  We study the low energy states of finite spin chains with isotropic
(Heisenberg) and anisotropic (XY and Ising-like) exchange interaction with
uniform and non-uniform coupling constants. We show that for an odd number of
sites a spin cluster qubit can be defined in terms of the ground state doublet.
This qubit is remarkably insensitive to the placement and coupling anisotropy
of spins within the cluster. One- and two-qubit quantum gates can be generated
by magnetic fields and inter-cluster exchange, and leakage during quantum gate
operation is small. Spin cluster qubits inherit the long decoherence times and
short gate operation times of single spins. Control of single spins is hence
not necessary for the realization of universal quantum gates.
