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UC Berkeley researchers measure smallest force ever quantified

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JULY 02, 2014

Researchers from Lawrence Berkeley National Laboratory and UC Berkeley published a paper last week on their measurement of the smallest force ever quantified.

Researchers applied a known force to a mass of extremely cold atoms and measured the resulting force using light. Using the measurement of this resulting force, they ultimately quantified the original force. Thus researchers demonstrated their ability to measure very small forces.

Shining too much light on an object to see its motion results in an effect known as quantum back-action, in which the measurement itself disturbs the motion of the object and prevents it from recording the force accurately.

The limit on measurement accuracy caused by quantum back-action is called the standard quantum limit. The force, at a miniscule 42 yoctonewtons, was closer to this limit than any other force recorded previously. There are more than a billion yoctonewtons in one newton and about 4.45 newtons in a pound of force.

“If you’ve got two very tiny things that are only nanometers apart … there are some theories that say Newtonian physics isn’t enough to describe that,” said Sydney Schreppler, a graduate student in the physics department at UC Berkeley and the lead author of the paper in Science. “So if you want to be able to measure that and test those theories, you need to be able to measure very tiny forces.”

Physicists studying gravitational waves aim to measure at the standard quantum limit, according to campus associate professor of physics Dan Stamper-Kurn, who led the research group.

“When they get there, they may be able to observe gravity waves,” Stamper-Kurn said. “It could be a new form of astronomy, looking at not just light but gravity waves, which could allow us to hear objects in the distant universe.”

Precise quantum measurements could also have applications in the field of biophysics and could be used to quantify the very small forces of attraction between proteins or DNA molecules. Any technology for measuring forces has a natural limit, depending in this system on the intensity of light used to measure the forces.

“There’s a fundamental limit of quantum mechanics,” said Nicolas Spethmann, a postdoctoral scholar at UC Berkeley and co-author of the paper.

Using a better light detector as well as colder gas may bring the sensitivity of the measurement closer to the standard quantum limit, Schreppler said.

“Back in 1980, there was a paper by a group of people that are working on these large gravitational wave interferometers, and they talked about the (standard quantum limit) and how it would become a problem for their measurement of small forces,” she said. “We think five years from now we will be at the standard quantum limit.”

Contact Madeleine Pauker at 

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JULY 02, 2014


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