Toshi Sumida

ZZ-Free bSWAP Gates under Strong Driving in Far-Detuned Fixed-Frequency Transmons

  1. Akinori Machino,
  2. Kazuhisa Ogawa,
  3. Yuto Ikegai,
  4. Takafumi Miyanaga,
  5. Koichiro Ban,
  6. Nilton F. G. Filho,
  7. Hidehisa Shiomi,
  8. Toshi Sumida,
  9. Shuhei Tamate,
  10. Yutaka Tabuchi,
  11. Yasunobu Nakamura,
  12. and Makoto Negoro
Far-detuned frequency allocations in fixed-frequency superconducting processors mitigate spectral crowding but constrain cross-resonance interactions at practical drive amplitudes.
Here, we demonstrate that a single strong drive induces a large XX−YY interaction in the far-detuned regime, while a weak second tone tunes the accompanying ZZ phase. The interaction realizes a 150-ns bSWAP‾‾‾‾‾‾‾‾√ gate, which can directly prepare a maximally entangled state from the ground state. For the implementations on the same qubit pair at matched drive strengths, cross resonance gives a shorter CNOT operation, while bSWAP‾‾‾‾‾‾‾‾√ gives a 33% shorter SWAP operation with a higher IRB fidelity. The multilevel theory further predicts a regime where stronger driving can yield a speed advantage for CNOT as well, a possibility absent in the two-level model. As an application of the XX−YY interaction, we also demonstrate two-spin squeezing beyond the standard quantum limit through two-axis countertwisting. The interaction realized here complements cross resonance and offers a route to efficient gate synthesis and quantum simulation of interacting spin systems.
arxiv pdf

QuBE/Qubex: an integrated hardware-software system for superconducting qubit experiments with broadband control

  1. Akinori Machino,
  2. Kazuhisa Ogawa,
  3. Takefumi Miyoshi,
  4. Hidehisa Shiomi,
  5. Shinichi Morisaka,
  6. Ryo Matsuda,
  7. Nilton F. G. Filho,
  8. Koichiro Ban,
  9. Takafumi Miyanaga,
  10. Keisuke Koike,
  11. Ryutaro Ohira,
  12. Toshi Sumida,
  13. Yoshinori Kurimoto,
  14. Yuuya Sugita,
  15. Yosuke Ito,
  16. Yasunari Suzuki,
  17. Peter A. Spring,
  18. Shiyu Wang,
  19. Hiroto Mukai,
  20. Arvind Mamgain,
  21. Shuhei Tamate,
  22. Yutaka Tabuchi,
  23. Yasunobu Nakamura,
  24. and Makoto Negoro
Achieving high-fidelity operation in large-scale superconducting qubit systems requires not only control hardware with broad frequency coverage, low crosstalk, and tight synchronization
but also software that coordinates system configuration, experiment execution, and data analysis. Here we present an integrated qubit-control system that combines broadband microwave hardware with a pulse-level software stack for scalable superconducting qubit experiments. The hardware provides broadband microwave coverage, including an instantaneous span of up to 1.6 GHz from a control output, while the software reduces setup and calibration overhead through automated configuration and built-in experiment workflows. We validate the system on a 64-qubit fixed-frequency transmon chip through full-chip frequency identification and representative demonstrations, including multi-unit far-detuned cross-resonance calibration and benchmarking that yields a measured two-qubit gate fidelity of 98.34%, and multilevel readout beyond the computational subspace. By disclosing the hardware architecture and releasing the software stack as open source, this work provides an inspectable hardware-software foundation for scalable superconducting qubit control experiments.
arxiv pdf