Quenched narrow-line laser cooling of ${}^{40}\mathrm{Ca}$ to near the photon recoil limit

We present a cooling method that should be generally applicable to atoms with narrow optical transitions. This technique uses velocity-selective pulses to drive atoms towards a zero-velocity dark state and then quenches the excited state to increase the cooling rate. We demonstrate this technique of quenched narrow-line cooling by reducing the 1D temperature of a sample of neutral ${}^{40}\mathrm{Ca}$ atoms. We cool selected velocities with the ${}^1{S}_0{(4s}^2{)}^3{P}_1\mathrm{}\left(4s4p\right)\mathrm{}$ 657-nm intercombination line and quench with the ${}^3{P}_1{\mathrm{}\left(4s4p\right)\mathrm{}}^1{S}_0\mathrm{}\left(4s5s\right)\mathrm{}$ intercombination line at 553 nm, which increases the cooling rate eightfold. Limited only by available quenching laser power, we have transferred 18% of the atoms from our initial 2-mK velocity distribution and achieved temperatures as low as 4 μK, corresponding to a $v_{\mathrm{rms}}$ of 2.8 cm/s or 2 recoils at 657 nm. This cooling technique, which is closely related to Raman cooling, can be extended to three dimensions.