  We propose a mechanism of long-range coherent coupling between nuclear spins
to be used as qubits in solid-state semiconductor-heterojunction quantum
information processing devices. The coupling is via localized donor electrons
which in turn interact with the two-dimensional electron gas. An effective
two-spin interaction Hamiltonian is derived and the coupling strength is
evaluated. We also discuss mechanisms of qubit decoherence and consider
possibilities for gate control of the interaction between neighboring qubits.
The resulting quantum computing scheme retains all the gate-control and
measurement aspects of earlier approaches, but allows qubit spacing at
distances of order 100nm, attainable with the present-day semiconductor device
technologies.
