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arXiv:2412.20734 · Physical Review Applied

Ferromagnetic traps for quasicontinuous operation of optical nanofiber interfaces

Ruijuan Liu, Jun Bo Wu, Yuan Heng Jiang, Yanting Zhao, Saijun Wu

Published Jul 31, 2025• 2 citations
arXivHTMLPDF

Abstract

A soft ferromagnetic plate uniformizes tesla-level fields generated by attached permanent magnets, producing a smooth and electronically tunable surface field on the opposite side. By arranging $n$ precisely fabricated rectangular plates, a nearly ideal magnetic quadrupole field with a substantial gradient can be created at the center. This robust and rapidly tunable field configuration is well suited for two-dimensional magneto-optical trapping (2D MOT) and magnetic guiding of cold atoms. By aligning an optical nanofiber (ONF) along the zero-field line of a planar 2D MOT in a two-plate assembly, we demonstrate quasicontinuous, field-free operation of the quantum optical interface without switching off the magnetic field. Transient transmission spectroscopy with nanosecond laser pulses is performed on the ${}^{87}\mathrm{Rb}\phantom{\rule{0.2em}{0ex}}D2$ line at a measurement repetition rate as high as 250 kHz. The observed line broadening, while not yet fully understood, is partially attributed to residual magnetic fields in the $n=2$ assembly. Through additional measurements and simulations, we verify that these residual fields can be fully eliminated in an $n=4$ assembly, resulting in an ultrastraight 2D trap that supports uniform light-atom interaction over exceptionally long, field-free distances. We extend our discussion to $n=6$, $n=8$ designs with similar uniformity but multiple zero-field lines. With its strong gradient for magnetic trapping, the ferromagnetic devices also enable quantum optical scenarios featuring interactions between coguided atoms and photons at ONF interfaces.

DOI
10.1103/413r-dn5p

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We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of (85)Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 10(15) m(-2). In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.
We present a magnetic trapping scheme for cold Rb 87 atoms based on light-induced fictitious magnetic fields generated by the evanescent field of an optical nanofiber (ONF) integrated with optical tweezers. We calculate and compare the trapping potentials for both Gaussian and Laguerre-Gaussian modes of the tweezer beam, combined with a quasilinearly polarized ONF-guided field. Based on the optical powers in the tweezer and ONF modes, we analyze the trap depths and the positions of the potential minima from the nanofiber surface. We show that, by varying the optical powers in the two fields, the trap position can be tuned over several hundred nanometers while simultaneously influencing the trap depth and trap frequencies. Such control over the atom-surface position is essential for studying distance-dependent effects on atoms trapped near a dielectric surface and optimizing atom-photon interfaces for quantum technology applications.
Abstract Cold Rydberg atoms, known for their long lifetimes and strong dipole-dipole interactions that lead to the Rydberg blockade phenomenon, are among the most promising platforms for quantum simulations, quantum computation and quantum networks. However, a major limitation to the performance of Rydberg atom-based platforms is dephasing, which can be caused by atomic motion within the trap. Here, we propose a trap for 87Rb cold atoms that confines both the electronic ground state and a Rydberg state, engineered to minimize the differential light shifts between the two states. This is achieved by combining a fictitious magnetic field induced by optical nanofiber (ONF) guided light and an external bias magnetic field. We calculate trap potentials for the cases of one- and two-guided modes with quasi-linear and quasi-circular polarizations, and calculate trap depths and trap frequencies for different values of laser power and bias fields. Moreover, we discuss the impact of the quadrupole polarisability of the Rydberg atoms on the trap potential and demonstrate how the size of a Rydberg atom influences the ponderomotive potential generated by the nanofiber-guided light field. This work expands on the idea of light-induced fictitious magnetic field traps and presents a practical approach for creating quantum networks using Rydberg atoms integrated with ONFs to generate 1D atom arrays.
This project aims to develop scientific as well as technical foundations towards realization of a novel platform for light-matter interaction based on tapered optical nanofibers that enable to interface neutral atoms in their vicinity. A setup for magneto-optical trap (MOT) has been built. Tapered nanofibers (TNFs) of sub-wavelength diameter, with transmission of 95 percent have been fabricated and characterized. Next, a nanofiber is transferred into the MOT setup and installed inside the vacuum chamber. The MOT cloud and nanofiber are made to spatially overlap inside ultra-high vacuum (UHV). Coupling of fluorescence of MOT into the guided mode of TNF has been observed. This clearly shows the coupling of atoms with the fiber-guided mode and that our interface works. In addition, 65-70 percent absorption of a free space beam by the atomic cloud has been observed and the relevant MOT parameters are obtained from it. Our efforts are directed towards observing interaction of atoms with the fiber-guided light via evanescent field. Attempts have been made to observe absorption of a probe beam through the nanofiber by the MOT cloud and techniques for switching on-off the MOT beams, the probe beam and the magnetic field are developed for that purpose. Ultimately, the goal will be to trap atoms in the vicinity of a nanofiber enabling to reach a large optical depth accompanied by the ability to reach high intensities for low optical power. I will present my work on the development of this MOT-nanofiber interface - a new promising platform for research in nonlinear quantum optics, quantum information science and technology, quantum communication and hybrid quantum system.
Magneto-optical traps (MOTs) are a workhorse technology for neutral-atom quantum sensing, simulation, and computing. Here, we demonstrate a MOT optimized for inertial quantum sensing and precision metrology applications. Our MOT traps 4×1084\times10^84×108 cesium (Cs) atoms in a robust, modular system that achieves stable operation through two design features: (1) we use divergent cooling laser beams to reduce unwanted reflections and (2) optical elements are mounted in a fiber-coupled, compact, and modular cage rigidly attached to the vacuum chamber. Following polarization-gradient cooling (PGC), the system produces atom samples with temperatures below 10 μμμK, similar to those achieved in conventional MOTs that use collimated laser beams. In addition, we observe trapping of 2×1072\times10^72×107 Cs atoms in a non-conventional MOT geometry, where the cooling laser beams are diagonal to the principal axis of the quadrupole magnetic field.
We propose a trap for cold neutral atoms using a fictitious magnetic field induced by a nanofiber-guided light field in conjunction with an external magnetic bias field. In close analogy to magnetic side-guide wire traps realized with current-carrying wires, a trapping potential can be formed when applying a homogeneous magnetic bias field perpendicular to the fiber axis. We discuss this scheme in detail for laser-cooled cesium atoms and find trap depths and trap frequencies comparable to the two-color nanofiber-based trapping scheme but with one order of magnitude lower power of the trapping laser field. Moreover, the proposed scheme allows one to bring the atoms closer to the nanofiber surface, thereby enabling efficient optical interfacing of the atoms with additional light fields. Specifically, optical depths per atom, σ 0 / A eff , of more than 0.4 are predicted, making this system eligible for nanofiber-based nonlinear and quantum optics experiments.