Daichi Sugiyama

Control the qubit-qubit coupling with double superconducting resonators

  1. Hui Wang,
  2. Rui Wang,
  3. Daichi Sugiyama,
  4. and J. S. Tsai
We experimentally studied the switching off processes in the double-resonator coupler superconducting quantum this http URL both frequency and time-domain, we observed the variation
of qubit-qubit effective coupling by tuning qubits’frequencies. According to the measurement results, by just shifting qubits‘ frequencies smaller than 50 MHz, the effective qubit-qubit coupling strength can be tuned from switching off point to two qubit gate point (effective coupling larger than 5 MHz) in double-resonator superconducting quantum circuit. The double-resonator coupler superconducting quantum circuit has the advantage of simple fabrications, introducing less flux noises, reducing occupancy of dilution refrigerator cables, which might supply a promising platform for future large-scale superconducting quantum processors.
arxiv pdf

Realisation of Protected Cat Qutrit via Engineered Quantum Tunnelling

  1. Sangil Kwon,
  2. Daisuke Hoshi,
  3. Toshiaki Nagase,
  4. Daichi Sugiyama,
  5. Hiroto Mukai,
  6. Kengo Takemura,
  7. Rintaro Kojima,
  8. Yu Zhou,
  9. Shohei Watabe,
  10. Fumiki Yoshihara,
  11. and Jaw-Shen Tsai
Engineering quantum tunnelling in phase space has emerged as a viable method for creating a protected qubit with biased-noise properties. A promising approach is to combine a Kerr nonlinearity
with multi-photon transitions, resulting in a system known as a Kerr parametric oscillator (KPO). In this work, we implement a three-photon KPO and explore its potential as a protected qutrit. We confirm quantum coherence by demonstrating three-photon Rabi oscillations and performing direct Wigner function measurements that reveal three-component cat-like states. We observe breathing-like dynamics in phase space, arising from exotic temporal interference between the qutrit and excited states. The frequency of this interference corresponds to the energy gap between the qutrit and excited manifolds, thereby providing an experimental hallmark of qutrit space protection. We also identify a higher-order pump term as the main mechanism suppressing photon occupation; mitigating this term is necessary to maximize protection. Our findings elucidate the basic quantum properties of the three-photon KPO and establish the first step toward its use as an alternative qutrit platform.
arxiv pdf