Researchers detect tiniest of force ever measured

What could be the smallest force that can be applied to an object? For now, scientists have measured a force of 42 yoctonewtons, which is the smallest force measured so far.

A yoctonewton is one septillionth of a newton and there are approximately 3 x 1,023 yoctonewtons in one ounce of force.

Using a combination of lasers and a unique optical trapping system that provides a cloud of ultra cold atoms, the force has been detected by researchers at Berkeley Lab and University of California (UC) Berkeley.

"We applied an external force to the centre-of-mass motion of an ultra cold atom cloud in a high-finesse optical cavity and measured the resulting motion optically," said physicist Dan Stamper-Kurn.

"When the driving force was resonant with the cloud's oscillation frequency, we achieved a sensitivity that is consistent with theoretical predictions and only a factor of four above the Standard Quantum Limit, the most sensitive measurement that can be made," Stamper-Kurn added.

If you want to confirm the existence of gravitational waves, space-time ripples predicted by Albert Einstein in his Theory of General Relativity, or want to determine to what extent the Law of Gravity on the macroscopic scale continues to apply at the microscopic scale, you need to detect and measure forces and motions that are almost incomprehensively tiny.

As measurements of force and motion reach quantum levels in sensitivity, however, they bump up against a barrier imposed by the Heisenberg uncertainty principle, in which the measurement itself perturbs the motion of the oscillator, a phenomenon known as "quantum back-action".

This barrier is called the Standard Quantum Limit (SQL).

Over the past couple of decades, a wide array of strategies have been deployed to minimise quantum back-action and get ever closer to the SQL, but the best of these techniques fell short by six to eight orders of magnitude.

"We measured force with a sensitivity that is the closest ever to the SQL," said lead author Sydney Schreppler.

"We were able to achieve this sensitivity because our mechanical oscillator is composed of only 1,200 atoms," she explained.

The findings appeared in the journal Science.