Next-generation ultralow-noise mechanical sensors defined and controlled by light

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Mechanical technologies are everywhere in society, from oscillators in timekeeping devices to accelerometers and electronic filters in automobiles and cell phones. They also represent an indispensable tool for fundamental and applied science: using tiny mechanical systems, it is possible to "feel around" surfaces at the atomic scale, detect chemical mass changes with single-proton resolution, and sense element-specific magnetic "tugs" from nanoscale clusters of nuclei (even creating a 3d map). In the field of optomechanics, we have learned to exploit the forces exerted by light to gain an unprecedented level of control over these systems at all size scales, leading to entirely new functionalities for next-generation sensors, including those in which the laws of quantum mechanics play a central role. ******Our research aims to realise ultralow-noise micromechanical sensors that are reconfigured and controlled by light in unique ways. To this end, we fabricate delicate "micro-trampolines" exhibiting a world-record combination of force sensitivity and optical performance, such that the radiation force from an average of just one photon -- the smallest quantity of light allowed by nature -- will exert a profound influence over their mechanical trajectories. Here we propose to capitalise upon this breakthrough to demonstrate strong single-photon control, access quantum states of motion, and generate quantum "squeezed" light useful for enhancing interferometers (such as those used to detect gravitational waves). Additionally, by partially levitating related mechanical elements, we will even further enhance their sensitivities by essentially replacing their primary mechanical supports with light (a low-noise alternative to flexible materials). Finally, we will pursue a qualitatively new system in which light strongly controls the spatial distribution of oscillating mass: by optically perturbing a periodic structure ("phononic crystal"), we can smoothly tune the spatial extent of oscillating mass from the centimetre scale to the micron scale in situ -- a level of control that is currently unheard of. Furthermore, due to a collective enhancement effect, a larger device is predicted to exhibit a larger response to a given quantity of light (despite its larger mass) enabling a truly macroscopic response to single-photon light levels in a chip-scale device. In addition to creating new types of reconfigurable mechanical sensing technologies, these complementary efforts build toward fundamental studies of quantum motion at the macro scale, mechanical transduction of quantum information between a variety of "quantum bit" ("qubit") technologies and light (e.g., for long-distance quantum-secured communication), and the detection of zeptonewton forces (equivalent to the gravitational pull between two loaves of bread separated by 100 km).