Thermal-Gradient Cooling of Atomic Vapor Fluid

The pursuit of high optical depth and long coherence time in atomic ensembles faces a fundamental thermodynamic constraint: heating enhances light-atom coupling via increased density but degrades coherence through thermal broadening, while laser cooling preserves coherence at the cost of density loss. Here, we demonstrate a non-equilibrium strategy that spatially achieves a negative correlation between density and temperature via controlled thermal-gradient transport. By engineering a temperature gradient via laser-cooling in a hot vapor cell, we drive a convective atomic fluid that expels hot atoms at the boundary while confining low-temperature atoms in the central region. This dynamic process sustains a density of 10^22m^-3 and a temperature of tens of kelvins at the center. A theoretical scheme based on the Boltzmann-type transport equation is established, which gives Navier-Stokes equations for non-equilibrium thermal-gradient atomic fluid. The results of numerical simulation indicate that this scheme can enhance the optical depth while reducing the temperature of the system, establishing a route to bypass equilibrium thermodynamics in room-temperature atom-light interactions, boosting high-performance quantum metrology and quantum information applications.