Long-distance transmon coupler with CZ gate fidelity above 99.8%

  1. Fabian Marxer,
  2. Antti Vepsäläinen,
  3. Shan W. Jolin,
  4. Jani Tuorila,
  5. Alessandro Landra,
  6. Caspar Ockeloen-Korppi,
  7. Wei Liu,
  8. Olli Ahonen,
  9. Adrian Auer,
  10. Lucien Belzane,
  11. Ville Bergholm,
  12. Chun Fai Chan,
  13. Kok Wai Chan,
  14. Tuukka Hiltunen,
  15. Juho Hotari,
  16. Eric Hyyppä,
  17. Joni Ikonen,
  18. David Janzso,
  19. Miikka Koistinen,
  20. Janne Kotilahti,
  21. Tianyi Li,
  22. Jyrgen Luus,
  23. Miha Papic,
  24. Matti Partanen,
  25. Jukka Räbinä,
  26. Jari Rosti,
  27. Mykhailo Savytskyi,
  28. Marko Seppälä,
  29. Vasilii Sevriuk,
  30. Eelis Takala,
  31. Brian Tarasinski,
  32. Manish J. Thapa,
  33. Francesca Tosto,
  34. Natalia Vorobeva,
  35. Liuqi Yu,
  36. Kuan Yen Tan,
  37. Juha Hassel,
  38. Mikko Möttönen,
  39. and Johannes Heinsoo
Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate
a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-Z gate with (99.81±0.02)% fidelity.

Unimon qubit

  1. Eric Hyyppä,
  2. Suman Kundu,
  3. Chun Fai Chan,
  4. András Gunyhó,
  5. Juho Hotari,
  6. Olavi Kiuru,
  7. Alessandro Landra,
  8. Wei Liu,
  9. Fabian Marxer,
  10. Akseli Mäkinen,
  11. Jean-Luc Orgiazzi,
  12. Mario Palma,
  13. Mykhailo Savytskyi,
  14. Francesca Tosto,
  15. Jani Tuorila,
  16. Vasilii Vadimov,
  17. Tianyi Li,
  18. Caspar Ockeloen-Korppi,
  19. Johannes Heinsoo,
  20. Kuan Yen Tan,
  21. Juha Hassel,
  22. and Mikko Möttönen
Superconducting qubits are one of the most promising candidates to implement quantum computers. The superiority of superconducting quantum computers over any classical device in simulating
random but well-determined quantum circuits has already been shown in two independent experiments and important steps have been taken in quantum error correction. However, the currently wide-spread qubit designs do not yet provide high enough performance to enable practical applications or efficient scaling of logical qubits owing to one or several following issues: sensitivity to charge or flux noise leading to decoherence, too weak non-linearity preventing fast operations, undesirably dense excitation spectrum, or complicated design vulnerable to parasitic capacitance. Here, we introduce and demonstrate a superconducting-qubit type, the unimon, which combines the desired properties of high non-linearity, full insensitivity to dc charge noise, insensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. We measure the qubit frequency, ω01/(2π), and anharmonicity α over the full dc-flux range and observe, in agreement with our quantum models, that the qubit anharmonicity is greatly enhanced at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13-ns single-qubit gates on two qubits with (ω01,α)=(4.49 GHz,434 MHz)×2π and (3.55 GHz,744 MHz)×2π, respectively. The energy relaxation time T1≲10 μs is stable for hours and seems to be limited by dielectric losses. Thus, future improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible quantum advantage with noisy systems.

Microwave Quantum Link between Superconducting Circuits Housed in Spatially Separated Cryogenic Systems

  1. Paul Magnard,
  2. Simon Storz,
  3. Philipp Kurpiers,
  4. Josua Schär,
  5. Fabian Marxer,
  6. Janis Luetolf,
  7. Jean-Claude Besse,
  8. Mihai Gabureac,
  9. Kevin Reuer,
  10. Abdulkadir Akin,
  11. Baptiste Royer,
  12. Alexandre Blais,
  13. and Andreas Wallraff
Superconducting circuits are a strong contender for realizing quantum computing systems, and are also successfully used to study quantum optics and hybrid quantum systems. However,
their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks either spanning different cryogenics systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on-demand with average transfer and target state fidelities of 85.8 % and 79.5 %, respectively, between the two nodes of this elementary network. Cryogenic microwave links do provide an opportunity to scale up systems for quantum computing and create local area quantum communication networks over length scales of at least tens of meters.