B. Royer

Real-time quantum error correction beyond break-even

  1. V. V. Sivak,
  2. A. Eickbusch,
  3. B. Royer,
  4. S. Singh,
  5. I. Tsioutsios,
  6. S. Ganjam,
  7. A. Miano,
  8. B. L. Brock,
  9. A. Z. Ding,
  10. L. Frunzio,
  11. S. M. Girvin,
  12. R. J. Schoelkopf,
  13. and M. H. Devoret
The ambition of harnessing the quantum for computation is at odds with the fundamental phenomenon of decoherence. The purpose of quantum error correction (QEC) is to counteract the
natural tendency of a complex system to decohere. This cooperative process, which requires participation of multiple quantum and classical components, creates a special type of dissipation that removes the entropy caused by the errors faster than the rate at which these errors corrupt the stored quantum information. Previous experimental attempts to engineer such a process faced an excessive generation of errors that overwhelmed the error-correcting capability of the process itself. Whether it is practically possible to utilize QEC for extending quantum coherence thus remains an open question. We answer it by demonstrating a fully stabilized and error-corrected logical qubit whose quantum coherence is significantly longer than that of all the imperfect quantum components involved in the QEC process, beating the best of them with a coherence gain of G=2.27±0.07. We achieve this performance by combining innovations in several domains including the fabrication of superconducting quantum circuits and model-free reinforcement learning.
arxiv pdf

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