Optical frequency measurement of the 1S-3S two-photon transition in hydrogen

This article reports the first optical frequency measurement of the $1\mathrm{S}-3\mathrm{S}$ transition in hydrogen. The excitation of this transition occurs at a wavelength of 205 nm which is obtained with two frequency doubling stages of a titanium sapphire laser at 820 nm. Its frequency is measured with an optical frequency comb. The second-order Doppler effect is evaluated from the observation of the motional Stark effect due to a transverse magnetic field perpendicular to the atomic beam. The measured value of the $1\mathrm{S}_{1/2}(F=1)-3\mathrm{S}_{1/2}(F=1)$ frequency splitting is $2 922 742 936.729 (13) \mathrm{MHz}$ with a relative uncertainty of $4.5\times10^{-12}$. After the measurement of the $1\mathrm{S}-2\mathrm{S}$ frequency, this result is the most precise of the optical frequencies in hydrogen

Metrological features of the rubidium two-photon standards of the BNM-LPTF and Kastler Brossel Laboratories

We have built three optical frequency standards based on the two-photon transition of rubidium at 778 nm, and analysed their performance over a period of more than three years. We discuss some systematic effects that could lead to the reproducibility we observe, and point out the possible improvements of the devices. We also examine the short and long term stabilities of the systems, and show that we have reached their ultimate performances