Coupling between CaWO₄ phonons and Er³⁺ dopants

We investigate the lattice dynamics of CaWO₄, a promising host crystal for erbium-based quantum memories, using inelastic neutron scattering together with density-functional perturbation theory. The measured phonon dispersion along the (100), (001), and (101) reciprocal space direction reveals phonon bands extending up to 130 meV, with a gap between 60 and 80 meV, in good agreement with our calculations. From a symmetry analysis of the phonon eigenmodes, we identify eight Raman-active modes that can couple directly to the Er³⁺ crystal-field operators, including a low-energy B_g mode at 9.1 meV that is expected to play a dominant role in phonon-assisted spin-lattice relaxation. These results provide a microscopic description of the phonon bath in CaWO₄ and establish a basis for engineering phononic environments to mitigate the loss of stored quantum states and optimize Er-doped CaWO₄ for quantum-memory applications.