  Universal quantum computation requires the implementation of arbitrary
control operations on the quantum register. In most cases, this is achieved by
external control fields acting selectively on each qubit to drive single-qubit
operations. In combination with a drift Hamiltonian containing interactions
between the qubits, this allows the implementation of any required gate
operation. Here, we demonstrate an alternative scheme that does not require
local control for all qubits: we implement one- and two-qubit gate operations
on a set of target qubits indirectly, through a combination of gates on
directly controlled actuator qubits with a drift Hamiltonian that couples
actuator and target qubits. Experiments are performed on nuclear spins, using
radio-frequency pulses as gate operations and magnetic-dipole couplings for the
drift Hamiltonian.
