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 5<i>S</i>_1/2(<i>F</i>=1)→5<i>D</i>_5/2(<i>F</i>^<i>'</i>=1) transition of ⁸⁷<i>Rb</i>, 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.