  We study the optimal quantum control of heteronuclear two-qubit systems
described by a Hamiltonian containing both nonlocal internal drift and local
control terms. We derive an explicit formula to compute the minimum time
required to steer the system from an initial state to a specified final state.
As applications the minimal time to implement Controlled-NOT gate, SWAP gate
and Controlled-U gate is calculated in detail. The experimental realizations of
these quantum gates are explicitly presented.
