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Accurate position and posture measurements of the freely-falling test massare crucial for the success of spaceborne gravitational wave detectionmissions. This paper presents a novel laboratory-developed test mass motionreadout that utilizes quadrant photodetectors to measure the translation andtilt of a test mass. Departing from conventional methods like Zeeman effect orAOM frequency shift modulation, the readout system employs the phase locking oftwo lasers to generate the dual-frequency heterodyne source. Notably, theout-of-loop sensitivity of the phase locking reaches below 30 pm/Hz1/2 withinthe frequency band of 1 mHz and 10 Hz. The system comprises three measurementinterferometers and one reference interferometer, featuring a symmetric designthat enables measurements of up to six degrees of freedom based onpolarization-multiplexing and differential wavefront sensing. Ground-simulatedexperimental results demonstrate that the proposed system has achieved ameasurement sensitivity of 4 pm/Hz1/2 and 2 nrad/Hz1/2 at 1 Hz, a resolution of5 nm and 0.1 urad, a range of 200 um and 600 urad, respectively. These findingsshowcase the system's potential as an alternative method for preciselymonitoring the motion of test masses in spaceborne gravitational wave detectionmissions and other applications requiring accurate positioning andmulti-degrees-of-freedom sensing.

Six-degrees-of-freedom test mass readout via optical phase-locking heterodyne interferometry