Cavity-QED measurements of the Sr 87 millihertz optical clock transition and determination of its natural linewidth

Abstract

We demonstrate the direct quantum nondemolition detection of a millihertz linewidth optical atomic transition. We observe the modification of the phase and amplitude of a probe field interacting with strontium atoms, which provides a direct spectroscopic signal to which a laser could be frequency stabilized. To investigate this measurement capability, we demonstrate an approach to determining the intrinsic natural lifetime of exceptionally long-lived optical excited states. Such transitions are key to the performance of state-of-the-art atomic clocks, have potential applications in searches for fundamental physics and gravitational wave detectors, as well as quantum many-body phenomena. Here, we determine the ratio of the challenging to measure and poorly known ultranarrow linewidth transition (P03 to S01 in Sr87) to that of another narrow well-known transition (P13 to S01) by coupling the two transitions to a single optical cavity and performing interleaved nondestructive measurements of the interaction strengths of the atoms with cavity modes near each transition frequency. We use this approach to determine the natural linewidth of the clock transition P03 to S01 in Sr87 to be γ0/(2π)=1.35(3) mHz or τ=118(3) s. The 30-μHz resolution implies that we could detect states with lifetimes just below 2 h, and with straightforward future improvements, we could detect states with lifetimes up to 15 h, using measurement trials that last only a few hundred milliseconds, eliminating the need for long storage times in optical potentials. This work opens the path to nondestructive direct spectroscopy of ultranarrow transition for continuous frequency measurements and laser stabilization.