Abstract
The displacement of micro-electro-mechanical-systems
(MEMs) cantilevers is used to measure a variety of phe-
nomena in devices ranging from force microscopes for
single spin detection[1] to biochemical sensors[2] to un-
cooled thermal imaging systems[3]. The displacement
readout is often performed optically with segmented de-
tectors or interference measurements. Until recently, var-
ious noise sources have limited the minimum detectable
displacement in MEMs systems, but it is now possible
to minimize all other sources[4] so that the noise level
of the coherent light eld, called the shot noise limit
(SNL), becomes the dominant source. Light sources dis-
playing quantum-enhanced statistics below this limit are
available[5, 6], with applications in gravitational wave
astronomy[7] and bioimaging[8], but direct displacement
measurements of MEMS cantilevers below the SNL have
been impossible until now. Here, we demonstrate the
rst direct measurement of a MEMs cantilever displace-
ment with sub-SNL sensitivity, thus enabling ultratrace
sensing, imaging, and microscopy applications. By com-
bining multi-spatial-mode quantum light sources with a
simple di erential measurement, we show that sub-SNL
MEMs displacement sensitivity is highly accessible com-
pared to previous e orts that measured the displacement
of macroscopic mirrors with very distinct spatial struc-
tures crafted with multiple optical parametric ampli ers
and locking loops[9]. We apply this technique to a com-
mercially available microcantilever in order to detect dis-
placements 60% below the SNL at frequencies where the
microcantilever is shot-noise-limited. These results sup-
port a new class of quantum MEMS sensor whose ulti-
mate signal to noise ratio is determined by the correla-
tions possible in quantum optics systems.
