Clock-jitter noise reduction by sideband arm locking for space-borne gravitational wave detectors

The space-borne gravitational wave (GW) detection mission endeavors to uncover more abundant GW sources within the frequency range of 0.1 mHz to 1 Hz. Given the inherently feeble nature of GW signals, they are often overshadowed by lots of noise sources. Among these, laser frequency noise and clock-jitter noise stand out as the primary obstacles in space-borne GW detection. To mitigate these noises, a postprocessing technique known as time-delay interferometry (TDI) has been proposed. Additionally, arm locking has emerged as a promising method to attenuate laser frequency noise by several orders of magnitude, thereby alleviating the challenges posed by TDI. Both theoretical and experimental evidence underscores the viability of this method. In this paper, we extend the application of arm locking to address the clock-jitter noise. In the space-borne GW detection, an interspacecraft clock transfer chain leveraging electro-optic modulator (EOM) sideband modulation is employed to facilitate the comparison of clock noises between spacecrafts. This comparison data can serve as an error signal and be used to control the clock signal, which is similar to the intersatellite laser phase-locking technology. Furthermore, we propose locking the clock frequency to the arm length of the GW detector, and delve into the methodology and identify the typical noise sources. Our findings reveal that within the scientific frequency band, clock-jitter noise can be suppressed by at least three orders of magnitude, thereby satisfying the stringent requirements of space-borne GW detection. At the same time, it also means that a highly stable clock can be effectively generated from a hardware perspective.