Spalled Erbium-doped CaWO₄ Films for Integrated Quantum Repeater Applications

Developing telecom-compatible spin-photon interfaces is essential for realizing scalable quantum repeater-based networks. Erbium ions (Er³⁺) offer a unique combination of a telecom C-band optical transition and an effective spin-1/2 ground state. Er ions doped into bulk CaWO₄, with its low nuclear spin concentration, is a leading platform. It has shown spin coherence times up to 23 ms [1] and demonstrated spin-photon entanglement [2].

However, bulk crystals pose limitations for device fabrication and integration. Developing thin Er³⁺-doped films is therefore crucial in enabling CMOS compatibility. One approach is to grow such films on Silicon with Molecular Beam Epitaxy [3-5]. Another approach is to use controlled spalling of thin films out of bulk substrate.

Spalling enables the release of thin films while preserving bulk-like properties. Earlier, we demonstrated controlled spalling of SiC [6] while preserving divacancies' quantum properties. In this work, we have optimized spalling parameters such as bath stress to enable spalling of medium-hard materials like CaWO₄, achieving spalled Er³⁺:CaWO₄ films with thicknesses of 5-10 microns from bulk crystals. We examined the optical properties of Er ions in the spalled films and reported quasi-bulk-like optical lifetimes, with T₁ of 5.37(14) ms on Y₁-Z₁ transition. To further investigate these effects, we investigate the optical linewidths and spin coherence times to identify the mechanisms contributing to broadening and decoherence.