Erbium quantum memory platform with long optical coherence via back-end-of-line deposition on foundry-fabricated photonics

Realizing scalable quantum interconnects necessitates the integration of solid-state quantum memories with foundry photonics processes. While prior photonic integration efforts have relied upon specialized, laboratory-scale fabrication techniques, this work demonstrates the monolithic integration of a quantum memory platform with low-loss foundry photonic circuits via back-end-of-line deposition. We deposited thin films of titanium dioxide (${\mathrm{Ti}\mathrm{O}}_{2}$) doped with erbium ($\mathrm{Er}$) onto silicon nitride nanophotonic waveguides and studied $\mathrm{Er}$ optical coherence at subkelvin temperatures with photon echo techniques. We suppressed optical dephasing through ex-situ oxygen annealing and optimized measurement conditions, which yielded an optical coherence time of $64\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}$ (a 5-kHz homogeneous linewidth) and slow spectral diffusion of 27 kHz over 4 ms, results that are comparable to state-of-the-art $\mathrm{Er}$ nanophotonic devices. Combined with second-long electron spin lifetimes and demonstrated electrical control of $\mathrm{Er}$ emission, our findings establish $\mathrm{Er}$:${\mathrm{Ti}\mathrm{O}}_{2}$ on foundry photonics as a manufacturable platform for ensemble and single-ion quantum memories.