Taeyoon Kim

Ultracoherent superconducting cavity-based multiqudit platform with error-resilient control

  1. Taeyoon Kim,
  2. Tanay Roy,
  3. Xinyuan You,
  4. Andy C. Y. Li,
  5. Henry Lamm,
  6. Oleg Pronitchev,
  7. Mustafa Bal,
  8. Sabrina Garattoni,
  9. Francesco Crisa,
  10. Daniel Bafia,
  11. Doga Kurkcuoglu,
  12. Roman Pilipenko,
  13. Paul Heidler,
  14. Nicholas Bornman,
  15. David van Zanten,
  16. Silvia Zorzetti,
  17. Roni Harnik,
  18. Akshay Murthy,
  19. Shaojiang Zhu,
  20. Changqing Wang,
  21. Andre Vallieres,
  22. Ziwen Huang,
  23. Jens Koch,
  24. Anna Grassellino,
  25. Srivatsan Chakram,
  26. Alexander Romanenko,
  27. and Yao Lu
Superconducting radio-frequency (SRF) cavities offer a promising platform for quantum computing due to their long coherence times and large accessible Hilbert spaces, yet integrating
nonlinear elements like transmons for control often introduces additional loss. We report a multimode quantum system based on a 2-cell elliptical shaped SRF cavity, comprising two cavity modes weakly coupled to an ancillary transmon circuit, designed to preserve coherence while enabling efficient control of the cavity modes. We mitigate the detrimental effects of the transmon decoherence through careful design optimization that reduces transmon-cavity couplings and participation in the dielectric substrate and lossy interfaces, to achieve single-photon lifetimes of 20.6 ms and 15.6 ms for the two modes, and a pure dephasing time exceeding 40 ms. This marks an order-of-magnitude improvement over prior 3D multimode memories. Leveraging sideband interactions and novel error-resilient protocols, including measurement-based correction and post-selection, we achieve high-fidelity control over quantum states. This enables the preparation of Fock states up to N=20 with fidelities exceeding 95%, the highest reported to date to the authors‘ knowledge, as well as two-mode entanglement with coherence-limited fidelities reaching up to 99.9% after post-selection. These results establish our platform as a robust foundation for quantum information processing, allowing for future extensions to high-dimensional qudit encodings.
arxiv pdf

Fast ZZ-Free Entangling Gates for Superconducting Qubits Assisted by a Driven Resonator

  1. Ziwen Huang,
  2. Taeyoon Kim,
  3. Tanay Roy,
  4. Yao Lu,
  5. Alexander Romanenko,
  6. Shaojiang Zhu,
  7. and Anna Grassellino
Engineering high-fidelity two-qubit gates is an indispensable step toward practical quantum computing. For superconducting quantum platforms, one important setback is the stray interaction
between qubits, which causes significant coherent errors. For transmon qubits, protocols for mitigating such errors usually involve fine-tuning the hardware parameters or introducing usually noisy flux-tunable couplers. In this work, we propose a simple scheme to cancel these stray interactions. The coupler used for such cancellation is a driven high-coherence resonator, where the amplitude and frequency of the drive serve as control knobs. Through the resonator-induced-phase (RIP) interaction, the static ZZ coupling can be entirely neutralized. We numerically show that such a scheme can enable short and high-fidelity entangling gates, including cross-resonance CNOT gates within 40 ns and adiabatic CZ gates within 140 ns. Our architecture is not only ZZ free but also contains no extra noisy components, such that it preserves the coherence times of fixed-frequency transmon qubits. With the state-of-the-art coherence times, the error of our cross-resonance CNOT gate can be reduced to below 1e-4.
arxiv pdf