning,
when nearby trains that could disturb the
measurement weren’t running.
