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

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 ⁸⁷Rb, 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.