Taro Yamashita

Ultralow-power coherent qubit control using AQFP logic at millikelvin temperatures

  1. Hiroto Mukai,
  2. Akiyoshi Tomonaga,
  3. Rui Wang,
  4. Yu Zhou,
  5. Taro Yamashita,
  6. Nobuyuki Yoshikawa,
  7. Jaw-Shen Tsai,
  8. and Naoki Takeuchi
Qubit controllers are essential for scaling superconducting quantum processors, but implementing them at the 10 mK stage of a dilution refrigerator remains challenging due to stringent
cooling constraints. Here we report an ultralow-power qubit controller using adiabatic quantum-flux-parametron (AQFP) logic, termed an AQFP-multiplexed qubit controller with virtual Z gates (AQFP QC-VZ). The AQFP QC-VZ generates multi-tone microwave pulses for qubit control with an ultralow power dissipation of 111 pW per qubit. By combining microwave and time-division multiplexing, the AQFP QC-VZ enables parallel application of X and virtual Z gates to multiple qubits using only a few control lines from room temperature. We demonstrate coherent single-qubit gates at the 10 mK stage using an AQFP mixer, a core component of the AQFP QC-VZ, without observable degradation in coherence.
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