A. Kandala

Quantum crosstalk cancellation for fast entangling gates and improved multi-qubit performance

  1. K. X. Wei,
  2. E. Magesan,
  3. I. Lauer,
  4. S. Srinivasan,
  5. D. F. Bogorin,
  6. S. Carnevale,
  7. G. A. Keefe,
  8. Y. Kim,
  9. D. Klaus,
  10. W. Landers,
  11. N. Sundaresan,
  12. C. Wang,
  13. E. J. Zhang,
  14. M. Steffen,
  15. O. E. Dial,
  16. D. C. McKay,
  17. and A. Kandala
Quantum computers built with superconducting artificial atoms already stretch the limits of their classical counterparts. While the lowest energy states of these artificial atoms serve
as the qubit basis, the higher levels are responsible for both a host of attractive gate schemes as well as generating undesired interactions. In particular, when coupling these atoms to generate entanglement, the higher levels cause shifts in the computational levels that leads to unwanted ZZ quantum crosstalk. Here, we present a novel technique to manipulate the energy levels and mitigate this crosstalk via a simultaneous AC Stark effect on coupled qubits. This breaks a fundamental deadlock between qubit-qubit coupling and crosstalk, leading to a 90ns CNOT with a gate error of (0.19 ± 0.02) % and the demonstration of a novel CZ gate with fixed-coupling single-junction transmon qubits. Furthermore, we show a definitive improvement in circuit performance with crosstalk cancellation over seven qubits, demonstrating the scalability of the technique. This work paves the way for superconducting hardware with faster gates and greatly improved multi-qubit circuit fidelities.
arxiv pdf

Demonstration of a High-Fidelity CNOT for Fixed-Frequency Transmons with Engineered ZZ Suppression

  1. A. Kandala,
  2. K. X. Wei,
  3. S. Srinivasan,
  4. E. Magesan,
  5. S. Carnevale,
  6. G. A. Keefe,
  7. D. Klaus,
  8. O. Dial,
  9. and D. C. McKay
Improving two-qubit gate performance and suppressing crosstalk are major, but often competing, challenges to achieving scalable quantum computation. In particular, increasing the coupling
to realize faster gates has been intrinsically linked to enhanced crosstalk due to unwanted two-qubit terms in the Hamiltonian. Here, we demonstrate a novel coupling architecture for transmon qubits that circumvents the standard relationship between desired and undesired interaction rates. Using two fixed frequency coupling elements to tune the dressed level spacings, we demonstrate an intrinsic suppression of the static ZZ, while maintaining large effective coupling rates. Our architecture reveals no observable degradation of qubit coherence (T1,T2>100 μs) and, over a factor of 6 improvement in the ratio of desired to undesired coupling. Using the cross-resonance interaction we demonstrate a 180~ns single-pulse CNOT gate, and measure a CNOT fidelity of 99.77(2)% from interleaved randomized benchmarking.
arxiv pdf