Magneto-Optical Trapping and Cooling of Strontium Atoms down to the Photon Recoil Temperature

We report narrow-line laser cooling and trapping of strontium atoms down to the photon recoil temperature. ⁸⁸Sr atoms precooled by the broad ¹S₀-¹P₁ transition at 461 nm were further cooled in a magneto-optical trap using the spin-forbidden transition ¹S₀-³P₁ at 689 nm. We have thus obtained an atomic sample with a density over 10¹²cm^-3 and a minimum temperature of 400 nK, corresponding to a maximum phase space density of 10^-2 which is 3 orders of magnitude larger than the value that has been obtained by magneto-optical traps to date. This scheme provides us an opportunity and system to study quantum statistical properties of degenerate fermions as well as bosons.