NbTiN Nanowire Resonators for Spin-Photon Coupling on Solid Neon

Electrons floating on a solid neon exhibit long charge coherence times, making them attractive for hybrid quantum systems. When combined with high-quality, high-impedance superconducting resonators and a local magnetic field gradient, this platform enables strong charge–photon and spin–charge coupling-key ingredients for scalable spin qubit architectures. In this work, we demonstrate that NbTiN nanowire resonators maintain high quality factors around 10^5 after depositing solid neon onto the resonators and subsequently loading electrons onto the neon surface, validating their suitability for electrons-on-neon platforms. Building on these experimental results, we theoretically analyze micromagnet designs and coupling strategies that can enable spin-photon interactions in this platform. Our analysis outlines performance targets for next-generation devices, showing that, at the charge sweet spot, spin qubit gate fidelities exceeding 99.99% for single-qubit operations and 99.9% for two-qubit operations are achievable with natural neon.

💡 Research Summary

The authors present a comprehensive study of NbTiN nanowire superconducting resonators integrated with electrons floating on a solid neon substrate, aiming to establish a scalable spin‑photon quantum platform. Three resonators with widths and gaps of 100 nm, 200 nm, and 300 nm were fabricated on high‑resistivity silicon using a 20 nm NbTiN film (Tc ≈ 10.7 K). At 10 mK the devices exhibit internal quality factors Q_int≈2.3×10⁵, external Q_ext≈(2–4)×10⁴ and total Q_tot≈(2–3)×10⁴, corresponding to a loss rate κ/2π≈0.1 MHz.

Neon was deposited at 25 K, forming thin layers of ≈160 nm (Resonator 1) and ≈270 nm (Resonator 2). The presence of neon slightly increased Q_int, likely because the higher dielectric constant reduces participation of two‑level system (TLS) loss. Electrons were subsequently loaded onto the neon surface by pulsed filament emission. As the electron surface density increased, the resonant frequency shifted downwards by up to –0.9 % while Q_int remained unchanged, indicating that the electron layer introduces a dispersive shift without appreciable additional loss. Drude‑type conductivity modeling reproduces the frequency shift for surface densities n_e≈(0.4–0.8)×10⁹ cm⁻², but underestimates the measured loss, suggesting that surface disorder and localization effects dominate over simple scattering.

To enable spin‑photon coupling, the paper proposes adding Co/Ti/NbTiN micro‑electrodes at both ends of Resonator 1. The cobalt layer acts as a ferromagnet; an external magnetic field B_ext applied along the y‑axis magnetizes the Co, generating a local magnetic field gradient ∂B_z/∂y. Finite‑element simulations show that with a Co thickness of 65 nm, Ti spacer of 196 nm, neon thickness 160 nm, and electron height 2.5 nm, the optimal distance between the Co centre and the electron (Δz≈146 nm) yields a gradient of 0.36 mT nm⁻¹. This gradient, combined with a charge‑photon coupling g_c≈2π·100 MHz (achieved thanks to the high kinetic inductance of NbTiN), produces a spin‑photon coupling strength g_s≈2π·7 MHz, comfortably exceeding the resonator loss κ and the spin decoherence rate γ for realistic parameters.

Using these coupling rates, the authors calculate gate fidelities for a charge‑sweet‑spot operating point. A single‑qubit X‑gate can be performed in ~20 ns with >99.99 % fidelity, while a two‑qubit controlled‑phase gate can be completed in ~150 ns with >99.9 % fidelity. The analysis assumes natural neon (⁴⁰Ne) and predicts electron spin coherence times approaching 1 s at temperatures ~10 mK, making spin decoherence negligible compared with photon loss.

Overall, the work demonstrates that NbTiN nanowire resonators retain high Q after solid‑neon deposition and electron loading, that the electron layer provides a sizable, low‑loss dispersive shift, and that carefully engineered micro‑magnets can generate magnetic field gradients sufficient for strong spin‑photon coupling. The combination of long charge and spin coherence, high‑impedance resonators, and tunable magnetic gradients positions the electrons‑on‑neon system as a promising candidate for scalable quantum processors. Future directions include integrating DC bias lines for precise electron confinement, fabricating the proposed Co/Ti/NbTiN electrodes on the same chip, mitigating residual charge noise, and extending the architecture to multi‑qubit networks with error‑correcting capabilities.