Ultracool method helps observe smallest force ever

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Using a combination of lasers and an optical tracking system, the Lawrence Berkeley National Laboratory and the University of California, Berkeley, have detected the smallest force ever recorded. The research paper, published in Science on 27 June, said these results could help to detect gravitational waves.

By observing a cloud of around 1,200 ultra-cold rubidium atoms, the team measured a force of approximately 42 yoctonewtons (10-24 Newtons) by means of a mechanical oscillator. The device uses three light fields; 860 and 840nm standing waves for trapping the cloud and a 780nm probe beam.

The standing waves act on the cloud with an equal but opposite force, holding the atoms in place. By varying the 840nm light field, a centre of mass motion was induced in the atom cloud, which was then measured by the 780nm probe.

In a news release Sydney Schreppler, a member of the research group and lead author of the Science paper, said: ‘When we apply an external force to our oscillator it is like hitting a pendulum with a bat then measuring the reaction.’ He continued: ‘A key to our sensitivity and approaching the SQL is our ability to decouple the rubidium atoms from their environment and maintain their cold temperature. The laser light we use to trap our atoms isolates them from external environmental noise but does not heat them, so they can remain cold and still enough to allow us to approach the limits of sensitivity when we apply a force.’

When observing forces on this scale – close to the Standard Quantum Limit (SQL) – problems described by the Heisenberg uncertainty principle can cause a phenomenon known as ‘quantum back-action’. The action of observing a particle on this level can influence the results and unsurprisingly has proved a challenge for researchers in the past. Attempts before now have fallen short by six to eight orders of magnitude.

By using this method the group are now only four orders of magnitude away from the SQL. This is getting closer to the kind of sensitivity potentially required for observing gravitational waves and to observe the effects of macroscopic gravity on the quantum world.

Schreppler said in the release that it should be possible to get even closer to the SQL for force sensitivity through a combination of colder atoms and improved optical detection efficiency.