Diffusive spin transport

Information to be stored and transported requires physical carriers. The quantum bit of information (qubit) can for instance be realised as the spin 1/2 degree of freedom of a massive particle like an electron or as the spin 1 polarisation of a massless photon. In this lecture, I first use irreducible representations of the rotation group to characterise the spin dynamics in a least redundant manner. Specifically, I describe the decoherence dynamics of an arbitrary spin S coupled to a randomly fluctuating magnetic field in the Liouville space formalism. Secondly, I discuss the diffusive dynamics of the particle's position in space due to the presence of randomly placed impurities. Combining these two dynamics yields a coherent, unified picture of diffusive spin transport, as applicable to mesoscopic electronic devices or photons propagating in cold atomic clouds.