  Ensembles of dopants have widespread applications in quantum technology. The
miniaturization of corresponding devices is however hampered by dipolar
interactions that reduce the coherence at increased dopant density. We
theoretically and experimentally investigate this limitation. We find that
dynamical decoupling can alleviate, but not fully eliminate, the decoherence in
crystals with strong anisotropic spin-spin interactions. Our findings can be
generalized to all quantum systems with anisotropic g-factor used for quantum
sensing, microwave-to-optical conversion, and quantum memory.
