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

Grady Kestler, Khang Ton, Dmytro Filin, Charles Cheung, Philipp Schneeweiß, Thomas Hoinkes, Jürgen Volz, Marianna S. Safronova, Arno Rauschenbeutel, Julio T. Barreiro

Published Oct 12, 2023• 17 citations
Publisher

Abstract

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.

DOI
10.1103/prxquantum.4.040308

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We report the experimental realization of an optical trap that localizes single Cs atoms $\ensuremath{\simeq}215\text{ }\text{ }\mathrm{nm}$ from the surface of a dielectric nanofiber. By operating at magic wavelengths for pairs of counterpropagating red- and blue-detuned trapping beams, differential scalar light shifts are eliminated, and vector shifts are suppressed by $\ensuremath{\approx}250$. We thereby measure an absorption linewidth $\ensuremath{\Gamma}/2\ensuremath{\pi}=5.7\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ }\mathrm{MHz}$ for the Cs $6{S}_{1/2}$, $F=4\ensuremath{\rightarrow}6{P}_{3/2}$, ${F}^{\ensuremath{'}}=5$ transition, where ${\ensuremath{\Gamma}}_{0}/2\ensuremath{\pi}=5.2\text{ }\text{ }\mathrm{MHz}$ in free space. An optical depth $d\ensuremath{\simeq}66$ is observed, corresponding to an optical depth per atom ${d}_{1}\ensuremath{\simeq}0.08$. These advances provide an important capability for the implementation of functional quantum optical networks and precision atomic spectroscopy near dielectric surfaces.

arXiv:1203.5108•Physical Review Letters•Jul 19, 2012•358 citations
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.
Laser trapping and interfacing of laser-cooled atoms in an optical fiber network is an important capability for quantum information science. Following the pioneering work of Balykin et al. and Vetsch et al., we propose a robust method of trapping single Cesium atoms with a two-color state-insensitive evanescent wave around a dielectric nanofiber. Specifically, we show that vector light shifts (i.e., effective inhomogeneous Zeeman broadening of the ground states) induced by the inherent ellipticity of the forward-propagating evanescent wave can be effectively canceled by a backward-propagating evanescent wave. Furthermore, by operating the trapping lasers at the magic wavelengths, we remove the differential scalar light shift between ground and excited states, thereby allowing for resonant driving of the optical D2 transition. This scheme provides a promising approach to trap and probe neutral atoms with long trap and coherence lifetimes with realistic experimental parameters.
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.
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.
The feasibility and potential performance of optical clocks based on all-optical atom chips using neutral strontium atoms have been investigated. The key benefits of all-optical atom chips are their ability to trap alkali-earth atoms, and how easily these atoms can be manipulated and examined. The planar configuration of the chip enables ring photonic resonators to be incorporated into its structure, making it a promising candidate for active optical atomic clocks. Using the 'magic wavelengths' of the trapping optical fields (813 nm and 390 nm for strontium atoms) minimises frequency shifts in the intercombination clock transition caused by variations in optical field intensity. However, this frequency shift can be compensated for using optical AC Stark shifts, which are generated by the trapping optical fields and have frequencies below the 'magic frequencies'. Furthermore, it has been demonstrated that atom chip clocks can operate using solely repulsive, blue-detuned evanescent light fields when the attraction of the trapped atoms towards the photonic waveguide is realised through van der Waals forces alone. According to estimates, this could result in the short-term fractional frequency stability of the strontium atom-chip optical clocks being limited to ~10^{-16} per second, with an accuracy of about 5 x 10^{-15} for fractional intensity stability in the trapping light fields of 0.001.
We report the experimental realization of an optical trap that localizes single Cs atoms $\ensuremath{\simeq}215\text{ }\text{ }\mathrm{nm}$ from the surface of a dielectric nanofiber. By operating at magic wavelengths for pairs of counterpropagating red- and blue-detuned trapping beams, differential scalar light shifts are eliminated, and vector shifts are suppressed by $\ensuremath{\approx}250$. We thereby measure an absorption linewidth $\ensuremath{\Gamma}/2\ensuremath{\pi}=5.7\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ }\mathrm{MHz}$ for the Cs 6S1/26{S}_{1/2}6S1/2​, $F=4\ensuremath{\rightarrow}6{P}_{3/2}$, ${F}^{\ensuremath{'}}=5$ transition, where ${\ensuremath{\Gamma}}_{0}/2\ensuremath{\pi}=5.2\text{ }\text{ }\mathrm{MHz}$ in free space. An optical depth $d\ensuremath{\simeq}66$ is observed, corresponding to an optical depth per atom ${d}_{1}\ensuremath{\simeq}0.08$. These advances provide an important capability for the implementation of functional quantum optical networks and precision atomic spectroscopy near dielectric surfaces.