Ville Bergholm

Long-distance transmon coupler with CZ gate fidelity above 99.8%

  1. Fabian Marxer,
  2. Antti Vepsäläinen,
  3. Shan W. Jolin,
  4. Jani Tuorila,
  5. Alessandro Landra,
  6. Caspar Ockeloen-Korppi,
  7. Wei Liu,
  8. Olli Ahonen,
  9. Adrian Auer,
  10. Lucien Belzane,
  11. Ville Bergholm,
  12. Chun Fai Chan,
  13. Kok Wai Chan,
  14. Tuukka Hiltunen,
  15. Juho Hotari,
  16. Eric Hyyppä,
  17. Joni Ikonen,
  18. David Janzso,
  19. Miikka Koistinen,
  20. Janne Kotilahti,
  21. Tianyi Li,
  22. Jyrgen Luus,
  23. Miha Papic,
  24. Matti Partanen,
  25. Jukka Räbinä,
  26. Jari Rosti,
  27. Mykhailo Savytskyi,
  28. Marko Seppälä,
  29. Vasilii Sevriuk,
  30. Eelis Takala,
  31. Brian Tarasinski,
  32. Manish J. Thapa,
  33. Francesca Tosto,
  34. Natalia Vorobeva,
  35. Liuqi Yu,
  36. Kuan Yen Tan,
  37. Juha Hassel,
  38. Mikko Möttönen,
  39. and Johannes Heinsoo
Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate
a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-Z gate with (99.81±0.02)% fidelity.
arxiv pdf

Arbitrary n-Qubit State Transfer Using Coherent Control and Simplest Switchable Local Noise

  1. Ville Bergholm,
  2. F.K. Wilhelm,
  3. and T. Schulte-Herbrueggen
We study the reachable sets of open n-qubit quantum systems, the coherent parts of which are under full unitary control, with time-modulable Markovian noise acting on a single qubit
as an additional degree of incoherent control. In particular, adding bang-bang control of amplitude damping noise (non-unital) allows the dynamic system to act transitively on the entire set of density operators. This means one can transform any initial quantum state into any desired target state. Adding switchable bit-flip noise (unital), on the other hand, suffices to explore all states majorised by the initial state. We have extended our open-loop optimal control package DYNAMO to also handle incoherent control so that these unprecedented reachable sets can systematically be exploited in experiments. We propose implementation by a GMon, a superconducting device with fast tunable coupling to an open transmission line, and illustrate how open-loop control with noise switching can accomplish all state transfers without the need for measurement-based closed-loop feedback schemes with a resettable ancilla.
arxiv pdf