  We propose a detector to read out the state of a single nuclear spin, with
potential application in future scalable NMR quantum computers. It is based on
a ``spin valve'' between bulk nuclear spin systems that is highly sensitive to
the state of the measured spin. We suggest a concrete realization of that
detector in a Silicon lattice. Transport of spin through the proposed spin
valve is analogous to that of charge through an electronic nanostructure, but
exhibits distinctive new features.
