Signal crosstalk in a flip-chip quantum processor

  1. Sandoko Kosen,
  2. Hang-Xi Li,
  3. Marcus Rommel,
  4. Robert Rehammar,
  5. Marco Caputo,
  6. Leif Grönberg,
  7. Jorge Fernández-Pendás,
  8. Anton Frisk Kockum,
  9. Janka Biznárová,
  10. Liangyu Chen,
  11. Christian Križan,
  12. Andreas Nylander,
  13. Amr Osman,
  14. Anita Fadavi Roudsari,
  15. Daryoush Shiri,
  16. Giovanna Tancredi,
  17. Joonas Govenius,
  18. and Jonas Bylander
Quantum processors require a signal-delivery architecture with high addressability (low crosstalk) to ensure high performance already at the scale of dozens of qubits. Signal crosstalk
causes inadvertent driving of quantum gates, which will adversely affect quantum-gate fidelities in scaled-up devices. Here, we demonstrate packaged flip-chip superconducting quantum processors with signal-crosstalk performance competitive with those reported in other platforms. For capacitively coupled qubit-drive lines, we find on-resonant crosstalk better than -27 dB (average -37 dB). For inductively coupled magnetic-flux-drive lines, we find less than 0.13 % direct-current flux crosstalk (average 0.05 %). These observed crosstalk levels are adequately small and indicate a decreasing trend with increasing distance, which is promising for further scaling up to larger numbers of qubits. We discuss the implication of our results for the design of a low-crosstalk, on-chip signal delivery architecture, including the influence of a shielding tunnel structure, potential sources of crosstalk, and estimation of crosstalk-induced qubit-gate error in scaled-up quantum processors.

Fast analytic and numerical design of superconducting resonators in flip-chip architectures

  1. Hang-Xi Li,
  2. Daryoush Shiri,
  3. Sandoko Kosen,
  4. Marcus Rommel,
  5. Lert Chayanun,
  6. Andreas Nylander,
  7. Robert Rehammer,
  8. Giovanna Tancredi,
  9. Marco Caputo,
  10. Kestutis Grigoras,
  11. Leif Grönberg,
  12. Joonas Govenius,
  13. and Jonas Bylander
In superconducting quantum processors, the predictability of device parameters is of increasing importance as many labs scale up their systems to larger sizes in a 3D-integrated architecture.
In particular, the properties of superconducting resonators must be controlled well to ensure high-fidelity multiplexed readout of qubits. Here we present a method, based on conformal mapping techniques, to predict a resonator’s parameters directly from its 2D cross-section, without computationally heavy simulation. We demonstrate the method’s validity by comparing the calculated resonator frequency and coupling quality factor with those obtained through 3D finite-element-method simulation and by measurement of 15 resonators in a flip-chip-integrated architecture. We achieve a discrepancy of less than 2% between designed and measured frequencies, for 6-GHz resonators. We also propose a design method that reduces the sensitivity of the resonant frequency to variations in the inter-chip spacing.

Transmon qubit readout fidelity at the threshold for quantum error correction without a quantum-limited amplifier

  1. Liangyu Chen,
  2. Hang-Xi Li,
  3. Yong Lu,
  4. Christopher W. Warren,
  5. Christian J. Križan,
  6. Sandoko Kosen,
  7. Marcus Rommel,
  8. Shahnawaz Ahmed,
  9. Amr Osman,
  10. Janka Biznárová,
  11. Anita Fadavi Roudsari,
  12. Benjamin Lienhard,
  13. Marco Caputo,
  14. Kestutis Grigoras,
  15. Leif Grönberg,
  16. Joonas Govenius,
  17. Anton Frisk Kockum,
  18. Per Delsing,
  19. Jonas Bylander,
  20. and Giovanna Tancredi
High-fidelity and rapid readout of a qubit state is key to quantum computing and communication, and it is a prerequisite for quantum error correction. We present a readout scheme for
superconducting qubits that combines two microwave techniques: applying a shelving technique to the qubit that effectively increases the energy-relaxation time, and a two-tone excitation of the readout resonator to distinguish among qubit populations in higher energy levels. Using a machine-learning algorithm to post-process the two-tone measurement results further improves the qubit-state assignment fidelity. We perform single-shot frequency-multiplexed qubit readout, with a 140ns readout time, and demonstrate 99.5% assignment fidelity for two-state readout and 96.9% for three-state readout – without using a quantum-limited amplifier.

Building Blocks of a Flip-Chip Integrated Superconducting Quantum Processor

  1. Sandoko Kosen,
  2. Hang-Xi Li,
  3. Marcus Rommel,
  4. Daryoush Shiri,
  5. Christopher Warren,
  6. Leif Grönberg,
  7. Jaakko Salonen,
  8. Tahereh Abad,
  9. Janka Biznárová,
  10. Marco Caputo,
  11. Liangyu Chen,
  12. Kestutis Grigoras,
  13. Göran Johansson,
  14. Anton Frisk Kockum,
  15. Christian Križan,
  16. Daniel Pérez Lozano,
  17. Graham Norris,
  18. Amr Osman,
  19. Jorge Fernández-Pendás,
  20. Anita Fadavi Roudsari,
  21. Giovanna Tancredi,
  22. Andreas Wallraff,
  23. Christopher Eichler,
  24. Joonas Govenius,
  25. and Jonas Bylander
We have integrated single and coupled superconducting transmon qubits into flip-chip modules. Each module consists of two chips – one quantum chip and one control chip –
that are bump-bonded together. We demonstrate time-averaged coherence times exceeding 90μs, single-qubit gate fidelities exceeding 99.9%, and two-qubit gate fidelities above 98.6%. We also present device design methods and discuss the sensitivity of device parameters to variation in interchip spacing. Notably, the additional flip-chip fabrication steps do not degrade the qubit performance compared to our baseline state-of-the-art in single-chip, planar circuits. This integration technique can be extended to the realisation of quantum processors accommodating hundreds of qubits in one module as it offers adequate input/output wiring access to all qubits and couplers.