  We propose and analyze a mechanism for rectification of spin transport
through a small junction between two spin baths or leads. For interacting baths
we show that transport is conditioned on the spacial asymmetry of the quantum
junction mediating the transport, and attribute this behavior to a gapped
spectral structure of the lead-system-lead configuration. For non-interacting
leads a minimal quantum model that allows for spin rectification requires an
interface of only two interacting two-level systems. We obtain approximate
results with a weak-coupling Born-master-equation in excellent agreement with
matrix-product-state calculations that are extrapolated in time by mimicking
absorbing boundary conditions. These results should be observable in controlled
spin systems realized with cold atoms, trapped ions, or in electrons in quantum
dot arrays.
