A Superconducting Qubit-Resonator Quantum Processor with Effective All-to-All Connectivity

  1. Michael Renger,
  2. Jeroen Verjauw,
  3. Nicola Wurz,
  4. Amin Hosseinkhani,
  5. Caspar Ockeloen-Korppi,
  6. Wei Liu,
  7. Aniket Rath,
  8. Manish J. Thapa,
  9. Florian Vigneau,
  10. Elisabeth Wybo,
  11. Ville Bergholm,
  12. Chun Fai Chan,
  13. Bálint Csatári,
  14. Saga Dahl,
  15. Rakhim Davletkaliyev,
  16. Rakshyakar Giri,
  17. Daria Gusenkova,
  18. Hermanni Heimonen,
  19. Tuukka Hiltunen,
  20. Hao Hsu,
  21. Eric Hyyppä,
  22. Joni Ikonen,
  23. Tyler Jones,
  24. Shabeeb Khalid,
  25. Seung-Goo Kim,
  26. Miikka Koistinen,
  27. Anton Komlev,
  28. Janne Kotilahti,
  29. Vladimir Kukushkin,
  30. Julia Lamprich,
  31. Alessandro Landra,
  32. Lan-Hsuan Lee,
  33. Tianyi Li,
  34. Per Liebermann,
  35. Sourav Majumder,
  36. Janne Mäntylä,
  37. Fabian Marxer,
  38. Arianne Meijer - van de Griend,
  39. Vladimir Milchakov,
  40. Jakub Mrożek,
  41. Jayshankar Nath,
  42. Tuure Orell,
  43. Miha Papič,
  44. Matti Partanen,
  45. Alexander Plyushch,
  46. Stefan Pogorzalek,
  47. Jussi Ritvas,
  48. Pedro Figuero Romero,
  49. Ville Sampo,
  50. Marko Seppälä,
  51. Ville Selinmaa,
  52. Linus Sundström,
  53. Ivan Takmakov,
  54. Brian Tarasinski,
  55. Jani Tuorila,
  56. Olli Tyrkkö,
  57. Alpo Välimaa,
  58. Jaap Wesdorp,
  59. Ping Yang,
  60. Liuqi Yu,
  61. Johannes Heinsoo,
  62. Antti Vepsäläinen,
  63. William Kindel,
  64. Hsiang-Sheng Ku,
  65. and Frank Deppe
In this work we introduce a superconducting quantum processor architecture that uses a transmission-line resonator to implement effective all-to-all connectivity between six transmon
qubits. This architecture can be used as a test-bed for algorithms that benefit from high connectivity. We show that the central resonator can be used as a computational element, which offers the flexibility to encode a qubit for quantum computation or to utilize its bosonic modes which further enables quantum simulation of bosonic systems. To operate the quantum processing unit (QPU), we develop and benchmark the qubit-resonator conditional Z gate and the qubit-resonator MOVE operation. The latter allows for transferring a quantum state between one of the peripheral qubits and the computational resonator. We benchmark the QPU performance and achieve a genuinely multi-qubit entangled Greenberger-Horne-Zeilinger (GHZ) state over all six qubits with a readout-error mitigated fidelity of 0.86.

Approximations in transmon simulation

  1. Tyler Jones,
  2. Kaiah Steven,
  3. Xavier Poncini,
  4. Matthew Rose,
  5. and Arkady Fedorov
Classical simulations of time-dependent quantum systems are widely used in quantum control research. In particular, these simulations are commonly used to host iterative optimal control
algorithms. This is convenient for algorithms which are too onerous to run in the loop with current-day quantum hardware, as well as for researchers without consistent access to said hardware. However, if the model used to represent the system is not selected carefully, an optimised control protocol may be rendered futile when applied to hardware. We present a series of models, ordered in a hierarchy of progressive approximation, which appear in quantum control literature. Significant model deviations are highlighted, with a focus on simulated dynamics under simple single-qubit protocols. The validity of each model is characterised experimentally by designing and benchmarking control protocols for an IBMQ cloud quantum device. This result demonstrates an error amplification exceeding 100%, induced by the application of a first-order perturbative approximation. Finally, an evaluation of simulated control dynamics reveals that despite the substantial variance in numerical predictions across the proposed models, the complexity of discovering local optimal control protocols appears invariant for a simple control scheme. The set of findings presented heavily encourage practitioners of this field to ensure that their system models do not contain assumptions that markedly decrease applicability to hardware in experimentally relevant control parameter regimes.

Quantum rifling: protecting a qubit from measurement back-action

  1. Daniel Szombati,
  2. Alejandro Gomez Frieiro,
  3. Clemens Müller,
  4. Tyler Jones,
  5. Markus Jerger,
  6. and Arkady Fedorov
Quantum mechanics postulates that measuring the qubit’s wave function results in its collapse, with the recorded discrete outcome designating the particular eigenstate the qubit
collapsed into. We show this picture breaks down when the qubit is strongly driven during measurement. More specifically, for a fast evolving qubit the measurement returns the time-averaged expectation value of the measurement operator, erasing information about the initial state of the qubit, while completely suppressing the measurement back-action. We call this regime `quantum rifling‘, as the fast spinning of the Bloch vector protects it from deflection into either of its two eigenstates. We study this phenomenon with two superconducting qubits coupled to the same probe field and demonstrate that quantum rifling allows us to measure either one of the two qubits on demand while protecting the state of the other from measurement back-action. Our results allow for the implementation of selective read out multiplexing of several qubits, contributing to efficient scaling up of quantum processors for future quantum technologies.