State-dependent potentials in a nanofiber-based two-color trap for cold atoms

TL;DR

This paper investigates the ac Stark shift and state-dependent potentials in a nanofiber-based two-color optical trap for cesium atoms, highlighting the effects of fictitious magnetic fields and proposing microwave cooling methods.

Contribution

It provides a detailed analysis of the hyperfine and magnetic interactions affecting trapping potentials, and suggests ways to avoid spin flips and enable microwave cooling.

Findings

State-dependent trapping potentials can cause spin flips, which can be mitigated with an external magnetic field.

The trapping configuration can be tuned to produce state-dependent displacement of potential minima.

Nonzero Franck-Condon factors enable the possibility of microwave cooling in the trap.

Abstract

We analyze the ac Stark shift of a cesium atom interacting with far-off-resonance guided light fields in the nanofiber-based two-color optical dipole trap realized by Vetsch \textit{et al.} [Phys. Rev. Lett. \textbf{104}, 203603, (2010)]. Particular emphasis is given to the fictitious magnetic field produced by the vector polarizability of the atom in conjunction with the ellipticity of the polarization of the trapping fields. Taking into account the ac Stark shift, the atomic hyperfine interaction, and a magnetic interaction, we solve the stationary Schr\"odinger equation at a fixed point in space and find Zeeman-state-dependent trapping potentials. In analogy to the dynamics in magnetic traps, a local degeneracy of these state-dependent trapping potentials can cause spin flips and should thus be avoided. We show that this is possible using an external magnetic field. Depending on the direction of this external magnetic field, the resulting trapping configuration can still exhibit state-dependent displacement of the potential minima. In this case, we find nonzero Franck-Condon factors between motional states of the potentials for different hyperfine-structure levels and propose the possibility of microwave cooling in a nanofiber-based two-color trap.