Hirotaka Terai

Enhancing Intrinsic Quality Factors Approaching 10 Million in Superconducting Planar Resonators via Spiral Geometry

  1. Yusuke Tominaga,
  2. Shotaro Shirai,
  3. Yuji Hishida,
  4. Hirotaka Terai,
  5. and Atsushi Noguchi
This study investigates the use of spiral geometry in superconducting resonators to achieve high intrinsic quality factors, crucial for applications in quantum computation and quantum
sensing. We fabricated Archimedean Spiral Resonators (ASRs) using domain-matched epitaxially grown titanium nitride (TiN) on silicon wafers, achieving intrinsic quality factors of Qi=(9.6±1.5)×106 at the single-photon level and Qi=(9.91±0.39)×107 at high power, significantly outperforming traditional coplanar waveguide (CPW) resonators. We conducted a comprehensive numerical analysis using COMSOL to calculate surface participation ratios (PRs) at critical interfaces: metal-air, metal-substrate, and substrate-air. Our findings reveal that ASRs have lower PRs than CPWs, explaining their superior quality factors and reduced coupling to two-level systems (TLSs).
arxiv pdf

Superconducting flux qubit operating at zero magnetic field

  1. Sunmi Kim,
  2. Leonid V. Abdurakhimov,
  3. Duong Pham,
  4. Wei Qiu,
  5. Hirotaka Terai,
  6. Sahel Ashhab,
  7. Shiro Saito,
  8. Taro Yamashita,
  9. and Kouichi Semba
The operation of a conventional superconducting flux qubit requires the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the
qubit loop, which makes the scaling of quantum circuits based on this type of qubits difficult. It has been proposed that, by inducing a pi phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as pi-junction, it is possible to realize a flux qubit operating at zero magnetic flux. We report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic pi-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero-flux-biased flux qubits can become a promising platform for quantum computing.
arxiv pdf

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.
arxiv pdf