Direct measurement of the 3P0 clock state natural lifetime in 87Sr

Abstract: Optical lattice clocks based on the narrow transition between 5s2-1S0 and 5s5p-3P0 levels in neutral strontium (Sr) are among the most precise and accurate measurement devices in existence. Although this transition is completely forbidden by selection rules, state mixing from the hyperfine interaction in 87Sr provides a weakly allowed transition that can be coherently driven with practical clock laser intensities. While the coherent interrogation times of optical clocks are typically set by the linewidth of the probe laser, this limitation can be overcome in synchronous differential comparisons between ensembles. In such measurements the natural lifetime of the 1S0 to 3P0 clock transition becomes the fundamental limiting factor to the duration of a single run of the experiment. However, a direct measurement of the decay rate of the clock excited state is quite challenging due to the competing effects of other loss channels such as Raman scattering, inelastic collisions and atom-loss due to background gas. In this work, we monitor the decay of Sr atoms trapped in an optical lattice and initialized in the 3P0 state. By making measurements of high and low density ensembles of both 87Sr and 88Sr across varying lattice trap depths, we isolate radiative decay, which accounts for a significant fraction of the observed decays at low depths. We obtain a natural radiative decay lifetime of 167+79-40s for the 3P0 clock state in 87Sr, a value that is consistent with previously reported direct and indirect measurements, along with theoretical predictions. We also introduce an additional measurement scheme that involves repeated measurements of the ground state population within a single experimental sequence, further confirming our model and the consistency of the measured rates.