Collective amplification and anisotropic narrowing of alignment signals in cesium vapor under strong spin exchange near zero magnetic field
Phys. Rev. A 114, 033717 – Published 14 September, 2026
DOI: https://doi.org/10.1103/z834-3rj6
Abstract
We present the results of an experimental study of the anomalous anisotropy of alignment signals in cesium vapors under strong spin-exchange conditions near zero magnetic field with linearly polarized optical pumping. We show that the anisotropy of the Hanle resonances in the plane perpendicular to the pump beam increases with Cs vapor number density: In one direction the widths remain broadened by spin exchange, whereas in the other they approach the spin-exchange relaxation-free limit. With a further increase in number density, additional nonlinear effects arise such as signal amplification, bistability, hysteresis, and memory. To explain these effects we construct an illustrative theoretical model incorporating spontaneous polarization effects under strong-spin-exchange conditions. The model qualitatively shows that the ultranarrow alignment resonances may originate from quadrupole anisotropy arising from the projection of spontaneous transverse orientation onto the detection axis. The unique properties of these resonances, such as their extremely small width and magnetic-field-controlled bistability with a long-term memory effect, make them promising for use in quantum sensing and information.
