Simultaneous preparation of two initial clock states in a thulium optical clock

Abstract

Due to the low sensitivity of the thulium optical clock to black-body radiation and good accuracy and stability estimations, it appears to be one of the most promising transportable optical clocks. One of the leading systematic effects for Tm clock transition, namely the second-order Zeeman effect, can be canceled by probing two clock transitions between different hyperfine levels of ground and metastable states during the clock operation. We prepare the atoms in $m_F=0$ state of both ground hyperfine levels simultaneously and excite alternately two clock transitions. Here we demonstrate efficient optical pumping into both target states via single-frequency radiation at 418.8 nm. The resulting population of $m_F=0$ states of ground hyperfine levels is 36% and 3.8% of the initial number of atoms with less than 4% and 0.4% on non-zero magnetic sublevels, correspondingly. We performed numerical simulations of the optical pumping process which is able to explain experimental results reasonably well.