Trapping atoms in a laser beam offers a new way to measure gravity

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By watching how atoms behave when they’re suspended in
midair, rather than in free fall, physicists have come up with a new way to measure
Earth’s gravity. 

Traditionally,
scientists have measured gravity’s influence on atoms by tracking how fast
atoms tumble down tall chutes. Such experiments can help test Einstein’s theory
of gravity and precisely
measure fundamental constants
(SN: 4/12/18).
But the meters-long tubes used in free-fall experiments can be unwieldy and
difficult to shield from environmental interference such as stray magnetic
fields. With a new tabletop setup, physicists can gauge the strength of Earth’s
gravity by monitoring atoms suspended a couple millimeters in the air by laser
light. 

This redesign, described in the Nov. 8 Science, could better probe the
gravitational forces exerted by small objects
. The technique also could
be used to measure slight gravitational variations at different places in the
world, which may help in mapping the seafloor or finding oil
and minerals underground
(SN: 2/12/08). 

Physicist Victoria Xu and colleagues at the University
of California, Berkeley began by launching a cloud of cesium atoms into the air
and using flashes of light to split each atom into a superposition state. In this
weird quantum limbo, each atom exists in two places at once: one version of the
atom hovering a few micrometers higher than the other. Xu’s team then trapped
these split cesium atoms in midair with light from a laser. 

Measuring the strength of gravity with atoms that are
held in place, rather than being tugged downward by a gravitational field, requires
tapping into the atoms’
wave-particle duality
(SN: 11/5/10).
That quantum effect means that, much as light waves can act like particles
called photons, atoms can act like waves. And for each cesium atom caught in
superposition, the higher version of the atom wave undulates a little faster
than its lower counterpart, due to the atoms’ slightly different positions in
Earth’s gravitational field. By tracking how fast the waviness of the two
versions of an atom gets out of sync, physicists can calculate the strength of
Earth’s gravity at that spot. 

“Very impressive,” says physicist Alan Jamison of MIT.
To him, one big promise of the new technique is more controlled measurements.
“It’s quite a challenge to work on these drop experiments, where you have a
10-meter-long tower,” he says. “Magnetic fields are hard to shield, and the
environment produces them all over the place — all the electrical systems in
your building, and so forth. Working in a smaller volume makes it easier to avoid
those environmental noises.” 

More compact equipment can also measure shorter-range
gravity effects, says study coauthor Holger Müller. “Let’s say you don’t want
to measure the gravity of the entire Earth, but you want to measure the gravity
of a small thing, such as a marble,” he says. “We just need to put the marble
close to our atoms [and hold it there]. In a traditional free-fall setup, the
atoms would spend a very short time close to our marble — milliseconds — and we
would get much less signal.” 

Physicist Kai Bongs of the University of Birmingham in
England imagines using the new kind of atomic gravimeter to investigate the
nature of dark matter or test a fundamental facet of Einstein’s theory of
gravity called the equivalence
principle
(SN: 4/28/17). Many unified theories of physics proposed
to reconcile quantum mechanics and Einstein’s theory of gravity — which are
incompatible — violate the equivalence principle in some way. “So looking for
violations might guide us to the grand unified theory,” he says. “That’s one of
the Holy Grails in physics.”

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