Reducing leakage of single-qubit gates for superconducting quantum processors using analytical control pulse envelopes

  1. Eric Hyyppä,
  2. Antti Vepsäläinen,
  3. Miha Papič,
  4. Chun Fai Chan,
  5. Sinan Inel,
  6. Alessandro Landra,
  7. Wei Liu,
  8. Jürgen Luus,
  9. Fabian Marxer,
  10. Caspar Ockeloen-Korppi,
  11. Sebastian Orbell,
  12. Brian Tarasinski,
  13. and Johannes Heinsoo
Improving the speed and fidelity of quantum logic gates is essential to reach quantum advantage with future quantum computers. However, fast logic gates lead to increased leakage errors
in superconducting quantum processors based on qubits with low anharmonicity, such as transmons. To reduce leakage errors, we propose and experimentally demonstrate two new analytical methods, Fourier ansatz spectrum tuning derivative removal by adiabatic gate (FAST DRAG) and higher-derivative (HD) DRAG, both of which enable shaping single-qubit control pulses in the frequency domain to achieve stronger suppression of leakage transitions compared to previously demonstrated pulse shapes. Using the new methods to suppress the ef-transition of a transmon qubit with an anharmonicity of -212 MHz, we implement RX(π/2)-gates with a leakage error below 3.0×10−5 down to a gate duration of 6.25 ns, which corresponds to a 20-fold reduction in leakage compared to a conventional Cosine DRAG pulse. Employing the FAST DRAG method, we further achieve an error per gate of (1.56±0.07)×10−4 at a 7.9-ns gate duration, outperforming conventional pulse shapes both in terms of error and gate speed. Furthermore, we study error-amplifying measurements for the characterization of temporal microwave control pulse distortions, and demonstrate that non-Markovian coherent errors caused by such distortions may be a significant source of error for sub-10-ns single-qubit gates unless corrected using predistortion.

Single-Shot Readout of a Superconducting Qubit Using a Thermal Detector

  1. András M. Gunyhó,
  2. Suman Kundu,
  3. Jian Ma,
  4. Wei Liu,
  5. Sakari Niemelä,
  6. Giacomo Catto,
  7. Vasilii Vadimov,
  8. Visa Vesterinen,
  9. Priyank Singh,
  10. Qiming Chen,
  11. and Mikko Möttönen
Measuring the state of qubits is one of the fundamental operations of a quantum computer. Currently, state-of-the-art high-fidelity single-shot readout of superconducting qubits relies
on parametric amplifiers at the millikelvin stage. However, parametric amplifiers are challenging to scale beyond hundreds of qubits owing to practical size and power limitations. Nanobolometers have properties that are advantageous for scalability and have recently shown sensitivity and speed promising for qubit readout, but such thermal detectors have not been demonstrated for this purpose. In this work, we utilize an ultrasensitive bolometer in place of a parametric amplifier to experimentally demonstrate single-shot qubit readout. With a modest readout duration of 13.9 μs, we achieve a single-shot fidelity of 0.618 which is mainly limited by the energy relaxation time of the qubit, T1=28 μs. Without the T1 errors, we find the fidelity to be 0.927. In the future, high-fidelity single-shot readout may be achieved by straightforward improvements to the chip design and experimental setup, and perhaps most interestingly by the change of the bolometer absorber material to reduce the readout time to the hundred-nanosecond level.

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.

Broadband Tunable Phase Shifter For Microwaves

  1. Jinli Zhang,
  2. Tianyi Li,
  3. Roope Kokkoniemi,
  4. Chengyu Yan,
  5. Wei Liu,
  6. Matti Partanen,
  7. Kuan Yen Tan,
  8. Ming He,
  9. Lu Ji,
  10. Leif Grönberg,
  11. and Mikko Möttönen
We implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different
frequencies, a unit transmission coefficient, |S21|=1, can be theoretically achieved along a curve where the phase shift is controllable by magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from from 0.07×π to 0.14×π radians along the full-transmission curve with the input frequency ranging from 6.00 to 6.28~GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.