We study the transfer of angular momentum of guided photons to a two-level atom with an electric quadrupole transition near an optical nanofiber. We show that the generation of the axial orbital torque of the driving guided field on the atom is governed by the internal-state selection rules for the quadrupole transition and by the angular momentum conservation law with the photon angular momentum given in the Minkowski formulation. We find that the torque depends on the photon angular momentum, the change in the angular momentum of the atomic internal state, and the quadrupole-transition Rabi frequency. We calculate numerically the torques for the quadrupole transitions between the sublevel $M=2$ of the hyperfine-structure level $5S_{1/2}F=2$ and the sublevels $M^{\prime }=0$, 1, 2, 3, and 4 of the hyperfine-structure level $4D_{5/2}{F}^{\prime }=4$ of a ${}^{87}\mathrm{Rb}$ atom. We show that the absolute value of the torque for the higher-order mode ${\mathrm{HE}}_{21}$ is larger than that of the torque for the fundamental mode ${\mathrm{HE}}_{11}$ except for the case $M^{\prime }-M=2$, where the torque for the mode ${\mathrm{HE}}_{21}$ is vanishing.

• Received 24 May 2022
• Accepted 7 July 2022

DOI:https://doi.org/10.1103/PhysRevA.106.013712

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society