  In recent years, levitated particles of optical traps in vacuum have shown
enormous potential in precision sensor development and searching for new
physics. The accuracy of the calibration relating the detected signal to
absolute displacement of the trapped particle is a critical factor for absolute
measurement performance. In this paper, we suggest and experimentally
demonstrate a novel calibration method for optical tweezers based on
free-falling particles in vacuum, where the gravitational acceleration is
introduced as an absolute reference. Our work provides a calibration protocol
with great certainty and traceability, which is significant in improving the
accuracy of precision sensing based on optically levitated particles.
