Anyone using a modern camera is implementing an optical position measurement. In an active autofocus camera, a pulse of infrared light is emitted from the camera, and the time taken for it to be reflected back to the camera is used to compute the distance between the object and the image plane. Imagine how difficult it would be to operate such a system if the object recoiled every time the infrared pulse was reflected from it. Heisenberg suggested that this is precisely what would happen if light were used to determine the position of a quantum object as accurately as his famous uncertainty principle would allow. On page 801 of this issue, Purdy et al. (1) demonstrate this quantum back-action effect in an optical measurement of the position of a macroscopic mirror.
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