TianQin is a future space-based gravitational wave (GW) observatory targeting the frequency window of 10⁻⁴–1 Hz. A large variety of GW sources are expected in this frequency band, including the merger of massive black hole binaries, the inspiral of extreme/intermediate mass ratio systems, stellar-mass black hole binaries, Galactic compact binaries, and so on. TianQin will consist of three Earth orbiting satellites on nearly identical orbits with orbital radii of about 10⁵ km. The satellites will form a normal triangle constellation whose plane is nearly perpendicular to the ecliptic plane. The TianQin project has been progressing smoothly following the ‘0123’ technology roadmap. In step ‘0’, the TianQin laser ranging station has been constructed and it has successfully ranged to all the five retro-reflectors on the Moon. In step ‘1’, the drag-free control technology has been tested and demonstrated using the TianQin-1 satellite. In step ‘2’, the inter-satellite laser interferometry technology will be tested using the pair of TianQin-2 satellites. The TianQin-2 mission has been officially approved and the satellites will be launched around 2026. In step ‘3’, i.e. the TianQin-3 mission, three identical satellites will be launched around 2035 to form the space-based GW detector, TianQin, and to start GW detection in space.

In this paper, we present a suppression method for the thermal drift of an ultra-stable laser interferometer. The detailed analysis on the Michelson interferometer indicates that the change in optical path length induced by temperature variation can be effectively reduced by choosing proper thickness and/or incident angle of a compensator. Taking the optical bench of the Laser Interferometer Space Antenna Pathfinder as an example, we analyze the optical bench model with a compensator and show that the temperature coefficient of this laser interferometer can be reduced down to 1 pm/K with an incident angle of 0.267828 rad. The method presented in this paper can be used in the design of ultra-stable laser interferometers, especially for space-based gravitational waves detection.

High-precision and wide-band microthrust testing and evaluation on the ground is key to achieving precision, drag-free control, which is extremely demanding for spaceborne gravitational-wave detection missions. However, unlike a stable space environment, complex ground vibrations inevitably affect the evaluation of microthrust. To reduce the interference of ground vibrations, this paper proposes a suppression solution based on a hanging pendulum with an adjustable mass center. The theoretical analysis shows that the response of the pendulum to ground vibration can be effectively reduced with a matched mass center. The dual-frequency modulation experiment verified the linear relationship between vibration noise and mass-center-to-swing-point distance. By setting the mass-center distance from the swing point to 0.2 mm, the noise of the sub-micro-Newton balance is better than $0.1\text{μ}\mathrm{N}{\mathrm{Hz}}^{-1/2}$ from 6 mHz to 20 Hz. This method effectively suppresses the interference of ground vibration, which helps us to obtain an accurate microthrust model for space gravitational-wave missions, while providing a solution for weak-force, ground testing over wide-band frequencies.

As ground-based gravitational wave (GW) detection has opened a new window for exploring the Universe, space-based GW detection will be the next frontier to deepen the understanding of galaxy evolution and massive black holes merging. TianQin-1 is a technology verification satellite for the space-based GW detector TianQin. One of the key technologies demonstrated in TianQin-1 is a high-precision and ultra-stable interferometer, with a noise floor lower than 100 pm / Hz 1 / 2 @ 0.1 Hz . In this paper, we report the long-term stability of the interferometer in the TianQin-1 satellite, which contains a quasi-monolithic optical bench made through hydroxide catalysis bonding (HCB). According to the results of the ground and in-orbit tests, the long-term stability of the interferometer for 13 months after launch has been evaluated. The resolution of the interferometer is better than 30 pm. The noise level of the interferometer is about 30 pm / Hz 1 / 2 @ 0.1 Hz and 10 pm / Hz 1 / 2 above 1 Hz, fulfilling the mission requirement of TianQin-1. The successful in-orbit operation has demonstrated the interferometer’s high precision and long-term stability based on the quasi-monolithic optical bench.