  We consider a CNOT gate operation under the influence of quantum bit-flip
noise and demonstrate that ac fields can change bit-flip noise into phase noise
and thereby improve coherence up to several orders of magnitude while the gate
operation time remains unchanged. Within a high-frequency approximation, both
purity and fidelity of the gate operation are studied analytically. The
numerical treatment with a Bloch-Redfield master equation confirms the
analytical results.
