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Optical and Quantum Sensing and Precision Metrology

Cavity-enhanced non-destructive measurements for determination of the strontium clock transition linewidth with 30 microhertz resolution

James K. Thompson

Published Mar 5, 2021• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2588451

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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.
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.
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.
We will describe a next-generation active atomic frequency reference based on super-radiant pulses of laser light from the ultra-narrow, 1 mHz linewidth, optical clock transition in an ensemble of cold 87Sr atoms. Light is stimulated from the millihertz linewidth transition by confining an ensemble of laser cooled atoms inside of a high finesse optical cavity. Such a light source has been proposed as a next-generation active atomic frequency reference, with the potential to enable high-precision optical frequency references to be used outside laboratory environments. We achieve a remarkable short term fractional frequency stability, 6.7 × 1016 at 1 s of averaging, absolute accuracy, 2 Hz (4 × 1015 fractional frequency), and high insensitivity to changes in the cavity length that limits the performance of todays more stable lasers. We will also discuss current work on cavity-enhanced dispersive measurements to perform high resolution spectroscopy and atom counting.
In this Letter we realize a narrow spectroscopic feature using a technique that we refer to as magnetically induced optical transparency. A cold ensemble of ^{88}Sr atoms interacts with a single mode of a high-finesse optical cavity via the 7.5 kHz linewidth, spin forbidden ^{1}S_{0} to ^{3}P_{1} transition. By applying a magnetic field that shifts two excited state Zeeman levels, we open a transmission window through the cavity where the collective vacuum Rabi splitting due to a single level would create destructive interference for probe transmission. The spectroscopic feature approaches the atomic transition linewidth, which is much narrower than the cavity linewidth, and is highly immune to the reference cavity length fluctuations that limit current state-of-the-art laser frequency stability.