Significance   With the rapid development of laser technology in the last century, microscopic optomechanical effects have gradually been discovered by researchers. In 1971, Arthur Ashkin in Bell Laboratory discovered the acceleration and trapping of particles by radiation pressure, and first proposed the concept of "optical potential wells", also known as "optical trap". In 1976, Ashkin achieved optical levitation of a fused quartz sphere in ultrahigh vacuum and pointed out its feasibility of high-precision sensing in low-damping environments. In 1986, Ashkin constructed an optical gradient potential trap using tightly focused beams to capture particles, which announced the birth of optical tweezers and raised a new era of levitated optomechanical sensing technology. Thanks to the pioneering work of Ashkin, and with the development of vacuum technology, levitated optomechanical sensing technology emerged. The technology has great characteristics of non-contact, high sensitivity, and feasible integration. Compared to previous quantum sensing based on the cold atom interference or nuclear magnetic resonance, this new technology involves larger particles with much more uniform atoms, which allows intuitive observation of particle morphology. Meanwhile, levitated optomechanical sensing technology enables ultra-high sensitive detection at room temperature without the need of the complex cryogenic environment. Therefore, the levitated optomechanical system can be considered as an "ideal platform" for precise measurements, where its accuracy is gradually approaching the standard quantum limits. The technology has also played significant roles in many cutting-edge fields including microscopic thermodynamics, dark-matter explorations, and macroscopic quantum state preparations.

  Progress   Firstly, we describe the basic theory of the levitated optomechanical sensing. Tested physical quantity can be measured by sensing the motion parameters of the optical-trapped particles. Relevant key sensing technologies contain the loading of the particles, the enhancement of the optical forces, the displacement detections, the calibration of the voltage coefficient and the feedback cooling. These specific technologies are remarkably developed in recent years. For instance, feedback cooling has achieved occupation numbers below 1, which opens the door to quantum ground-state at room temperature. During the last decades, levitated optomechanical sensing is widely used in the measurements of the basic physical quantities, such as the extremely weak forces, the accelerations, the microscopic mass, the residual electrical quantities, and ultra-small torques. We have listed the typical applications of levitated optomechanical sensing. It can realize a force sensitivity of ~10⁻²¹ N /\sqrt\mathrmH\mathrmz and acceleration sensitivity of ~100 ng /\sqrt\mathrmH\mathrmz . It also has achieved microscopic mass resolutions of 10⁻¹² gram and an electric intensity sensitivity of 1 μV/(cm·\sqrt\mathrmH\mathrmz ). When the particles are optically driven to high-speed rotation, accurate torque measurements can be achieved with a sensitivity of ~10⁻²⁹ N·m /\sqrt\mathrmH\mathrmz .

  Conclusions and Prospects   The trends of the technology are summarized and relevant suggestions are given. With the progress of its engineering, levitated optomechanical sensing is moving towards practical applications. The current levitated optomechanical sensing is developed in two routes of high-precision and integration. The former orients towards the demand for basic research, mainly using spatial optical components and pursuing lower noise floors. The latter orients towards practical applications using integrated optics and micro-nano processing. In the next step, we need to pay more attention to effective combination of levitated optomechanical sensing technology and other disciplines, and continue to strengthen the engineering practical research. We hope to achieve technical breakthroughs and practical applications of relevant sensors such as the light force accelerometers and the optomechanical gyroscopes.