Simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K on the D$_1$ line: Theory and Experiment

TL;DR

This paper demonstrates simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K using the D$_1$ line, combining experimental results with numerical simulations to understand the cooling mechanisms and achieve low temperatures suitable for quantum degeneracy.

Contribution

It provides the first combined experimental and theoretical analysis of sub-Doppler cooling of both species on the D$_1$ line, highlighting the role of ground state coherences and comparing D$_1$ and D$_2$ cooling.

Findings

Achieved temperatures of 44 μK for $^6$Li and 11 μK for $^{40}$K after 5 ms cooling.

Phase-space density close to 10^{-4} for both species.

Simulation accurately reproduces experimental results and elucidates physical mechanisms.

Abstract

We report on simultaneous sub-Doppler laser cooling of fermionic $^6$Li and $^{40}$K using the D$_1$ optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semi-classical Monte Carlo wavefunction method. The simulation takes into account the three dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the D$_1$ transitions. We discuss the physical mechanisms at play, we identify the important role of coherences between the ground state hyperfine levels and compare D$_1$ and D$_2$ sub-Doppler cooling. In 5 ms, the D$_1$ molasses phase largely reduces the temperature for both $^6$Li and $^{40}$K at the same time, with a final temperature of 44 $\mu$K and 11 $\mu$K, respectively. For both species this leads to a phase-space density close to $10^{-4}$. These conditions are well suited to directly load an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.