$\mathrm{\Lambda }$-Enhanced Gray Molasses Cooling of ${}^{85}$Rb Atoms in Tweezers Using the D${}_2$ Line

We demonstrate the implementation of $\mathrm{\Lambda }$-enhanced gray molasses cooling on the D${}_2$ line of ${}^{85}$Rb atoms in an optical tweezer array. This technique yields lower atomic temperatures of 4.0(2) $\mu$K compared to red-detuned polarization gradient cooling, and consequently extends the $T_2^{\ast }$ 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, as it only requires frequency and phase modulation control. Our experimental observations are corroborated by a numerical model based on a semi-classical force approach extended to a four-level system, including two hyperfine states of the upper manifold that are 120 MHz apart.