Optical backaction-evading measurement of a mechanical oscillator.

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Shomroni, Itay 
Qiu, Liu 
Nunnenkamp, Andreas  ORCID logo  https://orcid.org/0000-0003-2390-7636
Kippenberg, Tobias J 

Quantum mechanics imposes a limit on the precision of a continuous position measurement of a harmonic oscillator, due to backaction arising from quantum fluctuations in the measurement field. This standard quantum limit can be surpassed by monitoring only one of the two non-commuting quadratures of the motion, known as backaction-evading measurement. This technique has not been implemented using optical interferometers to date. Here we demonstrate, in a cavity optomechanical system operating in the optical domain, a continuous two-tone backaction-evading measurement of a localized gigahertz-frequency mechanical mode of a photonic-crystal nanobeam cryogenically and optomechanically cooled close to the ground state. Employing quantum-limited optical heterodyne detection, we explicitly show the transition from conventional to backaction-evading measurement. We observe up to 0.67 dB (14%) reduction of total measurement noise, thereby demonstrating the viability of backaction-evading measurements in nanomechanical resonators for optical ultrasensitive measurements of motion and force.

quant-ph, quant-ph
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Nat Commun
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Springer Science and Business Media LLC
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The Royal Society (uf130303)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (732894)
Engineering and Physical Sciences Research Council (EP/M506485/1)