  We propose a scheme for implementing quantum gates and entanglement between
spin qubits in the outer dots of a triple-dot system with an empty central dot.
The voltage applied to the central dot can be tuned to realize the gate. Our
scheme exemplifies the possibility of quantum gates outside the regime where
each dot has an electron, so that spin-spin exchange interaction is not the
only relevant mechanism. Analytic treatment is possible by mapping the problem
to a t-J model. The fidelity of the entangling quantum gate between the spins
is analyzed in the presence of decoherence stemming from a bath of nuclear
spins, as well as from charge fluctuations. Our scheme provides an avenue for
extending the scope of two qubit gate experiments to triple-dots, while
requiring minimal control, namely that of the potential of a single dot, and
may enhance the qubit separation to ease differential addressability.
