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.
Петренко et al. (Fri,) studied this question.