  Collective spins in thermal gases are at the core of a multitude of science
and technology applications. In most of them, the random thermal motion of the
particles is considered detrimental as it is responsible for decoherence and
noise. In conditions of diffusive propagation, thermal atoms can potentially
occupy various stable spatial modes in a glass cell. Extended or localized,
diffusive modes have different magnetic properties, depending on the boundary
conditions of the atomic cell, and can react differently to external
perturbations. Here we demonstrate that few of these modes can be selectively
excited, manipulated, and interrogated in atomic thermal vapours using laser
light. In particular, we individuate the conditions for the generation of modes
that are exceptionally resilient to undesirable effects introduced by optical
pumping, such as light shifts and power-broadening, which are often the
dominant sources of systematic errors in atomic magnetometers and
co-magnetometers. Moreover, we show that the presence of spatial inhomogeneity
in the pump, on top of the random diffusive atomic motion, introduces a
coupling that leads to a coherent exchange of excitation between the two
longest-lived modes. Our results indicate that systematic engineering of the
multi-mode nature of diffusive gases has great potential for improving the
performance of quantum technology applications based on alkali-metal thermal
gases, and promote these simple experimental systems as versatile tools for
quantum information applications.
