Clock transition for a future optical frequency standard with trapped atoms

Abstract

We report direct excitation of the strongly forbidden ${5s}^{2}{}^{1}{S}_{0}\ensuremath{-}5s5p{}^{3}{P}_{0}$ transition in ${}^{87}\mathrm{Sr}.$ Its frequency is 429 228 004 235(20) kHz. A resonant laser creates a small leak in a magneto-optical trap (MOT): atoms build up to the metastable ${}^{3}{P}_{0}$ state and escape the trapping process, leading to a detectable decrease in the MOT fluorescence. This line has a natural width of ${10}^{\ensuremath{-}3}\mathrm{Hz}$ and can be used for a new generation of optical frequency standards using atoms trapped in a light-shift-free dipole trap.