Our quantum spin transistor works with an arbitrary spin state at the input port (target spin) which is coherently transferred to the output port, if there are no excited spins in the gate, . However, if the gate contains an excited stationary spin (control spin), , it completely blocks the transfer of the target spin state between the input and output ports. In other words, we have coherent dynamics for the initial state of the system when the gate contains no spin excitation, but complete absence of dynamics for the state when the gate contains a single spin excitation. Our scheme thus realizes a quantum logic operation and it can be used to obtain spatially entangled states of target and control spins, as well as to create Schrödinger cat states for a large number of target spins.