Light-induced, fictitious magnetic trapping of cold alkali atoms using an optical tweezers-nanofiber hybrid platform

We present a magnetic trapping scheme for cold 87Rb atoms based on light-induced fictitious magnetic fields generated by the evanescent field of an optical nanofiber (ONF) integrated with an optical tweezers. We calculate and compare the trapping potentials for both Gaussian and Laguerre-Gaussian modes of the tweezers beam, combined with a quasi-linearly polarized ONF-guided field. Based on the optical powers in the tweezers 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 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.

💡 Research Summary

The authors present a novel hybrid platform—named OPTON (Optical Tweezers and Optical Nanofiber)—that traps laser‑cooled 87Rb atoms using light‑induced fictitious magnetic fields. The scheme exploits the vector light shift that remains when the scalar AC‑Stark shift is nulled at a “tune‑out” wavelength (≈ 790.2 nm for the 5S₁/₂ ground state). By combining the evanescent field of a quasi‑linearly polarized fundamental HE₁₁ mode of a silica nanofiber (radius 175 nm) with a tightly focused, circularly polarized optical tweezer beam (either a Gaussian or a Laguerre‑Gaussian LG₀₁ mode), a spatially varying fictitious magnetic field ( \mathbf{B}_{\text{fic}} ) is generated.

The vector light shift is proportional to the imaginary part of ( \mathbf{E}^\ast \times \mathbf{E} ); therefore, any non‑zero ellipticity in the local polarization creates a magnetic‑like interaction. The nanofiber mode provides a slowly varying azimuthal component, while the tightly focused tweezer contributes a strong y‑directed component (for σ⁺ polarization) and additional transverse components due to non‑paraxial focusing. The LG₀₁ mode, with its doughnut‑shaped intensity profile, pushes the peak fictitious field closer to the fiber surface compared with the Gaussian beam.

The total effective magnetic field experienced by the atom is the vector sum of three contributions: the fictitious field from the nanofiber, the fictitious field from the tweezer, and a static bias field (3 G along the z‑axis) that defines the quantization axis. The magnetic potential is ( U_{\text{mag}} = \mu_B g_F m_F |\mathbf{B}_{\text{eff}}| ). Because the fictitious field magnitude can be tuned by adjusting the optical powers in the nanofiber and tweezer beams, the position of the magnetic minimum—and thus the trap location—can be shifted continuously over a range of several hundred nanometers (≈ 200–600 nm) from the fiber surface.

In practice, the nanofiber wavelength is detuned slightly (to 787.9 nm) to introduce a small scalar polarizability, which adds an attractive scalar potential ( U_{\text{sc}} = -\frac{1}{4}\alpha_{\text{sc}}|E_{\text{ONF}}|^2 ). This modest scalar contribution deepens the trap by roughly (k_B \times 10) µK. At distances below ~100 nm, the van‑der‑Waals (vdW) interaction ( U_{\text{vdW}} = -C_3/(r-r_f)^3 ) becomes significant (with (C_3 = 3.36 \times 10^{-23}) mK·m³), pulling the atom toward the fiber surface. Consequently, the optimal trap depth and atom‑surface distance result from a balance among the magnetic, scalar, and vdW potentials.

Numerical simulations for a representative configuration (nanofiber power ≈ 5 mW, tweezer power = 0.3 mW) show a trap depth of (k_B \times 30) µK. The radial (azimuthal) trap frequency is about (2\pi \times 150) kHz, while the axial (along the fiber) frequency is (2\pi \times 80) kHz. Switching to the LG₀₁ tweezer mode at the same power increases the depth by ~20 % and moves the trap minimum ~100 nm closer to the fiber surface, owing to the tighter intensity gradient.

Because the fictitious magnetic field depends linearly on the optical intensities, rapid (µs‑scale) power modulation using acousto‑optic modulators can dynamically reposition the trap or change its depth. This capability enables real‑time studies of distance‑dependent surface effects, such as Casimir‑Polder forces, surface‑induced decoherence, or collective interactions mediated by the fiber. Moreover, the LG₀₁ configuration naturally creates a circular ring of atoms around the fiber, opening pathways to explore fiber‑mediated many‑body physics, chiral quantum optics, and the implementation of atom‑photon interfaces for quantum networks.

In summary, the paper demonstrates that light‑induced fictitious magnetic fields provide a versatile, all‑optical magnetic trapping mechanism that circumvents the need for real magnetic coils or complex two‑color evanescent‑field traps. By leveraging the complementary strengths of a nanofiber’s evanescent field and a tightly focused tweezer beam, the authors achieve fine spatial control, tunable trap depths, and high trap frequencies, establishing a powerful new tool for cold‑atom experiments near dielectric surfaces and for advancing quantum‑technology platforms.