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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Measuring nanomechanical motion with an imprecision below that at the standard quantum limit

G. Anetsberger, P. Verlot, E. Gavartin, O. Arcizet, Quirin P. Unterreithmeier, Eva Maria Weig, M. L. Gorodetsky, J. P. Kotthaus, Tobias Jan Kippenberg

Published May 1, 2011• 5 citations
Publisher

Abstract

Using optical readout, we demonstrate for the first time sub-SQL imprecision for nanomechanical motion at room temperature.
DOI
10.1109/cleoe.2011.5943378

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We report a room-temperature measurement of nanomechanical motion with an imprecision below the standard quantum limit (SQL), using cavity-enhanced optical near fields. Fundamental thermodynamical cavity frequency fluctuation limits the total imprecision to 3dB3 \mathrm{dB}3dB below the SQL at room temperature [corresponding to (530am/Hz)2(530 \mathrm{am}/\sqrt{\mathrm{Hz}}){}^{2}(530am/Hz​)2 in absolute units] and is expected to reduce to negligible values at moderate cryogenic temperatures. Coupling strengths exceeding those required to reach the SQL by more than two orders of magnitude are achieved, allowing a shot-noise limited imprecision more than 10dB10 \mathrm{dB}10dB below the SQL. The transducer, thus, in principle, allows access to the quantum backaction dominated regime, a prerequisite for exploring quantum backaction, measurement-induced squeezing, and obtaining sub-SQL sensitivity using backaction evading techniques.
Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.