Enhanced-coherence all-nitride superconducting qubit epitaxially grown on Si Substrate

  1. Sunmi Kim,
  2. Hirotaka Terai,
  3. Taro Yamashita,
  4. Wei Qiu,
  5. Tomoko Fuse,
  6. Fumiki Yoshihara,
  7. Sahel Ashhab,
  8. Kunihiro Inomata,
  9. and Kouichi Semba
We have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on Si substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation (resulting in fewer two-level fluctuators), feasibility for epitaxial tunnel barriers (further reducing energy relaxation and dephasing), and a larger superconducting gap of ∼5.2 meV for NbN compared to ∼0.3 meV for Al (suppressing the excitation of quasiparticles). Replacing conventional MgO by a Si substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time T1=16.3 μs and a spin-echo dephasing time T2=21.5 μs. These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to previously reported NbN-based qubits with MgO substrates (T1≈T2≈0.5 μs). These results are an important step towards constructing a new platform for superconducting quantum hardware.
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