Interferometric techniques to measure mechanical displacement are subject to constraints ultimately governed by quantum mechanics. In a “standard” measurement of a mechanically induced optical phase shift, there exists a trade-off between measurement imprecision and motion disturbance, known as standard quantum limit (SQL) [1]. For a mechanical resonator with susceptibility χm(Ω), this limit is SSQL(Ω) = ħ |χm(Ω)| (ħ reduced Planck constant), at any frequency Ω. In the last half-century, a number of systems, including advanced LIGO and ultracold atoms have progressed towards this limit, but excess sources of noise have yet prevented fully reaching the SQL. Here, we show measurements of an ultracoherent mechanical resonator performed at the SQL within 33% [2] and, for the first time, below the SQL by 1.5 dB[3]. The latter has been possible by a non-standard measurement scheme that explois quantum correlations between optical quadratures induced in the optomechanical device.

© 2019 IEEE

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