Erbium Quantum Memory Platform with Long Optical Coherence via Back-End of Line Deposition on Foundry-Fabricated Photonics

TL;DR

This work demonstrates the monolithic integration of erbium-doped titanium dioxide quantum memories with foundry-fabricated silicon nitride photonics, achieving long optical coherence times suitable for scalable quantum networks.

Contribution

It introduces a scalable, manufacturable platform for quantum memories by integrating erbium-doped TiO2 films onto silicon nitride photonic circuits using back-end-of-line deposition.

Findings

Optical coherence time of 64 μs achieved

Suppressed optical dephasing with oxygen annealing

Comparable to state-of-the-art erbium devices

Abstract

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 ($\mathrm{TiO_2}$) doped with erbium (Er) onto silicon nitride nanophotonic waveguides and studied Er optical coherence at sub-Kelvin 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 $\mu$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 erbium devices. Combined with second-long electron spin lifetimes and demonstrated electrical control of Er emission, our findings establish Er:$\mathrm{TiO_2}$ on foundry photonics as a manufacturable platform for ensemble and single-ion quantum memories.