The Standard Quantum Limit is imposed by the Heisenberg uncertainty principle, in which the measurement itself perturbs the motion of the oscillator, a phenomenon known as “quantum back-action.” (Image by Kevin Gutowski)

A team of researchers led by Dan Stamper-Kurn of UC Berkeley have announced that they have measured the smallest amount of force to date. Using a combination of lasers and optical trapping, they were able to detect 42 yoctonewtons. One yoctonewton is a septillionth (1/1024) of a Newton. Though scientists once believed the standard quantum limit (SQL) would be reached about 30 years ago, previous measurements within recent years have only come within a factor of 6-8 above SQL. These results come just a factor of 4 greater than the SQL. The team’s paper was published in Science.

“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,” Stamper-Kurn said in a press release. “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.”

The SQL is brought about by the Heisenberg uncertainty principle, which essentially states that two complimentary measurements (force and motion, in this case) can only get so precise without negatively affecting one another. In measuring the force optically, the equipment used will get to a point where they are perturbed more by the act of measuring than by the object it is supposed to be recording, due to “quantum back-action.” The SQL is most precise measurement of quantum force that can be taken before measurements are skewed.

The team used an mechanical oscillator created by two standing-wave light fields at slightly different wavelengths in order to optically trap a cloud of only 1,200 rubidium atoms. After the cloud was brought down near absolute zero, one of the light waves creates a force on the cloud by slightly altering the wave’s amplitude. A laser probe is then able to measure the infinitesimal amount of force.

“When we apply an external force to our oscillator it is like hitting a pendulum with a bat then measuring the reaction,” explained lead author Sydney Schreppler. “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.”

By using colder atoms and more advanced optical equipment, the researchers are confident that they can detect forces even closer to SQL. There are also different techniques that can be tried in order to avoid the quantum back-action that creates noise when measuring force. Advances on this front could also lead to improved atomic force microscopy.