Transverse Cooling of Cold Caesium Atoms in a Hollow-Core Fiber

Laser-cooled atoms confined with a dipole trap to a transverse area with a diameter of just a few um inside a photonic-bandgap hollow-core fiber offer a unique platform for studies of light-matter interactions and of effective photon-photon interactions mediated by atomic ensembles. However, when atoms initially cooled in free space to a temperature of ∼20-30 μK enter the hollow-core region of the fiber, they are accelerated due to the optical potential of the dipole trapping beam guided by the fiber. As a result, the atom temperature in the radial (transverse) direction inside the fiber can reach up to a few ~mK [1] as the atoms follow orbit-like trajectories in the fiber’s hollow core.

Cooling atoms radially (or in the transverse direction) can help confine them closer to the center [2] – thereby increasing the interaction between photons and atoms. Additionally, it can reduce the atom loss rate resulting from collisions with the wall of the fiber’s hollow core. Without sideways access to the atoms inside the fiber, Ref. [2] proposed and qualitatively demonstrated a technique to cool the fiber-confined rubidium atoms by utilizing the shifted dipole trapping potentials for the two hyperfine ground states (S1/2) and selectively pumping the atoms as they move radially. Here, we present quantitative results from the implementation of this protocol for transverse cooling of caesium atoms in our system.

[1]: Yoon, T. and Bajcsy, M., “Laser-cooled cesium atoms confined with a magic-wavelength dipole trap inside a hollow-core photonic-bandgap fiber,” Physical Review A 99(1), 023415 (2019)

[2]: Peyronel, T., Bajcsy, M., Hofferberth, S., Balic, V., Hafezi, M., Liang, Q., Zibrov, A., Vuletic, V., and Lukin, M. D., “Switching and counting with atomic vapors in photonic-crystal fibers,” IEEE Journal of Selected Topics in Quantum Electronics 18(6), 1747–1753 (2012)