  Nuclear spins support long lived quantum coherence due to weak coupling to
the environment, but are difficult to rapidly control using nuclear magnetic
resonance (NMR) as a result of the small nuclear magnetic moment. We
demonstrate a fast ~ 500 ns nuclear spin phase gate on a 14N nuclear spin qubit
intrinsic to a nitrogen-vacancy (NV) center in diamond. The phase gate is
enabled by the hyperfine interaction and off-resonance driving of electron spin
transitions. Repeated applications of the phase gate bang-bang decouple the
nuclear spin from the environment, locking the spin state for up to ~ 140
microseconds.
