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Optical and Quantum Sensing and Precision Metrology II

An active optical frequency reference using a pulsed superradiant laser

Juan Muniz Silva, Julia Cline, James K. Thompson, Matthew A. Norcia

Published Mar 1, 2019• 4 citations
Publisher

Abstract

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.

DOI
10.1117/12.2515582

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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.
Ultra stable frequency references such as the ones used in optical atomic clocks and for quantum metrology may be obtained by stabilizing a laser to an optical cavity that is stable over time. State-of-the-art frequency references are constructed in this way, but their stabilities are currently limited by thermally induced length fluctuations in the reference cavity. Several alternative approaches using the potential for frequency discriminating of highly forbidden narrow atomic transitions have been proposed in, e.g., [1] and [2]. In this proceeding we will present some of the ongoing experimental efforts derived from these proposals, to use cavity-enhanced interaction with atomic 88 Sr samples as a frequency reference for laser stabilization. Such systems can be realized using both passive and active approaches where either the atomic phase response is used as an error signal, or the narrow atomic transition itself is used as a source for a spectrally pure laser. Both approaches shows the promise of being able to compete with the current state of the art in stable lasers and have similar limitations on their ultimately achievable linewidths [1, 2].
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.
Abstract : We demonstrated that an optical laser can operate with less than 1 photons on average inside the cavity while also obtaining significant spectral narrowing of the emitted light relative to the single atom transition linewidth. We further explored the power output properties and threshold conditions of this laser. We made direct measurements that showed that the lasing frequency can be made greater than 10000 times less sensitive to the resonance frequency of the optical cavity than is the case for normal lasers or for passive optical reference cavities. Such a large reduction in sensitivity would essentially remove both thermal and technical mirror vibrations as a limitation on laser linewidth. Other studies included how phase information can be stored and manipulated inside of our laser, such that it may be possible to have both an active laser and a passive Ramsey-like sensor in the same setup. We considered the fundamental limits on the atomic quantum information that can be extracted from the laser light and found that it corresponded closely to the fundamental atomic standard quantum limit on phase estimation.
Today's narrowest linewidth lasers are limited by mirror motion in the reference optical resonator used to stabilize the laser's frequency.Recent proposals suggest that superradiant lasers based on narrow dipoleforbidden transitions in cold alkaline earth atoms could offer a way around this limitation.Such lasers operating on transitions with linewidth of order mHz are predicted to achieve output spectra orders of magnitude narrower than any currently existing laser.As a step towards this goal, we demonstrate and study a laser based on the 7.5-kHz linewidth dipole-forbidden 3 P 1 to 1 S 0 transition in laser-cooled and tightly confined 88 Sr.We can operate this laser in the bad-cavity or superradiant regime, where coherence is primarily stored in the atoms, or continuously tune to the more conventional good-cavity regime, where coherence is primarily stored in the light field.We show that the cold-atom gain medium can be repumped to achieve quasi-steady-state lasing.We also demonstrate up to an order of magnitude suppression in the sensitivity of laser frequency to changes in cavity length, verifying a key feature of the proposed narrow linewidth lasers.