Experimental Determination of theMg24I(3s3p)P23Lifetime

We present the first experimental determination of the electric-dipole forbidden $(3s3p)^{3}P_{2}\ensuremath{\rightarrow}(3{s}^{2})^{1}S_{0}$ ($M2$) transition rate in $^{24}\mathrm{Mg}$ and compare to state-of-the-art theoretical predictions. Our measurement exploits a magnetic trap isolating the sample from perturbations and a magneto-optical trap as an amplifier converting each $^{3}P_{2}\ensuremath{\rightarrow}^{1}S_{0}$ decay event into millions of photons readily detected. The transition rate is determined to be $(4.87\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ corresponding to a $^{3}P_{2}$ lifetime of ${2050}_{\ensuremath{-}110}^{+140}$ sec. This value is in agreement with recent theoretical predictions, and to our knowledge the longest lifetime ever determined in a laboratory environment.