  The strongly interacting matter created in relativistic heavy-ion collisions
possesses several conserved quantum numbers, such as baryon number,
strangeness, and electric charge. The diffusion process of these charges can be
characterized by a diffusion matrix that describes the mutual influence of the
diffusion of various charges. We derive the Kubo relations for evaluating
diffusion coefficients as elements of a diffusion matrix. We further
demonstrate that in the weak coupling limit, the diffusion matrix elements
obtained through Kubo relations reduce to those obtained from kinetic theory
with an appropriate identification of the relaxation times. We illustrate this
evaluation in a toy model of two interacting scalar fields with two conserved
charges.
