Er-Mediated Phononic Waveguide-QED Architecture for Scalable Microwave–Optical Quantum Transduction in Thin-Film Lithium Niobate

Er-Mediated Phononic Waveguide-QED Architecture for Scalable Microwave–Optical Quantum Transduction in Thin-Film Lithium Niobate Francis Procaccia, Independent Researcher, Santa Clara, CA We propose a distributed quantum transduction architecture in which 4 GHz phonons propagate along a phononic crystal bus waveguide in Er-doped thin-film lithium niobate and are locally converted to telecom optical photons at resonant cavity nodes co-localized with Er³⁺ ensembles. Unlike single-resonator transducers, the phonon-first bus enables spatial multiplexing of N independent transduction sites on one chip. A two-step waveguide-QED Hamiltonian resolves the transit-time problem that makes propagating-mode architectures categorically incapable of coherent coupling. A Nb/goldene bilayer at the IDT interface suppresses TLS losses for a 3–5× mechanical Q improvement toward the Qₘ ≥ 10⁵ strong-coupling threshold. ZEFOZ operation with ¹⁶⁷Er enrichment targets T₂ of 5–10 ms, giving a coherence-to-gate-time ratio exceeding 250, 000. The dominant unknown — the strain susceptibility G_ε for the Er³⁺ telecom transition in LiNbO₃, is identified and a complete DFPT + embedded-cluster (CASSCF/NEVPT2) computational framework to extract it ab initio is specified. A six-phase experimental roadmap with explicit go/no-go criteria is provided.