  As spin-based quantum technology evolves, the ability to manipulate spin with
non-magnetic fields is critical - both for the development of hybrid quantum
systems and for compatibility with conventional technology. Particularly useful
examples are electric fields, optical fields, and mechanical lattice
vibrations. The last of these represents direct spin-phonon coupling, which has
garnered fundamental interest as a potential mediator of spin-spin
interactions, but could also find applications in high-stability inertial
sensing. In this Letter, we demonstrate direct coupling between phonons and
nitrogen-vacancy (NV) center spins in diamond by inducing spin transitions with
mechanically-driven harmonic strain. The ability to control NV spins
mechanically can enhance NV-based quantum metrology, grant access to all
transitions within the spin-1 quantum state of the NV center, and provide a
platform to study spin-phonon interactions at the level of a few interacting
spins.
