Sabbir A. Khan

Sidewall Metrology Protocol for Superconducting Qubit Devices

  1. Jørgen Garnæs,
  2. Henrik S. Guttesen,
  3. Astrid T. Rømer,
  4. Oscar Danielsson,
  5. Emily P. Duggan,
  6. Zhenhai Sun,
  7. Asen Lyubenov Georgiev,
  8. Li Wei Chang,
  9. Morten Kjaergaard,
  10. Mark Bieler,
  11. Lukas Grünhaupt,
  12. and Sabbir A. Khan
Sidewall morphology is a potentially important factor influencing superconducting qubit performance, yet its quantitative characterization remains challenging due to the absence of
standardized measurement protocols. In this work, we develop a scanning probe microscopy-based measurement framework for the quantitative characterization of superconducting qubit sidewalls. The framework introduces a refined root mean square lateral sidewall roughness descriptor and a new curvature-equivalent radius for quantifying vertical sidewall morphology. The descriptors and measurement procedures are applied to different components of qubit devices, forming the basis of a proposed sidewall characterization protocol. This provides a foundation for quantitative comparisons of fabrication processes across laboratories, enabling future investigations of correlations between sidewall geometry and qubit performance.
arxiv pdf

Transparent Gatable Superconducting Shadow Junctions

  1. Sabbir A. Khan,
  2. Charalampos Lampadaris,
  3. Ajuan Cui,
  4. Lukas Stampfer,
  5. Yu Liu,
  6. S. J. Pauka,
  7. Martin E. Cachaza,
  8. Elisabetta M. Fiordaliso,
  9. Jung-Hyun Kang,
  10. Svetlana Korneychuk,
  11. Timo Mutas,
  12. Joachim E. Sestoft,
  13. Filip Krizek,
  14. Rawa Tanta,
  15. M.C. Cassidy,
  16. Thomas S. Jespersen,
  17. and Peter Krogstrup
Gate tunable junctions are key elements in quantum devices based on hybrid semiconductor-superconductor materials. They serve multiple purposes ranging from tunnel spectroscopy probes
to voltage-controlled qubit operations in gatemon and topological qubits. Common to all is that junction transparency plays a critical role. In this study, we grow single crystalline InAs, InSb and InAs1−xSbx nanowires with epitaxial superconductors and in-situ shadowed junctions in a single-step molecular beam epitaxy process. We investigate correlations between fabrication parameters, junction morphologies, and electronic transport properties of the junctions and show that the examined in-situ shadowed junctions are of significantly higher quality than the etched junctions. By varying the edge sharpness of the shadow junctions we show that the sharpest edges yield the highest junction transparency for all three examined semiconductors. Further, critical supercurrent measurements reveal an extraordinarily high ICRN, close to the KO−2 limit. This study demonstrates a promising engineering path towards reliable gate-tunable superconducting qubits.
arxiv pdf