We propose a new two--qubit phase gate for ultra--cold atoms confined in an
experimentally realized tilted double--well optical lattice [Sebby--Strabley et
al., Phys. Rev. A {\bf 73} 033605 (2006)]. Such a lattice is capable of
confining pairs of atoms in a two--dimensional array of double--well potentials
where control can be exercised over the barrier height and the energy
difference of the minima of the two wells (known as the ``tilt''). The four
lowest single--particle motional states consist of two pairs of motional states
in which each pair is localized on one side of the central barrier, allowing
for two atoms confined in such a lattice to be spatially separated qubits. We
present a time--dependent scheme to manipulate the tilt to induce tunneling
oscillations which produce a collisional phase gate. Numerical simulations
demonstrate that this gate can be performed with high fidelity.
