Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

  1. Janka Biznárová,
  2. Amr Osman,
  3. Emil Rehnman,
  4. Lert Chayanun,
  5. Christian Križan,
  6. Per Malmberg,
  7. Marcus Rommel,
  8. Christopher Warren,
  9. Per Delsing,
  10. August Yurgens,
  11. Jonas Bylander,
  12. and Anita Fadavi Roudsari
We demonstrate aluminum-on-silicon planar transmon qubits with time-averaged T1 energy relaxation times of up to 270μs, corresponding to Q = 5 million, and a highest observed value
of 501μs. We use materials analysis techniques and numerical simulations to investigate the dominant sources of energy loss, and devise and demonstrate a strategy towards mitigating them. The mitigation of loss is achieved by reducing the presence of oxide, a known host of defects, near the substrate-metal interface, by growing aluminum films thicker than 300 nm. A loss analysis of coplanar-waveguide resonators shows that the improvement is owing to a reduction of dielectric loss due to two-level system defects. We perform time-of-flight secondary ion mass spectrometry and observe a reduced presence of oxygen at the substrate-metal interface for the thicker films. The correlation between the enhanced performance and the film thickness is due to the tendency of aluminum to grow in columnar structures of parallel grain boundaries, where the size of the grain depends on the film thickness: transmission electron microscopy imaging shows that the thicker film has larger grains and consequently fewer grain boundaries containing oxide near this interface. These conclusions are supported by numerical simulations of the different loss contributions in the device.

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.

Quantum approximate optimization of the exact-cover problem on a superconducting quantum processor

  1. Andreas Bengtsson,
  2. Pontus Vikstål,
  3. Christopher Warren,
  4. Marika Svensson,
  5. Xiu Gu,
  6. Anton Frisk Kockum,
  7. Philip Krantz,
  8. Christian Križan,
  9. Daryoush Shiri,
  10. Ida-Maria Svensson,
  11. Giovanna Tancredi,
  12. Göran Johansson,
  13. Per Delsing,
  14. Giulia Ferrini,
  15. and Jonas Bylander
Present-day, noisy, small or intermediate-scale quantum processors—although far from fault-tolerant—support the execution of heuristic quantum algorithms, which might enable
a quantum advantage, for example, when applied to combinatorial optimization problems. On small-scale quantum processors, validations of such algorithms serve as important technology demonstrators. We implement the quantum approximate optimization algorithm (QAOA) on our hardware platform, consisting of two transmon qubits and one parametrically modulated coupler. We solve small instances of the NP-complete exact-cover problem, with 96.6\% success probability, by iterating the algorithm up to level two.