Demonstration of chronometric leveling using transportable optical clocks beyond laser coherence limit

An optical clock network requires the establishment of an optical frequency transmission link between multiple optical clocks, utilizing narrow-linewidth lasers. Despite achieving link noise levels of 10^-20, the final accuracy is limited by the phase noise of the clock laser. Correlation spectroscopy has been developed to transmit frequency information between two optical clocks directly, enabling optical clock comparison beyond the phase noise limit of clock lasers, and significantly enhancing the measurement accuracy or shortening the measurement time. In this article, two compact transportable ^40Ca^+ clocks are employed to accomplish a correlation spectroscopy comparison, demonstrating a 10-cm level measurement accuracy of chronometric leveling using a mediocre clock laser with linewidth of 200 Hz. The relative frequency instability reaches 6. 010^-15//s, which is about 20 times better than the result with Rabi spectroscopy using the same clock laser. This method circumvents the harsh requirements on the performance of the clock laser, so that an ordinary stable laser can also be employed in the construction of an optical clock network, which is essential for field applications, especially for chronometric leveling.