  Quantum registers that combine the attractive properties of different types
of qubits are useful for many different applications. They also pose a number
of challenges, often associated with the large differences in coupling
strengths between the different types of qubits. One example is the
non-resonant effect that alternating electromagnetic fields have on the
transitions of qubits that are not targeted by the specific gate operation. The
example being studied here is known as Bloch-Siegert shift. Unless these shifts
are accounted for and, if possible, compensated, they can completely destroy
the information contained in the quantum register. Here we study this effect
quantitatively in the important example of the nitrogen vacancy (NV) center in
diamond and demonstrate how it can be eliminated.
