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Observation of quantum-measurement backaction with an ultracold atomic gas


Current research on micromechanical resonators strives for quantum-limited detection of the motion of macroscopic objects. Prerequisite to this goal is the observation of measurement backaction consistent with quantum metrology limits. However, thermal noise currently dominates measurements and precludes ground-state preparation of the resonator. Here, we establish the collective motion of an ultracold atomic gas confined tightly within a Fabry–Perot optical cavity as a system for investigating the quantum mechanics of macroscopic bodies. The cavity-mode structure selects a particular collective vibrational motion that is measured by the cavity’s optical properties, actuated by the cavity optical field and subject to backaction by the quantum force fluctuations of this field. Experimentally, we quantify such fluctuations by measuring the cavity-light-induced heating of the intracavity atomic ensemble. These measurements represent the first observation of backaction on a macroscopic mechanical resonator at the standard quantum limit.

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Figure 1: Experimental schematic diagram.
Figure 2: Cavity-based observation of evaporative atomic losses due to cavity-light-induced diffusive heating.
Figure 3: Cavity heating of the collective atomic motion in a strongly coupled Fabry–Perot cavity over spontaneous emission dominated free-space heating.


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We thank T. Purdy and S. Schmid for early contributions to the experimental apparatus, and S. M. Girvin, J. Harris, H. J. Kimble, H. Mabuchi and M. Raymer for helpful discussions. This work was supported by AFOSR, DARPA and the David and Lucile Packard Foundation.

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K.W.M., K.L.M. and S.G. contributed experimental work, data analysis and theoretical work to the article and supplemental information. D.M.S.-K. contributed project guidance, data analysis and theoretical work.

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Correspondence to Dan M. Stamper-Kurn.

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Murch, K., Moore, K., Gupta, S. et al. Observation of quantum-measurement backaction with an ultracold atomic gas. Nature Phys 4, 561–564 (2008).

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