Blue-detuned magneto-optical trap

We present the properties and advantages of a new magneto-optical trap (MOT) where blue-detuned light drives `type-II’ transitions that have dark ground states. Using ${}^{87}$Rb, we reach a radiation-pressure-limited density exceeding ${10}^{11}$cm${}^{-3}$ and a temperature below 30$\mu$K. The phase-space density is higher than in normal atomic MOTs, and a million times higher than comparable red-detuned type-II MOTs, making it particularly attractive for molecular MOTs which rely on type-II transitions. The loss of atoms from the trap is dominated by ultracold collisions between Rb atoms. For typical trapping conditions, we measure a loss rate of $1.8(4)\times {10}^{-10}$cm${}^3$s${}^{-1}$.