Kohei Matsuura

Photon-noise-tolerant dispersive readout of a superconducting qubit using a nonlinear Purcell filter

  1. Yoshiki Sunada,
  2. Kenshi Yuki,
  3. Zhiling Wang,
  4. Takeaki Miyamura,
  5. Jesper Ilves,
  6. Kohei Matsuura,
  7. Peter A. Spring,
  8. Shuhei Tamate,
  9. Shingo Kono,
  10. and Yasunobu Nakamura
Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout.
Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesired dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we fabricated is shown to be enhanced by a factor of three relative to a device with a linear filter. The measurement rate is enhanced by another factor of three by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a QND fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.
arxiv pdf

All-microwave manipulation of superconducting qubits with a fixed-frequency transmon coupler

  1. Shotaro Shirai,
  2. Yuta Okubo,
  3. Kohei Matsuura,
  4. Alto Osada,
  5. Yasunobu Nakamura,
  6. and Atsushi Noguchi
All-microwave control of fixed-frequency superconducting quantum computing circuits is advantageous for minimizing the noise channels and wiring costs. Here we introduce a swap interaction
between two data transmons assisted by the third-order nonlinearity of a coupler transmon under a microwave drive. We model the interaction analytically and numerically and use it to implement an all-microwave controlled-Z gate. The gate based on the coupler-assisted swap transition maintains high drive efficiency and small residual interaction over a wide range of detuning between the data transmons.
arxiv pdf