A. Remm

Demonstration of an All-Microwave Controlled-Phase Gate between Far Detuned Qubits

  1. S. Krinner,
  2. P. Kurpiers,
  3. B. Royer,
  4. P. Magnard,
  5. I. Tsitsilin,
  6. J.-C. Besse,
  7. A. Remm,
  8. A. Blais,
  9. and A. Wallraff
A challenge in building large-scale superconducting quantum processors is to find the right balance between coherence, qubit addressability, qubit-qubit coupling strength, circuit complexity
and the number of required control lines. Leading all-microwave approaches for coupling two qubits require comparatively few control lines and benefit from high coherence but suffer from frequency crowding and limited addressability in multi-qubit settings. Here, we overcome these limitations by realizing an all-microwave controlled-phase gate between two transversely coupled transmon qubits which are far detuned compared to the qubit anharmonicity. The gate is activated by applying a single, strong microwave tone to one of the qubits, inducing a coupling between the two-qubit |f,g⟩ and |g,e⟩ states, with |g⟩, |e⟩, and |f⟩ denoting the lowest energy states of a transmon qubit. Interleaved randomized benchmarking yields a gate fidelity of 97.5±0.3% at a gate duration of 126ns, with the dominant error source being decoherence. We model the gate in presence of the strong drive field using Floquet theory and find good agreement with our data. Our gate constitutes a promising alternative to present two-qubit gates and could have hardware scaling advantages in large-scale quantum processors as it neither requires additional drive lines nor tunable couplers.
arxiv pdf

Benchmarking Coherent Errors in Controlled-Phase Gates due to Spectator Qubits

  1. S. Krinner,
  2. S. Lazar,
  3. A. Remm,
  4. C. K. Andersen,
  5. N. Lacroix,
  6. G. J. Norris,
  7. C. Hellings,
  8. M. Gabureac,
  9. C. Eichler,
  10. and A. Wallraff
A major challenge in operating multi-qubit quantum processors is to mitigate multi-qubit coherent errors. For superconducting circuits, besides crosstalk originating from imperfect
isolation of control lines, dispersive coupling between qubits is a major source of multi-qubit coherent errors. We benchmark phase errors in a controlled-phase gate due to dispersive coupling of either of the qubits involved in the gate to one or more spectator qubits. We measure the associated gate infidelity using quantum process tomography. In addition, we point out that, due to coupling of the gate qubits to a non-computational state during the gate, two-qubit conditional phase errors are enhanced. Our work is important for understanding limits to the fidelity of two-qubit gates with finite on/off ratio in multi-qubit settings.
arxiv pdf