Joona Andersson

Efficient and accurate two-qubit-gate operation in a high-connectivity transmon lattice utilizing a tunable coupling to a shared mode

  1. Tuure Orell,
  2. Hao Hsu,
  3. Joona Andersson,
  4. Jani Tuorila,
  5. Frank Deppe,
  6. and Hsiang-Sheng Ku
Increasing connectivity and decreasing qubit-state delocalization without compromising the speed and accuracy of elementary gate operations are topical challenges in the development
of large-scale superconducting quantum computers. In this theoretical work, we study a special honeycomb qubit lattice where each qubit inside a unit cell is coupled to every other one via two dedicated tunable couplers and a common central element. This results in an effective multi-mode interaction enabling tunable, on-demand, all-to-all connectivity between each qubit pair within the unit cell. We provide a thorough analysis of the unit cell, including a proposal for a novel and efficient conditional-Z gate scheme which takes advantage of the effective multi-mode coupling. We develop an experimentally viable pulse protocol for a single-step gate implementation which considerably improves the gate speed compared to the previous two-qubit-gate realizations suggested for architectures utilizing a center mode. We also show numerical results on how the presence of spectator qubits affects the average two-qubit-gate fidelity, and analyse how the multi-mode coupling structure mitigates the delocalization-induced crosstalk during simultaneous single-qubit gates within the unit cell. We also provide analytical estimates for the errors caused by relaxation and dephasing during a two-qubit-gate operation, including noise terms for the multi-mode coupling structure. Our multi-mode coupling architecture results in a good balance between increased connectivity and available parallelism, especially when several interacting unit cells form a quantum processing unit. We anticipate that the obtained results pave the way towards high-connectivity quantum processors with efficient and low-overhead quantum algorithms.
arxiv pdf

Mitigating crosstalk errors for simultaneous single-qubit gates on a superconducting quantum processor

  1. Jaap J. Wesdorp,
  2. Eric Hyyppä,
  3. Joona Andersson,
  4. Janos Adam,
  5. Rohit Beriwal,
  6. Ville Bergholm,
  7. Saga Dahl,
  8. Simone Diego Fasciati,
  9. Alejandro Gomez Friero,
  10. Zheming Gao,
  11. Daria Gusenkova,
  12. Andrew Guthrie,
  13. Johannes Heinsoo,
  14. Tuukka Hiltunen,
  15. Keiran Holland,
  16. Amin Hosseinkhani,
  17. Sinan Inel,
  18. Joni Ikonen,
  19. Shan W. Jolin,
  20. Kristinn Juliusson,
  21. Seung-Goo Kim,
  22. Anton Komlev,
  23. Roope Kokkoniemi,
  24. Otto Koskinen,
  25. Joonas Kylmälä,
  26. Alessandro Landra,
  27. Julia Lamprich,
  28. Magdalena Lehmuskoski,
  29. Nizar Lethif,
  30. Per Liebermann,
  31. Tianyi Li,
  32. Aleksi Lintunen,
  33. Fabian Marxer,
  34. Kunal Mitra,
  35. Jakub Mrożek,
  36. Lucas Ortega,
  37. Miha Papič,
  38. Matti Partanen,
  39. Alexander Plyushch,
  40. Stefan Pogorzalek,
  41. Michael Renger,
  42. Jussi Ritvas,
  43. Sampo Saarinen,
  44. Indrajeet Sagar,
  45. Matthew Sarsby,
  46. Mykhailo Savytskyi,
  47. Ville Selinmaa,
  48. Ivan Takmakov,
  49. Brian Tarasinski,
  50. Francesca Tosto,
  51. David Vasey,
  52. Panu Vesanen,
  53. Jeroen Verjauw,
  54. Alpo Välimaa,
  55. Nicola Wurz,
  56. Hsiang-Sheng Ku,
  57. Frank Deppe,
  58. Juha Hassel,
  59. Caspar Ockeloen-Korppi,
  60. Wei Liu,
  61. Jani Tuorila,
  62. Chun Fai Chan,
  63. Attila Geresdi,
  64. and Antti Vepsäläinen
Single-qubit gates on superconducting quantum processors are typically implemented using microwave pulses applied through dedicated control lines. However, these microwave pulses may
also drive other qubits due to crosstalk arising from capacitive coupling and wavefunction overlap in systems with closely spaced transition frequencies. Crosstalk and frequency crowding increase errors during simultaneous single-qubit operations relative to isolated gates, thus forming a major bottleneck for scaling superconducting quantum processors. In this work, we combine model-based qubit frequency optimization with pulse shaping to demonstrate crosstalk error mitigation in single-qubit gates on a 49-qubit superconducting quantum processor. We introduce and experimentally verify an analytical model of simultaneous single-qubit gate error caused by microwave crosstalk that depends on a given pulse shape. By employing a model-based optimization strategy of qubit frequencies, we minimize the crosstalk-induced error across the processor and achieve a mean simultaneous single-qubit gate fidelity of 99.96% for a 16-ns gate duration, approaching the mean individual gate fidelity. To further reduce the simultaneous error and required qubit frequency bandwidth on high-crosstalk qubit pairs, we introduce a crosstalk transition suppression (CTS) pulse shaping technique that minimizes the spectral energy around transitions inducing leakage and crosstalk errors. Finally, we combine CTS with model-based frequency optimization across the device and experimentally show a systematic reduction in the required qubit frequency bandwidth for high-fidelity simultaneous gates, supported by simulations of systems with up to 1000 qubits. By alleviating constraints on qubit frequency bandwidth for parallel single-qubit operations, this work represents an important step for scaling towards larger quantum processors.
arxiv pdf