Characterizing and Mitigating Flux Crosstalk in Superconducting Qubits-Couplers System

  1. Chen-Hsun Ma,
  2. Myrron Albert Callera Aguila,
  3. Nien-Yu Li,
  4. Li-Chieh Hsiao,
  5. Yi-Shiang Huang,
  6. Yen-Chun Chen,
  7. Teik-Hui Lee,
  8. Chin-Chia Chang,
  9. Jyh-Yang Wang,
  10. Ssu-Yen Huang,
  11. Hsi-Sheng Goan,
  12. Chiao-Hsuan Wang,
  13. Cen-Shawn Wu,
  14. Chii-Dong Chen,
  15. and Chung-Ting Ke
Superconducting qubits have achieved exceptional gate fidelities, exceeding the error-correction threshold in recent years. One key ingredient of such improvement is the introduction
of tunable couplers to control the qubit-to-qubit coupling through frequency tuning. Moving toward fault-tolerant quantum computation, increasing the number of physical qubits is another step toward effective error correction codes. Under a multiqubit architecture, flux control (Z) lines are crucial in tuning the frequency of the qubits and couplers. However, dense flux lines result in magnetic flux crosstalk, wherein magnetic flux applied to one element inadvertently affects neighboring qubits or couplers. This crosstalk obscures the idle frequency of the qubit when flux bias is applied, which degrades gate performance and calibration accuracy. In this study, we characterize flux crosstalk and suppress it in a multiqubit-coupler chip with multi-Z lines without adding additional readout for couplers. By quantifying the mutual flux-induced frequency shifts of qubits and couplers, we construct a cancellation matrix that enables precise compensation of non-local flux, demonstrating a substantial reduction in Z-line crosstalk from 56.5permilleto 0.13permille which is close to statistical error. Flux compensation corrects the CZ SWAP measurement, leading to a symmetric map with respect to flux bias. Compared with a crosstalk-free calculated CZ SWAP map, the measured map indicates that our approach provides a near-zero crosstalk for the coupler-transmon system. These results highlight the effectiveness of our approach in enhancing flux crosstalk-free control and supporting its potential for scaling superconducting quantum processors.

Photon-Number Dependent Hamiltonian Engineering for Cavities

  1. Chiao-Hsuan Wang,
  2. Kyungjoo Noh,
  3. José Lebreuilly,
  4. S. M. Girvin,
  5. and Liang Jiang
Cavity resonators are promising resources for quantum technology, while native nonlinear interactions for cavities are typically too weak to provide the level of quantum control required
to deliver complex targeted operations. Here we investigate a scheme to engineer a target Hamiltonian for photonic cavities using ancilla qubits. By off-resonantly driving dispersively coupled ancilla qubits, we develop an optimized approach to engineering an arbitrary photon-number dependent (PND) Hamiltonian for the cavities while minimizing the operation errors. The engineered Hamiltonian admits various applications including canceling unwanted cavity self-Kerr interactions, creating higher-order nonlinearities for quantum simulations, and designing quantum gates resilient to noise. Our scheme can be implemented with coupled microwave cavities and transmon qubits in superconducting circuit systems.