Smallest ever force measured by lasers and ultracold atoms (Wired UK)

VinothChandar / CC BY 2.0



Physicists at the Lawrence Berkeley National Laboratory and the
University of California have successfully measured a force of 42
yoctonewtons — the smallest ever.

To get an idea of how small that is, one newton is roughly the
force exerted on your hand by an apple due to gravity. A
yoctonewton is 0.000000000000000000000001 times that amount -
unimaginably tiny, in other words.

More interestingly, it’s only a factor of four above the standard
quantum limit
– a number that represents the most sensitive
measurement that it’s possible to make by our current understanding
of physics.

The team did it with a combination of lasers and an optical
trapping system comprised of a cloud of very cold atoms. Or as Dan
Stamper-Kurn, one of the physicists involved explains: “We applied
an external force to the center-of-mass motion of an ultracold atom
cloud in a high-finesse optical cavity and measured the resulting
motion optically. When the driving force was resonant with the
cloud’s oscillation frequency, we achieved a sensitivity that is
consistent with theoretical predictions.” Yup.

That super-accurate setup should allow physicists to test
whether Newtonian gravity applies in the quantum world, and could
also help us improve our most accurate microscopes.

In the meantime, colder atoms and better optical detection
should allow the researchers to get even closer to the standard
quantum limit. But before you get too impressed by that, you should
know that we’re behind schedule. “A scientific paper in 1980
predicted that the SQL might be reached within five years,” said
Sydney Schreppler, lead author of the paper
describing the results
 in Science.



“It took about 30 years longer than predicted, but we now have
an experimental set-up capable of reaching very close to the

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