Circle fit optimization for resonator quality factor measurements: point redistribution for maximal accuracy

  1. Paul G. Baity,
  2. Connor Maclean,
  3. Valentino Seferai,
  4. Joe Bronstein,
  5. Yi Shu,
  6. Tania Hemakumara,
  7. and Martin Weides
The control of material loss mechansims is playing an increasingly important role for improving coherence times in superconducting quantum devices. Such material losses can be characterized
through the measurement of planar superconducting resonators, which reflect losses through the resonance’s quality factor Ql. The resonance quality factor consists of both internal (material) losses as well as coupling losses when resonance photons escape back into the measurement circuit. The combined losses are then described as Q−1l=Q−1c+Q−1i, where Qc and Qi reflect the coupling and internal quality factors of the resonator, respectively. To separate the relative contributions of Qi and Qc to Ql, diameter-correcting circle fits use algebraic or geometric means to fit the resonance signal on the complex plane. However, such circle fits can produce varied results, so to address this issue, we use a combination of simulation and experiment to determine the reliability of a fitting algorithm across a wide range of quality factor values from Qi≪Qc to Qc≪Qi. In addition, we develop a novel measurement protocol that can not only reduce fitting errors by factors ≳2 but also mitigates the influence of the measurement background on the fit results. This technique can be generalized for other resonance systems beyond superconducting resonators.

Highly coherent superconducting qubits from a subtractive junction fabrication process

  1. Alexander Stehli,
  2. Jan David Brehm,
  3. Tim Wolz,
  4. Paul Baity,
  5. Sergey Danilin,
  6. Valentino Seferai,
  7. Hannes Rotzinger,
  8. Alexey V. Ustinov,
  9. and Martin Weides
Josephson tunnel junctions are the centerpiece of almost any superconducting electronic circuit, including qubits. Typically, the junctions for qubits are fabricated using shadow evaporation
techniques to reduce dielectric loss contributions from the superconducting film interfaces. In recent years, however, sub-micron scale overlap junctions have started to attract attention. Compared to shadow mask techniques, neither an angle dependent deposition nor free-standing bridges or overlaps are needed, which are significant limitations for wafer-scale processing. This comes at the cost of breaking the vacuum during fabrication, but simplifies integration in multi-layered circuits, implementation of vastly different junction sizes, and enables fabrication on a larger scale in an industrially-standardized process. In this work, we demonstrate the feasibility of a subtractive process for fabrication of overlap junctions. We evaluate the coherence properties of the junctions by employing them in superconducting transmon qubits. In time domain experiments, we find that both, the qubit life- and coherence time of our best device, are on average greater than 20 μs. Finally, we discuss potential improvements to our technique. This work paves the way towards a more standardized process flow with advanced materials and growth processes, and constitutes an important step for large scale fabrication of superconducting quantum circuits.