  Negatively charged group-IV defects in diamond show great potential as
quantum network nodes due to their efficient spin-photon interface. However,
reaching sufficiently long coherence times remains a challenge. In this work,
we demonstrate coherent control of germanium vacancy center (GeV) at
millikelvin temperatures and extend its coherence time by several orders of
magnitude to more than 20 ms. We model the magnetic and amplitude noise as an
Ornstein-Uhlenbeck process, reproducing the experimental results well. The
utilized method paves the way to optimized coherence times of group-IV defects
in various experimental conditions and their successful applications in quantum
technologies
