  Quantum sensors, qubits sensitive to external fields, have become powerful
detectors for various small acoustic and electromagnetic fields. A major key to
their success have been dynamical decoupling protocols which enhance
sensitivity to weak oscillating (AC) signals. Currently, those methods are
limited to signal frequencies below a few MHz. Here we harness a
quantum-optical effect, the Mollow triplet splitting of a strongly driven
two-level system, to overcome this limitation. We microscopically understand
this effect as a pulsed dynamical decoupling protocol and find that it enables
sensitive detection of fields close to the driven transition. Employing a
nitrogen-vacancy center, we detect GHz microwave fields with a signal strength
(Rabi frequency) below the current detection limit, which is set by the
center's spectral linewidth 1/T2*. Pushing detection sensitivity to the much
lower 1/T2 limit, this scheme could enable various applications, most
prominently coherent coupling to single phonons and microwave photons.
