Improving the absolute accuracy of the gravitational wave detectors by combining the photon pressure and gravity field calibrators

The absolute accuracy of the estimated parameters of gravitational wave sources will be fundamentally limited by the calibration uncertainties of the detectors in upcoming observation runs with the increased number of source statistics. Photon calibrators have so far been the primary tools for the absolute calibration of a test-mass displacement, relying on the measurement of the photon pressure. The current technological limit of the absolute calibration uncertainty for gravitational-wave amplitudes is limited to a few percent, due to the uncertainty in the laser power standard maintained by the metrology institutes. To reduce this uncertainty, this article proposes a novel calibration method that combines a photon calibrator and a gravity field calibrator. The gravity field calibrator achieves modulation of the displacement of the test mass by generating a gravity gradient. In previous studies, uncertainty in the distance between the test mass and the gravity field calibrator has proven a serious source of systematic error. To suppress this uncertainty, we propose a novel method that uses a combination of quadrupole and hexapole mass distributions in the gravity field calibrator. We estimate the absolute uncertainty associated with the method to be as low as 0.17%, which is 10 times less than that of previous methods.