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arXiv:2404.13210

Laser cooling $^{88}$Sr to microkelvin temperature with an integrated-photonics system

Andrew R. Ferdinand, Luo, Zheng, Sindhu Jammi, Zachary L. Newman, Grisha Spektor, Okan Koksal, Parth Patel, Daniel Sheredy, William D. M. Lunden, Akash Rakholia, Travis C. Briles, Wenqi Zhu, Martin M. Boyd, Amit Kumar Agrawal, Scott B. Papp

Published Apr 19, 2024• 2 citations
arXivHTMLPDF

Abstract

We report on experiments generating a magneto-optical trap (MOT) of 88-strontium ($^{88}$Sr) atoms at microkelvin temperature, using integrated-photonics devices. With metasurface optics integrated on a fused-silica substrate, we generate six-beam, circularly polarized, counter-propagating MOTs on the blue broad-line, 461 nm, and red narrow-line, 689 nm, Sr cooling transitions without bulk optics. By use of a diverging beam configuration, we create up to 10 mm diameter MOT beams at the trapping location. To frequency stabilize and linewidth narrow the cooling lasers, we use fiber-packaged, integrated nonlinear waveguides to spectrally broaden a frequency comb. The ultra-coherent supercontinuum of the waveguides covers 650 nm to 2500 nm, enabling phase locks of the cooling lasers to hertz level linewidth. Our work highlights the possibility to simplify the preparation of an ultracold 88Sr gas for an optical-lattice clock with photonic devices. By implementing a timing sequence for control of the MOT lasers and the quadrupole magnetic-field gradient, we collect atoms directly from a thermal beam into the blue MOT and continuously cool into a red MOT with dynamic detuning and intensity control. There, the red MOT temperature is as low as $2~μ$K and the overall transfer efficiency up to 16%. We characterize this sequence, including an intermediate red MOT with modulated detuning. Our experiments demonstrate an integrated photonics system capable of cooling alkaline-earth gases to microkelvin temperature with sufficient transfer efficiencies for adoption in scalable optical clocks and quantum sensors.

DOI
10.48550/arxiv.2404.13210

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We report on experiments generating a magneto-optical trap (MOT) of 88-strontium (88^{88}88Sr) atoms at microkelvin temperature, using integrated-photonics devices. With metasurface optics integrated on a fused-silica substrate, we generate six-beam, circularly polarized, counter-propagating MOTs on the blue broad-line, 461 nm, and red narrow-line, 689 nm, Sr cooling transitions without bulk optics. By use of a diverging beam configuration, we create up to 10 mm diameter MOT beams at the trapping location. To frequency stabilize and linewidth narrow the cooling lasers, we use fiber-packaged, integrated nonlinear waveguides to spectrally broaden a frequency comb. The ultra-coherent supercontinuum of the waveguides covers 650 nm to 2500 nm, enabling phase locks of the cooling lasers to hertz level linewidth. Our work highlights the possibility to simplify the preparation of an ultracold 88Sr gas for an optical-lattice clock with photonic devices. By implementing a timing sequence for control of the MOT lasers and the quadrupole magnetic-field gradient, we collect atoms directly from a thermal beam into the blue MOT and continuously cool into a red MOT with dynamic detuning and intensity control. There, the red MOT temperature is as low as 2 μ2~μ2 μK and the overall transfer efficiency up to 16%. We characterize this sequence, including an intermediate red MOT with modulated detuning. Our experiments demonstrate an integrated photonics system capable of cooling alkaline-earth gases to microkelvin temperature with sufficient transfer efficiencies for adoption in scalable optical clocks and quantum sensors.
Optical atomic clocks provide exceptionally accurate and precise signals for timekeeping and precision measurements, but they require high-power, free-space laser configurations that limit scalability. We introduce and explore a scalable infrastructure for strontium (Sr) optical-lattice clocks that incorporates co-design of atomic-beam slowing and a magneto-optical trap (MOT) from an effusion source, generation of complex, three-dimensional free-space laser configurations with a photonic integrated circuit (PIC) and metasurface (MS) optics, and laser stabilization to a frequency-comb supercontinuum generated with integrated nonlinear photonics. With these elements, we realize MOTs of all stable strontium isotopes (84^{84}84Sr, 86^{86}86Sr, 87^{87}87Sr, 88^{88}88Sr) with populations commensurate with natural abundances, demonstrating precise beam control and robustness. Access to laser-cooled alkaline-earth atoms with scalable integrated photonics enables system engineering for optical clocks, quantum sensing, and quantum information, and our experiments demonstrate extensible technologies that advance toward a Sr optical clock largely free of bulk optics.
We report the experimental realization of strontium magneto-optical trap (MOT) operating on the intercombination transition 1 S 0 − 3 P 1 at 689 nm, namely red MOT. A 689 nm laser used for cooling and trapping is injection locked to a master laser, whose linewidth is narrowed to 150 Hz by locking to a high finesse optical reference cavity. 88 Sr atoms pre-cooled and trapped by the broad 1 S 0 − 1 P 1 transition at 461 nm are transferred to the red MOT with the help of a time sequence controller. The transfer ratio is about 20% and the red MOT's temperature is estimated to be less than 20 μK by the time-of-flight (TOF) image analysis.
Laser cooling has become a powerful tool for producing quantum gases and has enabled the study of a wide range of phenomena only accessible at ultracold temperatures. This thesis will describe the implementation and characterization of a magneto-optical trap (MOT) for laser cooling strontium to microkelvin temperatures operating on the narrow 689nm transition (the “red” MOT). The red MOT is the second cooling stage required to create quantum degenerate gases of strontium, bridging the temperatures and densities produced by the first cooling stage (the 461 nm “blue” MOT) and the shallow conservative traps for evaporation to quantum degeneracy.
Optical lattice clocks generate pristine timing signals, which advance understanding of physics and open new application opportunities. However, the existing experimental apparatus for such clocks consists of numerous complicated subsystems, and are laborious to assemble. This Letter demonstrates the use of integrated photonics technologies to generate the free-space optical configuration needed to laser cool and trap atomic samples for a lattice clock and to create a frequency-comb supercontinuum to stabilize the lasers for the lattice clock. The authors assemble integrated photonics devices without active alignment, highlighting the potential for scalability in lattice-clock systems.
Ultracold polar molecules have emerged as promising platforms for quantum simulation, quantum information science, precision measurements of fundamental symmetry violations, and quantum chemistry. Over the past decade, rapid progress in direct laser cooling of molecules has enabled new capabilities such as magneto-optical traps (MOTs), sub-Doppler cooling, and optical dipole traps (ODTs), expanding the quantum toolbox with ultracold molecules of complex internal structures and strong dipolar interactions. This thesis presents the design, construction, and operation of an experiment on ultracold strontium monofluoride (88Sr19F) molecules, demonstrating state-of-the-art techniques for molecular laser cooling and trapping. The apparatus integrates a broad range of technical sub-systems, including cryogenics, ultra-high vacuum, optics, electronics, and automated laboratory control software. SrF molecules produced in a slowed cryogenic buffer gas beam are captured into a dual-frequency MOT. Subsequent Λ-enhanced gray molasses cooling reduces the molecule temperature to as low as 5 μK. A novel blue-detuned MOT is also implemented to further compress the molecular cloud, increasing the density by a factor of ∼ 400. At this high density, SrF molecules are efficiently loaded into an ODT, a tight, conservative trap with long lifetimes. Population transfer between different rotational levels is achieved via optical pumping and coherent microwave control, yielding ∼ 0.8 ms coherence time in Rabi oscillation and ∼ 90 % transfer efficiency. This thesis also describes the development of an ultracold rubidium (87Rb) atom experiment, in which a loading rate of ∼ 10^10 atoms/s into a radio-frequency (RF) MOT is demonstrated. A modular two-dimensional MOT serves as the Rb source and can be seamlessly integrated into the SrF apparatus, opening the possibility for future studies of co-trapping and sympathetic cooling of SrF molecules with atoms. The results presented in this thesis establish an experimental foundation for exploring new frontiers in ultracold molecular physics.
Obtaining ultracold alkaline earth(-like) atoms in space encounters the problem of performing narrow-line laser cooling in microgravity environments ( μ -gEs). This paper reports an analysis of the magneto-optical trap (MOT) based on the narrow-line transition in 88 Sr, while paying special attention to the role of the gravity. This analysis suggests the MOTs based on narrow-line transitions cannot be cold and dense enough in a μ -gE. We thus propose a strategy: that one can use a dual-frequency MOT to realize a low-temperature, high density, and high transfer efficiency, narrow-line red MOT in μ -gEs.