State-Insensitive Trapping of Alkaline-Earth Atoms in a Nanofiber-Based Optical Dipole Trap

TL;DR

This paper demonstrates a state-insensitive optical dipole trap for strontium-88 atoms using a nanofiber, enabling low-temperature trapping and high-resolution spectroscopy, advancing quantum technology applications with alkaline-earth atoms.

Contribution

The work introduces a novel double magic wavelength trapping scheme for strontium-88 in a nanofiber-based trap, achieving state insensitivity near a predicted wavelength and enabling new quantum experiments.

Findings

Achieved trapping of strontium-88 at record low trap depths (~3 μK).

Verified state insensitivity near the magic wavelength of 435.827 nm.

Performed high-resolution spectroscopy confirming trap properties.

Abstract

Neutral atoms trapped in the evanescent optical potentials of nanotapered optical fibers are a promising platform for developing quantum technologies and exploring fundamental science, such as quantum networks and quantum electrodynamics. Building on the successful advancements with trapped alkali atoms, here we demonstrate a state-insensitive optical dipole trap for strontium-88, an alkaline-earth atom, using the evanescent fields of a nanotapered optical fiber. Leveraging the low laser-cooling temperatures of $\sim\!\!1~\mu$K readily achievable with strontium, we demonstrate trapping in record low trap depths corresponding to $\sim\!\!3~\mu$K. Further, employing a double magic wavelength trapping scheme, we realize state-insensitive trapping on the kilohertz-wide $5s^{2}\;^{1}\!S_{0}-5s5p\;^{3}\!P_{1,|m|=1}$ cooling transition, which we verify by performing near-surface high-resolution spectroscopy of the atomic transition. This allows us to experimentally find and verify the state insensitivity of the trap nearby a theoretically predicted magic wavelength of 435.827(25) nm. Given the non-magnetic ground state and low collisional scattering length of strontium-88, this work also lays the foundation for developing versatile and robust matter-wave atomtronic circuits over nanophotonic waveguides.