  We propose and analyze a scheme for conditional state transfer in a
Heisenberg $XXZ$ spin chain which realizes a quantum spin transistor. In our
scheme, the absence or presence of a control spin excitation in the central
gate part of the spin chain results in either perfect transfer of an arbitrary
state of a target spin between the weakly coupled input and output ports, or
its complete blockade at the input port. We also present a possible realization
of the corresponding spin chain with a one-dimensional ensemble of cold atoms
with strong contact interactions.
