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arXiv:2609.20741

Effective Conservation and Bistability of Atomic Alignment under Strong Spin~Exchange

Anton K. Vershovskii

Published Sep 17, 2026• 0 citations
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Abstract

We present a phenomenological model of anomalous alignment signals in dense cesium vapor under linearly polarized pumping and fast spin exchange near zero magnetic field. Despite the absence of a conservation law for rank-2 angular momentum, our recent experiments reveal anisotropic narrow resonances, hysteresis, and bistability. We attribute these effects to a stretched state forming a collective mode in which orientation and alignment are bidirectionally coupled. This mode acts as a reservoir, preserving the essential properties of alignment despite rapid spin exchange.

DOI
10.48550/arxiv.2609.20741

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We present a phenomenological model of anomalous alignment signals in dense cesium vapor under linearly polarized pumping and fast spin exchange near zero magnetic field. Despite the absence of a conservation law for rank-2 angular momentum, our recent experiments reveal anisotropic narrow resonances, hysteresis, and bistability. We attribute these effects to a stretched state forming a collective mode in which orientation and alignment are bidirectionally coupled. This mode acts as a reservoir, preserving the essential properties of alignment despite rapid spin exchange.
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 concentration: 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 concentration, additional nonlinear effects arise, such as signal amplification, bistability, hysteresis, and memory. To explain these effects we construct a illustrative theoretical model incorporating spontaneous polarization effects under strong spin exchange conditions. The model qualitatively shows that the ultra-narrow 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.
The results of a study of the dynamics of atomic moments alignment in cesium under optical pumping by linearly polarized resonant light in ultra-weak magnetic field are presented. It is shown that there are alignment components whose relaxation does not depend on spin-exchange broadening. The effect of suppression of spin-exchange relaxation in zero magnetic fields is detected, which is similar in its manifestations to the SERF (Spin-Exchange Relaxation Free) effect observed in orientation signals. This observation is interesting from the standpoint of general theory, since the law of conservation of angular momentum responsible for maintaining orientation in the SERF mode should not guarantee the preservation of alignment under the same conditions. A comparison with theoretically calculated parameters of SERF resonances in orientation is given. A qualitative explanation of the observed effect is presented.
Hydrogen-like alkali atoms with a single valence electron are the most common objects in quantum optics and, at the same time, serve as essential tools of the field. Under conditions of optical pumping, strong spin-exchange and ultra-weak magnetic field (spin-exchange relaxation free mode, SERF), ensembles of such atoms in the gas phase can demonstrate not only the absence of spin-exchange relaxation, but also nonlinear collective effects. We present experimental evidence that the alignment, i.e. the quadrupole momentum, can not only be preserved under SERF conditions, but also coexist and interact with the orientation, i.e. the dipole momentum. We also show that this interaction leads to bistability: a small change in conditions can cause the medium to transition to a different steady state, an effect characterized by hysteresis. The combination of properties of this effect opens up a wide range of applications as optical keys or memory elements with a storage time of hundreds of seconds in tasks of quantum information and cryptography.
We have observed spontaneous atomic spin polarization of a Cs vapor undergoing rapid spin exchange in a cell when illuminated with plane-polarized 894 nm ( $D$1) light. The spin polarization shows marked hysteresis in switching between two stable orientations, fully aligned parallel or antiparallel to an external magnetic field, as the circular polarization of the pump light is varied about zero. We show that our observations agree with theoretical expectations and point out possible applications of spontaneous polarization.
An effect is demonstrated where off-resonant optical pumping is enhanced by spin-exchange collisions, which enables a high degree of spin orientation to be obtained in alkali-metal atoms. The experiment with room-temperature cesium vapor employs radio-frequency spectroscopy under conditions of the nonlinear Zeeman effect that allow the populations of the individual Zeeman sublevels to be determined. The improved efficiency of atomic orientation leads to a significant reduction of population relaxation out of the state with maximum magnetic quantum number mmm (stretched state), which is observed as a narrowing of radio-frequency resonances. Spin-exchange collisions play a crucial role in this effect although the applied atomic density $\ensuremath{\sim}101010{}^{11}$ cm${}^{\ensuremath{-}3}$ corresponds to collision rates not greater than several tens of Hz. A simplified theoretical model has been developed to reproduce the measurement results numerically.
We report results of measurements and theoretical modeling of spontaneous spin polarization and associated hysteresis in an optically pumped Cs vapor. We observe hysteresis over the full range (20--450 torr) of N2{\mathrm{N}}_{2}N2​ buffer gas pressures tested, and on two hyperfine components of the D1D1D1 transition, so long as both electronic spin exchange and optical pumping of a resolved ground state hyperfine level take place faster than spin relaxation. On the $F=\stackrel{\ensuremath{\rightarrow}}{3}{F}_{e}=4$ transition, the spin angular momentum is gained from the linearly polarized pump light, while on the $\stackrel{\ensuremath{\rightarrow}}{3}3$ transition, it comes instead from the buffer gas during collisional quenching. The density-matrix theoretical model we have developed includes the effects of coherence in the excited state. It reproduces all observed features of spontaneous polarization, including hysteresis curves in quantitative agreement with the data across a broad range of experimental parameters.