Quantum-enhanced sensing is a powerful technique in which nonclassical states are used to optimize the sensitivity of a measurement. Although squeezed light in particular has proven to be an attractive resource for enhancing mechanical displacement sensing, the fundamental limits of the enhancement have not yet been encountered. Here we use a microwave cavity optomechanical system to observe the squeezing-dependent radiation pressure noise that necessarily accompanies any improvement in the measurements precision and ultimately limits the measurements noise performance. We also exploit this radiation pressure force to realize a quantum nondemolition (QND) measurement of the light, enhancing our detection of the squeezing. Our results fully illustrate how squeezed light interacts with mechanical oscillators, which will be relevant for the next generation of advanced gravitational wave observatories.
optomechanics, squeezing, quantum nondemolition