  We demonstrate the control of the alpha-proton nuclear spin, I=1/2, coupled
to the stable radical CH(COOH)2, S=1/2, in a gamma-irradiated malonic acid
single crystal using only microwave pulses. We show that, depending on the
state of the electron spin mS=+/-1/2, the nuclear spin can be locked in a
desired state or oscillate between mI=+1/2 and mI=-1/2 on the nanosecond time
scale. This approach provides a fast and efficient way of controlling nuclear
spin qubits and also enables the design of switchable spin-based quantum gates
by addressing only the electron spin.
