Sub-Doppler Cooling of Neutral Atoms in a Grating Magneto-Optical Trap

The recent demonstration of a grating magneto-optical trap (GMOT) for $^{87}$Rb presents an advancement in the field of atom traps [1]. The system requires only a single beam and three planar diffraction gratings to form an accessible cloud of cold atoms above the plane of the diffractors. Here we demonstrate further sup-Doppler cooling of the atoms to a temperature of 7.6(0.6) $\mu$K through a multi-stage, far-detuned MOT in conjunction with optical molasses. A decomposition of the electric field into polarization components for this geometry does not yield a mapping onto standard sub-Doppler cooling configurations. With numerical simulations, we find that the polarization composition of the GMOT optical field, which includes both $\sigma$- and $\pi$-polarized light, does indeed produce sub-Doppler temperatures. We also discuss the integrability of the GMOT with an optical nanofiber trap as a step towards creating a hybrid quantum system that couples atoms to superconducting circuits. \\[4pt] [1] M. Vangeleyn \textit{et al}., Opt. Lett. \textbf{35}, 3453 (2010).