Motion-selective coherent population trapping by Raman sideband cooling along two paths in a $Λ$ configuration

Abstract

We report our experiment on sideband cooling with two Raman transitions in a Λ configuration that allows selective coherent population trapping (CPT) of the motional ground state. The cooling method is applied to 87 Rb atoms in a circularly-polarized one-dimensional optical lattice. Owing to the vector polarizability, the vibration frequency of a trapped atom depends on its Zeeman quantum number, and CPT resonance for a pair of bound states in the Λ configuration depends on their vibrational quantum numbers. We call this scheme motion-selective coherent population trapping (MSCPT) and it is a trapped-atom analogue to the velocity-selective CPT developed for free He atoms. We observe a pronounced dip in temperature near a detuning for the Raman beams to satisfy the CPT resonance condition for the motional ground state. Although the lowest temperature we obtain is ten times the recoil limit owing to the large Lamb-Dicke parameter of 2.3 in our apparatus, the experiment demonstrates that MSCPT enhances the effectiveness of Raman sideband cooling and enlarges the range of its application. Discussions on design parameters optimized for MSCPT on 87 Rb atoms and opportunities provided by diatomic polar molecules, whose Stark shift shows strong dependence on the rotational quantum number, are included.

Motion-selective coherent population trapping for subrecoil cooling of optically trapped atoms outside the Lamb-Dicke regime

“…However, reducing γ 12 to γ 0 12 /10 is challenging. In our previous work on 7 Li using rf fields [5], we achieved γ 0 12 /4, limited by the magnetic field noise. The final limit on γ 12 for an atom in an optical lattice originates from the scattering rate R OL of the lattice photons [14].…”

“…Other problems are the slow cooling process and technical burdens to implement complex cooling beams and to control the phase noise from various sources. Under these challenges, a proof-of-principle experiment was successfully carried out in our laboratory to demonstrate the feasibility and effectiveness of the idea [7], and an improved version of the experiment is in progress. Longer term, we plan to apply the MSCPT scheme to cool a diatomic polar molecule in an optical trap.…”

“…In spite of these difficulties, we recently demonstrated effectiveness of the MSCPT scheme in an experiment using 87 Rb atoms in a 1D optical lattice [7]. We observed CPT phenomena driven by a pair of stimulated Raman transitions and, by employing the MSCPT scheme, achieved lower temperature than that obtained by RSC.…”