Temperature-shift suppression scheme for two-photon two-color rubidium vapor clocks

Compact optical clocks are at the heart of many important applications involving navigation, position and timing. Employing a vapor cell that is either slightly heated or cooled beyond room temperature to maintain a desirable vapor pressure, these compact clocks neither require a vacuum system nor laser cooling, and their size is typically best measured in liters, rather than cubic meters. However, such clocks only deliver a ‘poor’ fractional instabilities at the order of 10^-12 to 10^-15. This is mainly due to broad linewidths of clock transitions (e.g. a few 100 kHz for the 5S – 5D rubidium two photon transition), and more importantly the AC-Stark and the pressure-induced shifts driven by intensity fluctuations of the probe-laser and temperature variations of the vapor cell. Thus, reaching fractional frequency instabilities beyond 10^-15 impose technologically unrealistic long-term constraints on the laser power and the cell temperature. A scheme for rubidium vapor clocks was recently proposed to significantly reduce AC-Stark-induced shifts to the 5S_1/2 → 5D_5/2 two photon transition by using two lasers at 776 and 780 nm, instead of a single laser at 778 nm to probe the atoms. With an intermediate level closer to the 5P_3/2 level, this scheme allows a two-orders of magnitude increase in the signal to noise ratio that could allow operation of the clock at a lower cell temperature, where the atomic vapor becomes less dense. Unfortunately, we found that this two-color scheme adds a first-order contribution from the optical Doppler shift to the clock transition, which makes it slightly more sensitive to the cell temperature variations. Here, we propose to employ this residual Doppler shift against the frequency shifts induced by cell temperature variations and achieve a full cancellation for both of the shifts without using special gas mixtures. This could lead to a vapor clock that is insensitive to both, optical power fluctuations and temperature/pressure fluctuations of the atomic vapor, making liter-sized optical clocks with a stability of the previous-generation cold atomic clocks feasible.