  We propose a general theory of charge, spin, and orbital diffusion based on
Keldysh formalism. Our findings indicate that the diffusivity of orbital
angular momentum in metals is much lower than that of spin or charge due to the
strong orbital intermixing in crystals. Furthermore, our theory introduces the
concept of spin-orbit polarization by which a pure orbital (spin) current
induces a longitudinal spin (orbital) current, a process as efficient as spin
polarization in ferromagnets. Finally, we find that orbital currents undergo
momentum swapping, even in the absence of spin-orbit coupling. This theory
establishes several key parameters for orbital transport of direct importance
to experiments.
