Carlos A. Riofrío

A high-fidelity two-qubit gate for multimode superconducting P-mon qubits

  1. Frederik Pfeiffer,
  2. Federico A. Roy,
  3. Niklas J. Glaser,
  4. Julius Feigl,
  5. Leon Koch,
  6. Kevin Kiener,
  7. Gleb Krylov,
  8. Johannes Schirk,
  9. Christian M. F. Schneider,
  10. Lasse Södergren,
  11. Florian Wallner,
  12. Max Werninghaus,
  13. Carlos A. Riofrío,
  14. and Stefan Filipp
To scale superconducting quantum processors, it is essential to achieve long coherence times while engineering interactions that do not introduce additional decoherence channels. In
superconducting qubit systems, this can be realized using multimode circuits that feature a protected qubit mode alongside a distinct mediator mode. Building on this concept, our recently developed P-mon qubit provides intrinsic protection against decoherence from the readout environment. We extend this approach to controlled two-qubit interactions, by exploiting the mediator modes of P-mons for on-demand coupling. Because direct interactions between the qubit modes are strongly suppressed, unwanted ZZ-type interactions are significantly reduced to below 3.6(5) kHz in the idle state. When tuning the coupled mediator modes on resonance, the cross-Kerr interaction between the qubit and the hybridized mediator modes leads to a qubit-state dependent frequency shift. By selectively addressing these transitions, we implement a 180 ns long CZ gate and determine a fidelity of 99.62(4) %. These results represent a significant step toward a scalable superconducting architecture that maintains high performance at scale.
arxiv pdf

Efficient decoupling of a non-linear qubit mode from its environment

  1. Frederik Pfeiffer,
  2. Max Werninghaus,
  3. Christian Schweizer,
  4. Niklas Bruckmoser,
  5. Leon Koch,
  6. Niklas J. Glaser,
  7. Gerhard Huber,
  8. David Bunch,
  9. Franz X. Haslbeck,
  10. M. Knudsen,
  11. Gleb Krylov,
  12. Klaus Liegener,
  13. Achim Marx,
  14. Lea Richard,
  15. João H. Romeiro,
  16. Federico Roy,
  17. Johannes Schirk,
  18. Christian Schneider,
  19. Malay Singh,
  20. Lasse Södergren,
  21. Ivan Tsitsilin,
  22. Florian Wallner,
  23. Carlos A. Riofrío,
  24. and Stefan Filipp
To control and measure the state of a quantum system it must necessarily be coupled to external degrees of freedom. This inevitably leads to spontaneous emission via the Purcell effect,
photon-induced dephasing from measurement back-action, and errors caused by unwanted interactions with nearby quantum systems. To tackle this fundamental challenge, we make use of the design flexibility of superconducting quantum circuits to form a multi-mode element — an artificial molecule — with symmetry-protected modes. The proposed circuit consists of three superconducting islands coupled to a central island via Josephson junctions. It exhibits two essential non-linear modes, one of which is flux-insensitive and used as the protected qubit mode. The second mode is flux-tunable and serves via a cross-Kerr type coupling as a mediator to control the dispersive coupling of the qubit mode to the readout resonator. We demonstrate the Purcell protection of the qubit mode by measuring relaxation times that are independent of the mediated dispersive coupling. We show that the coherence of the qubit is not limited by photon-induced dephasing when detuning the mediator mode from the readout resonator and thereby reducing the dispersive coupling. The resulting highly protected qubit with tunable interactions may serve as a basic building block of a scalable quantum processor architecture, in which qubit decoherence is strongly suppressed.
arxiv pdf