Rare-earth oxide host engineering for telecom qubits

Telecom spin qubits in semiconductors are of great interest for developing quantum memories and repeaters in quantum communication networks, as they provide the needed telecom spin-photon interface compatible with long-distance optical fiber networks. Retaining their quantum properties over millisecond timescales or longer has remained a major hurdle. To this end, controlling and engineering the local host environment to enable long-lived coherent spins is key. Here, we discuss our recent advance in this area including exploration of a near-nuclear spin-free host material system with various form factors guided by first-principal calculations [1] towards the development of a felicitous spin-host platforms.

We explore erbium (Er³⁺) ions in epitaxially-grown CeO₂ (cerium dioxide) film on Si substrates. The near-zero (99.96% spin-free) nuclear spin environment provided by CeO₂ is critical for supporting long-lived spins with predicted long spin coherence [1]. We discuss our efforts on microstructural study [2] of the CeO₂ host as well as the optical and spin coherence properties of Er³⁺ [3]. The reduced nuclear spin noise in the host yields an electron spin coherence of 0.66 μs in the isolated ion limit and a millisecond long spin relaxation at T=3.6 K [3] with lower spin-phonon couplings compared with other rare-earth oxide host. Continued studies on Er³⁺ spin coherence at 15-300 mK indicate a spin coherence up to ~180 𝜇s at 15mK [4]. We combine experimental data and simulation to explore dominating dephasing processes. Besides thin films, we also explore the effects of form factors and surfaces on Er³⁺ spin coherence in nanocrystal CeO₂ [5]. Within these nanocrystals, the Er³⁺ spin coherence is limited by the nuclear spin bath of hydrogen atoms from the oleic acid ligands on the surface [5]. Ce³⁺ and O vacancies are prevalent on the surface of the nanocrystal, likely playing a role in the observed spin coherence and lifetime. These studies point to possible approaches to improve Er³⁺ spin coherence, and the potential of Er³⁺:CeO₂ for quantum networks applications.