  The displacement of micro-electro-mechanical-systems (MEMS) cantilevers is
used to measure a broad variety of phenomena in devices ranging from force
microscopes to biochemical sensors to thermal imaging systems. We demonstrate
the first direct measurement of a MEMS cantilever displacement with a noise
floor at 40% of the shot noise limit (SNL). By combining multi-spatial-mode
quantum light sources with a simple differential measurement, we show that
sub-SNL MEMS displacement sensitivity is highly accessible compared to previous
efforts that measured the displacement of macroscopic mirrors with very
distinct spatial structures crafted with multiple optical parametric amplifiers
and locking loops. These results support a new class of quantum MEMS sensor
with an ultimate signal to noise ratio determined by quantum correlations,
enabling ultra-trace sensing, imaging, and microscopy applications in which
signals were previously obscured by shot noise.
