Two-color Rubidium clock with AC Stark Shift Suppression

Navigation, positioning, and timing rely heavily on compact optical clocks. Warm vapor clocks utilize vapor cells near room temperature to maintain vapor pressure and thus do not require the complex cooling systems of cold atom clocks. They typically take up less space, measured in liters rather than cubic meters, and therefore could be more versatile than large cold atomic clocks. Despite those advantages, the AC-Stark and pressure-induced shift caused by temperature and probe laser power fluctuations result in these clocks producing a substantial fractional instability in the order of 10^-12 to 10^-15. With the standard one-color clock, reaching fractional instability beyond 10-15 is technologically impractical. However, a newly proposed scheme uses two lasers at 776 and 780 nm rather than one at 776 nm, which reduces AC-Stark shift of the 5S_1/2 →5D_5/2 two-photon transition [1]. This could allow the clock to operate at a lower cell temperature; unfortunately, this scheme adds first-order optical Doppler shift, making the transition more sensitive to cell temperature. We propose using the opposing signs of the first-order Doppler shift and temperature-induced frequency shift to cancel both shifts fully [2]. This cancellation would result in a warm vapor clock that is more resilient against small temperature, pressure, and optical power fluctuations in the vapor. This scheme could produce compact optical clocks that can outperform the previous generation of cold atomic clocks.