Critical dynamics in alkali spin vapor

Alkali vapor spin systems hold significant potential for quantum sensing applications. We report on the critical behavior of strongly interacting, warm cesium vapor driven by linearly polarized light, where the collective nature stems from local spin-exchange interactions. We experimentally observe the power-law dependence of the macroscopic spin magnetization on both the intensity of the driving light and the gas density, as well as the divergence of the susceptibility to an imbalance in the drive. We identify this phenomenon as a second-order, mean-field, magnetic phase transition and measure the phase diagram of the system. The spin response out of equilibrium – when crossing the phase transition – critically slows down to a level of a few seconds, more than two orders of magnitude longer than the bare 20-millisecond spin lifetime. We thus establish an accessible, well-controlled platform for investigating spatial and out-of-equilibrium properties of Ising-like systems and potentially for advancing the development of quantum sensors based on critical dynamics.