Thin-film calcium tungstate for scalable quantum communication networks

With its low nuclear-spin density and long coherence times, calcium tungstate (CaWO₄) has emerged as a promising host of erbium ions, enabling spin-photon entanglement for quantum repeater applications. However, current demonstrations primarily use bulk crystals, which limit device scalability.

Thin-film growth using scalable, industry-standard deposition techniques could overcome this limitation. To this end, Seth et al. investigated radio frequency (RF) magnetron sputtering of CaWO₄ on yttria-stabilized zirconia (YSZ) wafers, enabling future growth on Si.

“We chose erbium-doped CaWO₄, one of the most mature and promising erbium-doped oxide platforms, and paired it with RF magnetron sputtering, a scalable, complementary metal-oxide-semiconductor (CMOS)-compatible deposition technique already widely used in the semiconductor industry,” said author Sagar Kumar Seth. “The aim is to create thin films ready for building quantum technologies, grown efficiently at wafer scale with existing semiconductor tools.”

The team initially deposited calcium, tungsten, and oxygen atoms in an amorphous arrangement, before annealing the as-grown films in oxygen-rich conditions to drive CaWO₄ crystallization on YSZ. X-ray diffraction and transmission electron microscopy measurements demonstrated epitaxial growth, an indicator that YSZ serves as a promising lattice template. Optical characterization showed that the experimental techniques used here yield optical homogeneous linewidths of erbium ions at the same order of magnitude as other standard thin-film fabrication techniques, thus confirming the quantum coherency of erbium-doped CaWO₄ films.

The team describes their technique as a first step toward a scalable, CMOS-compatible platform aiming to match the quantum properties of bulk erbium-doped crystals.

“We hope to motivate further work on erbium-doped CaWO₄ toward demonstrating long-distance entanglement, as once this platform is proven to work as quantum repeater nodes, its compatibility with scalable manufacturing could make it an especially practical choice for long-distance quantum communication,” said Seth.

Source: “Microstructural and optical characterization of erbium-doped calcium tungstate films on yttria-stabilized zirconia,” by Sagar Kumar Seth, Bonnie Y. X. Lin, Ignas Masiulionis, Gregory D. Grant, Claire M. McDermott, F. Joseph Heremans, Jiefei Zhang, James M. LeBeau, David D. Awschalom, and Supratik Guha, Journal of Vacuum Science & Technology A (2026). The article can be accessed at https://doi.org/10.1116/6.0005722 .