Polarisation and temperature dependence of Er ³⁺ :CaWO ₄ —towards a solid-state rare-earth ion-doped quantum memory

Abstract In the development of high-performance optical quantum memories, Er 3+ :CaWO 4 has emerged as a compelling rare-earth ion-doped solid-state platform owing to its long optical coherence times and compatibility with the 1550nm telecommunications band. In this work, we systematically investigate how optical polarisation and temperature influence the absorption strength, central wavelength, and linewidth of the Z 1 → Y 1 and Z 1 → Y 2 transitions, with light incident along the crystal a and c axes. We identify the Z 1 → Y 1 transition at 1532.6 nm with the incident laser propagating along the c -axis at cryogenic temperatures (∼3 K) as particularly advantageous for memory operation. Under these conditions, the transition exhibits a stable central wavelength, narrower linewidth, polarisation-independent absorption, and an enhanced absorption cross-section, while residing within the telecommunication C-band. These combined attributes position the Z 1 → Y 1 line as a robust and suitable point for Er 3+ :CaWO 4 based quantum memories.