  The strong driving regime occurs when a quantum two-level system is driven
with an external field whose amplitude is greater or equal to the energy
splitting between the system's states, and is typically identified with the
breaking of the rotating wave approximation (RWA). We report an experimental
study, in which the spin of a single nitrogen-vacancy (NV) center in diamond is
strongly driven with microwave (MW) fields of arbitrary polarization. We
measure the NV center spin dynamics beyond the RWA, and characterize the
limitations of this technique for generating high-fidelity quantum gates. Using
circularly polarized MW fields, the NV spin can be harmonically driven in its
rotating frame regardless of the field amplitude, thus allowing rotations
around arbitrary axes. Our approach can effectively remove the RWA limit in
quantum-sensing schemes, and assist in increasing the number of operations in
QIP protocols.
