Λ -enhanced gray-molasses cooling of Rb 85 atoms in tweezers using the D 2 line

We demonstrate the implementation of Λ -enhanced gray molasses cooling on the D 2 line of Rb 85 atoms in an optical tweezer array. This technique yields lower atomic temperatures of 4.0(2) µK compared with red-detuned polarization gradient cooling, and consequently extends the T 2 * coherence time of the hyperfine clock qubit by a factor of 1.5. The method is alignment-free and can be readily implemented on laser beams used for magneto-optical trapping because it only requires frequency and phase modulation control. Our experimental observations are corroborated by a numerical model based on a semiclassical force approach extended to a four-level system, including two hyperfine states of the upper manifold that are 120 M Hz apart.