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.

Direct spectrum analysis using a threshold detector with application to a superconducting circuit

  1. Grégoire Ithier,
  2. Giovanna Tancredi,
  3. and Phil Meeson
We introduce a new and quantitative theoretical framework for noise spectral analysis using a threshold detector, which is then applied to a superconducting device: the Cavity BifurcationAmplifier (CBA). We show that this new framework provides direct access to the environmental noise spectrum with a sensitivity approaching the standard quantum limit of weak continuous measurements. In addition, the accessible frequency range of the spectrum is, in principle, limited only by the ring down time of the resonator. This on-chip noise detector is non-dissipative and works with low probing powers, allowing it to be operated at low temperatures (T<10mK). We exploit this technique for measuring the frequency fluctuations of the CBA and find a low frequency noise with an amplitude and spectrum compatible with a dielectric origin.[/expand]