  Modern quantum technologies rely crucially on techniques to mitigate quantum
decoherence; these techniques can be either passive, achieved for example via
materials engineering, or active, typically achieved via pulsed monochromatic
driving fields applied to the qubit. Using a solid-state defect spin coupled to
a microwave-driven spin bath, we experimentally demonstrate a decoherence
mitigation method based on spectral engineering of the environmental noise with
a polychromatic drive waveform, and show that it outperforms monochromatic
techniques. Results are in agreement with quantitative modeling, and open the
path to active decoherence protection using custom-designed waveforms applied
to the environment rather than the qubit.
