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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Trapping and Interfacing Laser-Cooled Strontium Atoms Using an Optical Nanofiber

Luca Göcke, Hector Letellier, Philipp Schneeweiß, Jürgen Volz, Arno Rauschenbeutel

Published Jun 23, 2025• 0 citations
Publisher

Abstract

We are in the process of building an experimental setup for trapping and optically interfacing laser-cooled strontium atoms using the evanescent field surrounding an optical nanofiber [1]. The nanofiber is produced from a standard step-index optical fiber in a heat-pull process. It features a waist diameter of 230 nm where light is still efficiently guided while a significant part of the light propagates in the form of an evanescent field surrounding the nanofiber. Atoms are trapped in a one-dimensional (1D) optical lattice formed by two fiber-guided light-fields, red- and blue-detuned with respect to the strong transition at a wavelength of 461 nm. The aim is to realize a compensated trap, where the wavelengths of the trapping fields are magic for the 7.4 kHz wide intercombination line at 689 nm. This will allow us to implement advanced schemes for loading atoms into the 1D optical lattice and to investigate the phenomenon of selective radiance [2], where the atoms themselves can act as the waveguide. Here we will present our compact design for trapping strontium atoms from a laser ablated source with a “hot MOT” (operated at 461 nm wavelength), then transfer them to a “cold MOT” (operated at the intercombination line) and to the nanofiber trap.

DOI
10.1109/cleo/europe-eqec65582.2025.11111537

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We are in the process of building an experimental setup for trapping and optically interfacing laser-cooled strontium atoms using the evanescent field surrounding an optical nanofiber [1]. The nanofiber is produced from a standard step-index optical fiber in a heat-pull process. It features a waist diameter of 230 nm where light is still efficiently guided while a significant part of the light propagates in the form of an evanescent field surrounding the nanofiber. Atoms are trapped in a one-dimensional (1D) optical lattice formed by two fiber-guided light-fields, red- and blue-detuned with respect to the strong transition at a wavelength of 461 nm. The aim is to realize a compensated trap, where the wavelengths of the trapping fields are magic for the 7.4 kHz wide intercombination line at 689 nm. This will allow us to implement advanced schemes for loading atoms into the 1D optical lattice and to investigate the phenomenon of selective radiance [2], where the atoms themselves can act as the waveguide. Here we will present our compact design for trapping strontium atoms from a laser ablated source with a “hot MOT” (operated at 461 nm wavelength), then transfer them to a “cold MOT” (operated at the intercombination line) and to the nanofiber trap.
In this thesis I present a developing project to elaborate a scientific platform towards the realization of light-matter interaction based on tapered optical nanofiber (TNF) that enables optical fields to interface with trapped neutral atoms in their vicinity. In such a configuration, the guided light exhibits a large portion of evanescent field into the vacuum thus laser cooled atoms can couple to the fiber mode. By trapping and cooling atoms around the nanofiber I investigated the interaction of the atoms with the field. My work presented in this thesis includes: Calculating the evanescent modes propagating through our fiber; Implementing fiber pulling setup using oxyhydrogen flame and analyzing nanofiber’s thermal dynamics and degradation; Constructing a Rubidium 87 MOT and characterizing its critical parameters; Realizing some atom-light interface through an experiment on coupling spontaneously emitted photons from MOT into TNF; Calculating two-color dipole trap parameters for our nanofiber.
Neutral atoms that are optically trapped using the evanescent fields surrounding optical nanofibers are a promising platform for developing quantum technologies and exploring fundamental science, such as quantum networks and many-body physics of interacting photons.Building on the successful advancements with trapped alkali atoms, here we trap strontium-88 atoms, an alkaline-earth element, in a state-insensitive, nanofiber-based optical dipole trap using the evanescent fields of an optical nanofiber.Employing a two-color, double magic-wavelength trapping scheme, we realize state-insensitive trapping of the atoms for the kilohertz-wide 5s 2 1 S 0 -5s5p 3 P 1,|m|=1 intercombination transition, which we verify by performing high-resolution spectroscopy for an atom-surface distance of about 300 nm.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, a necessary step to confirm precision atomic physics calculations.Alkaline-earth atoms also exhibit nonmagnetic ground states and ultranarrow linewidth transitions making them ideal candidates for atomic clocks and precision metrology applications, especially with state-insensitive traps.Additionally, given the low collisional scattering length specific to strontium-88, this work also lays the foundation for developing versatile and robust matter-wave atomtronic circuits over nanophotonic waveguides.
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 ∼ ⁣ ⁣1 μ\sim\!\!1~μ∼1 μK readily achievable with strontium, we demonstrate trapping in record low trap depths corresponding to ∼ ⁣ ⁣3 μ\sim\!\!3~μ∼3 μK. Further, employing a double magic wavelength trapping scheme, we realize state-insensitive trapping on the kilohertz-wide 5s2  1 ⁣S0−5s5p  3 ⁣P1,∣m∣=15s^{2}\;^{1}\!S_{0}-5s5p\;^{3}\!P_{1,|m|=1}5s21S0​−5s5p3P1,∣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.
Tapered optical fibers with a nanofiber waist are versatile tools for interfacing light and matter. In this context, laser-cooled atoms trapped in the evanescent field surrounding the optical nanofiber are of particular interest: They exhibit both long ground-state coherence times and efficient coupling to fiber-guided fields. Here, we demonstrate electromagnetically induced transparency, slow light, and the storage of fiber-guided optical pulses in an ensemble of cold atoms trapped in a nanofiber-based optical lattice. We measure a slow-down of light pulses to group velocities of 50 m/s. Moreover, we store optical pulses at the single photon level and retrieve them on demand in the fiber after 2 microseconds with an overall efficiency of (3.0 +/- 0.4) %. Our results show that nanofiber-based interfaces for cold atoms have great potential for the realization of building blocks for future optical quantum information networks.
Tapered optical fiber with subwavelength diameter waist, an optical nanofiber, provides a unique and versatile platform for manipulating atoms and photons. In the guided modes of the nanofibers, the optical field can be tightly confined in the transverse direction while enabling strong interaction with the surrounding medium in the evanescent region. Combining laser-cooled atoms with nanofibers has enabled surprising quantum optics experiments, e.g. the efficient channeling of emission from single atoms into the fiber-guided modes, spectroscopy of near-surface atoms, high optical depth with an array of atoms optically trapped around the nanofiber, atomic memories and Bragg reflectors for fiber-guided photons, chiral light-matter interaction etc. In addition, using moderate longitudinal confinement in nanofiber cavities has enabled strong coupling between a single atom and fiber-guided photons. In this article, I review some of the key experimental demonstrations on the “atom＋nanofiber” platform.
We have recently demonstrated a new experimental platform for trapping and optically interfacing laser-cooled cesium atoms. The scheme uses a two-color evanescent field surrounding an optical nanofiber to localize the atoms in a one-dimensional optical lattice 200 nm above the nanofiber surface [1]. In order to use this fiber-coupled ensemble of trapped atoms for applications in the context of quantum communication and quantum information processing, non-classical states of the atomic spins have to be prepared and should live long enough to allow one to apply successive quantum gates. The close proximity of the trapped atoms to the nanofiber surface and the strong polarization gradients of nanofiber-guided light fields are potentially important sources of decoherence. Here, we present our latest experimental results on the coherence properties of atomic spins in our nanofiber-based trap. Using a microwave field to drive the clock transition, we determine inhomogeneous and homogeneous dephasing times by Ramsey and spin echo techniques, respectively. Our results constitute the first measurement of the coherence properties of atoms trapped in the vicinity of a nanofiber and represent a fundamental step towards establishing nanofiber-based traps for cold atoms as a building block in a quantum network.