Ensembles of dopants have widespread applications in quantum information processing. However, miniaturization of corresponding devices is hampered by spin-spin interactions that reduce the coherence with increasing dopant density. Here, we investigate this limitation in erbium-doped crystals, in which these interactions are anisotropic and particularly strong. After implementing efficient spin initialization, microwave control, and readout of the spin ensemble, we demonstrate that the coherence limitation can be alleviated by dynamical decoupling. Our findings can be generalized to other dopants and hosts used for quantum sensors, microwave-to-optical conversion, and quantum memories.

Ensembles of erbium dopants can realize quantum memories and frequency converters that operate in the minimal-loss wavelength band of fiber optical communication. Their operation requires the initialization, coherent control and readout of the electronic spin state. In this work, we use a split-ring microwave resonator to demonstrate such control in both the ground and optically excited state. The presented techniques can also be applied to other combinations of dopant and host, and may facilitate the development of new quantum memory protocols and sensing schemes.