Integrated levitated optomechanical gravimeter

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Current highly sensitive gravimeters, such as superconducting spheres, atom interferometers, and torsion pendulums, suffer from high manufacture and maintenance cost (up to £400k), bulky size (as large as 2.5m^3) and slow measurement speed (typically 1 hour). Here we propose an exciting innovation in quantifying gravity, based on the frequency measurement of the gravity-induced precession in an optically levitated fast-spinning particle. This novel levitated optomechanical systems (LOMS) gravimeter can be fabricated on a silicon wafer with wafer-level vacuum encapsulation, making its footprint as small as one mm^2. The small size device is mass-producible with a fabrication cost potentially less than £4k. The proposed research uses the analogy of the precession of the Earth, a slow and continuous change in the orientation of the Earth's rotational axis induced by the gravity of the sun, to develop the novel gravimeter. In December 2018, our research for the first time revealed that the precessional motion also appears in sophisticatedly designed LOMS and that optical scattering techniques can precisely measure the frequency of precession [U9]. Our calculation predicts that levitated rotating particles of 10um diameter can achieve the sensitivity of 10^-9 g/sqrt(Hz) and a very fast-spinning particle (GHz reported in 2018 [x19]) can achieve 10^-11 g/sqrt(Hz) sensitivity, respectively. The novel gravimeter can also measure the acceleration due to the Einstein equivalence principle. Thanks to the ultra-high Quality-factor (7.7x10^11 demonstrated in 2017 [x3]) of the rotating particles, the novel sensor will have the potential to cover 11 orders of magnitude of acceleration measurement. Moreover, using the advanced silicon fabrication technique, we will be able to differentiate the centre-of-mass and the centre-of-optical-force of the levitated particle, in order to optimise the range of the gravity (or acceleration) induced torque, and correspondingly design the sensing range and sensitivity of the acceleration, e.g. 10^-6 m/s^2 to 10^5 m/s^2 to cover the seismic and mining health monitoring applications or 1 m/s^2 to 10^11 m/s^2 for fundamental physics research. The sensor only requires short integration times (1ns to 100s, depend on the precession frequency). Thus, it can complete the measurement very rapidly. This novel precession sensing principle can also be utilised to measure force, strain, charge and mass, with similar ultra-wide dynamic range and ultra-high sensitivity potentially.The innovative gravimeter (accelerometer) can be a powerful tool for investigating fundamental physics questions in gravitation, which are pressing and very hard to access experimentally due to the weakness of the gravitational interaction if compared to other interactions. The proposed research can also provide a platform for quantum manipulation of mesoscopic mechanical devices in the nano-scale regime and can serve as a testbed for theoretical predictions.Furthermore, our novel sensor can equipt the oil and gas industry with its applications in CO2-EOR and exploration. It can track temporal and spatial variations of the gravitational field and provide highly accurate information of mass redistribution below the surface. The prototype on-chip LOMS gravimeter has a small footprint so that it can be installed close to the drilling bit. Based on Newton's law of universal gravitation, the gravimeter has the potential to detect 1.5x10^7 kg mass redistribution above the ground, and 1.5x10^5 kg mass redistribution inside the wellbore. The sensitivity of the novel gravimeters installed inside wellbores can be four orders of magnitude better than that of the existing highly sensitive gravimeters. Our research also contributes to CSS, mineral exploration, structural safety monitoring for mining, earthquake warning, inertial navigation and geoscience, and can lead to significant cost savings in multiple industries.