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Physical Review Research

Cavity-QED measurements of the Sr87 millihertz optical clock transition and determination of its natural linewidth

Juan A. Muniz, Dylan J. Young, Julia Cline, James K. Thompson

Published May 27, 2021• 38 citations
Publisher

Abstract

The authors develop a nondestructive cavity-QED technique to measure observables along long-lived optical transitions, applying this technique to determine the lifetime of a millihertz-linewidth transition in 87-Sr used in next-gen atomic clocks.
DOI
10.1103/physrevresearch.3.023152

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Cavity-QED determination of the natural linewidth of the 87^{87}87Sr millihertz clock transition with 30μ\muμHz resolution
We present a new method for determining the intrinsic natural linewidth or 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 novel quantum many-body phenomena. With longer lifetime optical transitions, sensitivity is increased, but so far it has proved challenging to determine the natural lifetime of many of these long lived optical excited states because standard population decay detection techniques become experimentally difficult. Here, we determine the ratio of a poorly known ultranarrow linewidth transition (3^33P0_00​ to 1^11S0_00​ in 87^{87}87Sr) to that of another narrow well known transition (3^33P1_11​ to 1^11S0_00​) 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 3^33P0_00​ to 1^11S0_00​ in 87^{87}87Sr to be γ0/(2π)=1.35(3) \gamma_0/(2\pi) = 1.35(3)~γ0​/(2π)=1.35(3) mHz or τ=118(3) \tau= 118(3)~τ=118(3) s. The 30 μ~\mu μHz resolution implies that we could detect states with lifetimes just below 2 hours, and with straightforward future improvements, we could detect states with lifetimes up to 15 hours, using measurement trials that last only a few hundred milliseconds, eliminating the need for long storage times in optical potentials.
We will present a new technique for determining the lifetime of long-lived optically excited states. In particular, we are able to implement cavity-enhanced QND style measurements on two optical transitions in strontium as a way of determining the linewidth of the millihertz linewidth optical transition in strontium. The observation of the direct modification of the transmitted probe light also opens the door to direct frequency comparisons between the probe light and the optical transition frequency, perhaps providing an alternative to Ramsey spectroscopy and enhanced measurement bandwidth.
In my thesis work, I have explored a variety of ways in which narrow linewidth optical transitions can be utilized to extend the capabilities of precision measurement and quantum science. These tools include non-destructive atom counting techniques that may enhance the performance of optical lattice clocks, a novel form of cavity enhanced atomic spectroscopy appropriate for laser frequency stabilization, a new form of laser cooling with reduced reliance on spontaneous emission, several methods for enhancing the sensitivity of proposed atomic gravitational wave detectors, a newly observed form of cavity-mediated spin-spin interactions, and a new class of laser based on optical superradiance from narrow and ultra-narrow linewidth transitions. As part of the superradiant laser project, we have demonstrated the most precise active absolute frequency reference realized to date, which still has great potential for improvement.\nMany of these tools involve coupling atoms to an optical cavity, a field known as cavity quantum electrodynamics, or cavity QED. In my thesis work, a key focus was the extension of cavity QED techniques into the new regime of coupling large ensembles of atoms to a cavity using narrow linewidth optical transitions (in contrast to the previously explored regimes of microwave-frequency transitions or broad-linewidth optical transitions). To this end, I constructed a new experiment to couple an ensemble of atoms to an optical cavity using two optical transitions in atomic strontium, one with a narrow 7.5 kHz linewidth, and another with an ultra-narrow 1 mHz linewidth. This new system has allowed us to access a unique new regime of cavity QED, and a wealth of interesting and useful applications.
Abstract Narrow linewidth optical atomic transitions provide a valuable resource for frequency metrology, and form the basis of today's most precise and accurate clocks. Recent experiments have demonstrated that ensembles of atoms can be interfaced with the mode of an optical cavity using such transitions, and that atom-cavity interactions can dominate over decoherence processes even when the atomic transition that mediates the interactions is very weak. This scenario enables new opportunities for optical frequency metrology, including techniques for nondestructive readout and entanglement enhancement for optical lattice clocks, methods for cavity-enhanced laser frequency stabilization, and high-precision active optical frequency references based on superradiant emission. This tutorial provides a pedagogical description of the physics governing atom-cavity coupling with narrow linewidth optical transitions, and describes several examples of applications to optical frequency metrology.
Optical lattice clocks based on the narrow ( 5 s 2 ) S 0 1 − ( 5 s 5 p ) P 0 3 transition 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 Sr 87 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 S 0 1 – P 0 3 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 P 0 3 state. By making measurements of high and low density ensembles of both Sr 87 and Sr 88 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 − 40 + 79 s for the P 0 3 clock state in Sr 87 , a value that is consistent with previously reported measurements and theoretical predictions. We also introduce an additional measurement scheme that involves repeated measurements of the ground-state population within a single experimental sequence, validating our model and the consistency of the measured rates. We expect that the techniques introduced in this work are applicable to lifetime measurements of long-lived metastable states in other atoms and ions used for clocks and quantum computing. The results presented in this work inform performance estimates of proposed gravitational wave detectors and long-baseline atom interferometers making use of Sr 87 .