M. R. Vanner, J. Hofer, G. D. Cole, M. Aspelmeyer
Observing a physical quantity without disturbing it is a key capability for the control of individual quantum systems. Such `quantum non-demolition' (QND) measurements were first introduced in the 1970s in the context of gravitational wave detection to measure weak forces on test masses by high precision monitoring of their harmonic motion. At present, QND techniques have become an indispensable tool in quantum science for preparing, manipulating, and detecting quantum states of light, atoms, and other quantum systems. Here we experimentally perform QND measurements of the position of a mechanical oscillator by using short pulses of light, i.e. with a pulse duration much shorter than a period of mechanical oscillation. This technique enables us to perform both state preparation and full state tomography of the mechanical motional state. We have reconstructed mechanical states with a position uncertainty reduced to 19 pm, limited by the quantum fluctuations of the optical pulse, and we have performed `cooling by measurement' to reduce the mechanical mode temperature from an initial 1100 K to 16 K. Future improvements to this technique may allow for quantum squeezing of mechanical motion, even from room temperature, and reconstruction of non-classical states exhibiting negative regions in their phase-space quasi-probability distribution.
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http://arxiv.org/abs/1211.7036
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