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

Andrew R. Ferdinand, Zheng Luo+13 more|arXiv (Cornell University)|Apr 19, 2024

Optical properties and cooling technologies in crystalline materials4 citations

TL;DR

This paper demonstrates laser cooling of $^{88}$Sr to microkelvin temperatures using an integrated-photonics platform based on metasurface optics. By generating a broadband supercontinuum source with a dispersive wave at 689 nm, the system enables efficient magneto-optical trapping (MOT) and achieves sub-microkelvin temperatures via a narrow-line MOT, showcasing a compact, stable, and scalable approach to ultracold atom generation.

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.

Motivation & Objective

• To develop a compact, integrated photonics platform for generating laser cooling light for $^{88}$Sr.
• To enable microkelvin cooling of $^{88}$Sr using a single, on-chip source with multiple wavelengths.
• To demonstrate efficient magneto-optical trapping (MOT) using a broadband supercontinuum source centered at 689 nm.
• To validate the system's performance through experimental MOT imaging and heterodyne beat measurements.
• To establish a scalable, stable, and miniaturized alternative to bulk optical systems for ultracold atom experiments.

Proposed method

• Employing a metasurface-based optical system to shape and control the spectral and spatial properties of the laser beam.
• Generating a supercontinuum source via nonlinear propagation in a photonic chip, with a dispersive wave precisely tuned to 689 nm for $^{88}$Sr cooling.
• Using heterodyne beat measurement to verify the spectral purity and stability of the 689 nm cooling light relative to the supercontinuum.
• Implementing a multi-wavelength beam configuration to simultaneously address the $^{88}$Sr $^2$S$_{1/2}$ → $^2$P$_{3/2}$ transition at 689 nm.
• Conducting experimental sequences with false-color MOT images to compare broadband (BB) and single-frequency (SF) narrow-line MOT performance.
• Utilizing a multifunctional metasurface optic to generate the required MOT beam geometry from a single input source.

Experimental results

Research questions

• RQ1 Can an integrated-photonics platform based on metasurfaces generate a broadband, spectrally tailored supercontinuum suitable for $^{88}$Sr laser cooling?
• RQ2 What is the achievable temperature in a magneto-optical trap using a chip-integrated supercontinuum source at 689 nm?
• RQ3 How does the performance of a broadband MOT compare to a single-frequency narrow-line MOT in this integrated system?
• RQ4 To what extent does the metasurface-based beam shaping enable efficient and stable trapping of $^{88}$Sr atoms?
• RQ5 Can this platform achieve microkelvin cooling without external bulk optics or complex beam manipulation?

Key findings

• The system successfully generated a supercontinuum with a dispersive wave at 689 nm, matching the $^{88}$Sr cooling transition.
• Heterodyne beat measurement confirmed high spectral stability and coherence of the 689 nm cooling light relative to the broadband source.
• False-color MOT images demonstrated successful trapping using both broadband and single-frequency configurations.
• The integrated metasurface system enabled a compact, single-chip realization of a multi-wavelength MOT beam geometry.
• The experimental sequence confirmed the viability of microkelvin cooling of $^{88}$Sr using only on-chip components.
• The platform achieved stable, scalable, and miniaturized laser cooling, demonstrating a path toward chip-based quantum technologies.