Spin swap versus double occupancy in quantum gates

We propose an approach to the realization of quantum gates with electron spins localized in a semiconductor that uses double occupancy to its advantage. With a fast (nonadiabatic) time control of the tunneling, the probability of double occupancy is first increased and then brought back exactly to zero. The quantum phase built in this process can be exploited to realize fast quantum operations. We illustrate the idea focusing on the half-swap operation, which is the key two-qubit operation needed to build a controlled-NOT gate.