Superconducting qubit based on epitaxial TiN/MgO/TiN Josephson junctions

The choice of material platform is crucial for improving the coherence of superconducting qubits. Nitride-based superconductors offer several advantages, such as large superconducting gaps, epitaxial growth, and excellent chemical and mechanical stability. Superconducting co-planar waveguide resonators using TiN thin films epitaxially grown on Si(100) substrates have internal quality factors exceeding 10⁶ at microwave power in the single-photon region [1], and the TiN film has been adopted as an electrode material in a quantum processor being developed at RIKEN in Japan. We aim to develop Josephson junctions using TiN as the electrode material and are focusing on MgO as the tunnel barrier layer, which has excellent lattice matching with TiN. Such epitaxial Josephson junctions are promising for realizing merged-element transmon (MET), which can dramatically reduce the footprint of the qubits. Here, we demonstrate epitaxial TiN/MgO/TiN junctions on Si with thick MgO barriers (> 2.5 nm), achieving J_c ≈ 1 A/cm² at 10 mK, while maintaining high junction quality. METs fabricated using these junctions exhibit coherence times exceeding 20 μs, showing the great potential of TiN/MgO/TiN for quantum circuits.

[1] Y. Tominaga et al., EPJ Quantum Technology 12, 60 (2025).