Integrated levitated optomechanical gravimeter

目前的高灵敏度重力仪，如超导球体、原子干涉仪和扭摆，都存在制造和维护成本高（高达40万英镑）、体积庞大（高达2.5m^3）和测量速度慢（通常为1小时）的问题。在这里，我们提出了一项令人兴奋的量化重力的创新，基于在光学悬浮的快速旋转粒子中重力诱导进动的频率测量。这种新型的悬浮式光机械系统（LOMS）重力仪可以在硅片上制造，并采用晶片级真空封装，使其占地面积小至1 mm^2。这种小尺寸的设备可以批量生产，制造成本可能不到4000英镑。提出的研究利用地球进动的类比，即由太阳引力引起的地球自转轴方向缓慢而连续的变化，来开发新型重力仪。2018年12月，我们的研究首次揭示了在精心设计的LOMS中也存在岁差运动，并且光学散射技术可以精确测量岁差频率[U9]。我们的计算预测，直径为10um的悬浮旋转粒子可以达到10^-9 g/sqrt（Hz）的灵敏度，而非常快速旋转的粒子（2018年报道的GHz [x19]）可以分别达到10^-11 g/sqrt（Hz）的灵敏度。由于爱因斯坦等效原理，这种新型重力仪还可以测量加速度。由于旋转粒子的超高质量因子（2017年展示的7.7x10^11 [x3]），这种新型传感器将有可能覆盖11个数量级的加速度测量。此外，利用先进的硅制造技术，我们将能够区分悬浮粒子的质心和光力中心，以优化重力（或加速度）诱导扭矩的范围，并相应地设计加速度的传感范围和灵敏度，例如10^-6 m/s^2到10^5 m/s^2，以覆盖地震和采矿健康监测应用，或1 m/s^2到10^11 m/s^2用于基础物理研究。传感器只需要很短的积分时间（1ns到100s，取决于进动频率）。因此，它可以非常快速地完成测量。这种新颖的进动传感原理也可用于测量力、应变、电荷和质量，具有类似的超宽动态范围和超高灵敏度。创新的重力仪（加速度计）可以成为研究引力基本物理问题的有力工具，由于引力相互作用与其他相互作用相比薄弱，这些问题在实验上非常紧迫和难以获得。该研究还可以为纳米尺度下介观机械器件的量子操纵提供一个平台，并可以作为理论预测的测试平台。此外，我们的新型传感器可以为石油和天然气行业的二氧化碳eor和勘探提供装备。它可以跟踪重力场的时空变化，提供地表以下质量再分布的高精度信息。片上LOMS重力仪的原型占地面积小，因此可以安装在钻头附近。基于牛顿万有引力定律，该重力仪有可能探测到地面上方1.5 × 10^7 kg的质量再分布，以及井筒内1.5 × 10^5 kg的质量再分布。新型井内重力仪的灵敏度比现有的高灵敏度重力仪提高了4个数量级。我们的研究还有助于CSS、矿产勘探、采矿结构安全监测、地震预警、惯性导航和地球科学，并可以为多个行业节省大量成本。

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.