Demonstration of a rubidium optical clock with an improved long-term stability of 6.6×10 −15 at an integration time of 10000 seconds

We report a rubidium optical clock based on a new architecture in which a fiber laser is stabilized to the two-photon 5S 1/2 (F=1)→5D 5/2 (F ′ =1) transition of 87 Rb , a choice that reduces the external field sensitivity relative to commonly used transitions. Long-term frequency fluctuation is minimized by equilibrating the helium concentration in the vapor cell, thereby suppressing collisional shifts from helium permeation. The error signal is generated through third-harmonic demodulation for the suppression of the residual amplitude modulation. With these measures, the system reaches a fractional frequency instability of 3.5×10 −13 at 1 s and 6.6×10 −15 at 10,000 s. Its long-term stability matches that of a VCH-1008 passive hydrogen maser over the same averaging time, demonstrating the potential of this compact rubidium optical clock for precision timekeeping and navigation.