Optically measuring force near the standard quantum limit

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Science  27 Jun 2014:
Vol. 344, Issue 6191, pp. 1486-1489
DOI: 10.1126/science.1249850

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Measuring tiny forces with atomic clouds

For projects such as detecting gravity waves, physicists need to measure tiny forces precisely. Schreppler et al. developed an extremely sensitive method for force measurement. They applied a known force on a cloud of ultracold rubidium atoms in an optical cavity. The force caused the atoms to oscillate, and the researchers used optical measurements to monitor the motion. Under optimal conditions, the authors could measure forces with a level of sensitivity only four times worse than the fundamental limit imposed by the Heisenberg uncertainty principle.

Science, this issue p. 1486


The Heisenberg uncertainty principle sets a lower bound on the noise in a force measurement based on continuously detecting a mechanical oscillator’s position. This bound, the standard quantum limit, can be reached when the oscillator subjected to the force is unperturbed by its environment and when measurement imprecision from photon shot noise is balanced against disturbance from measurement back-action. 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 is resonant with the cloud’s oscillation frequency, we achieve a sensitivity that is a factor of 4 above the standard quantum limit and consistent with theoretical predictions given the atoms’ residual thermal disturbance and the photodetection quantum efficiency.

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