M.C. Cassidy

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

Demonstration of an ac Josephson junction laser

  1. M.C. Cassidy,
  2. A. Bruno,
  3. S. Rubbert,
  4. M. Irfan,
  5. J. Kammhuber,
  6. R. N. Schouten,
  7. A. R. Akhmerov,
  8. and L.P.Kouwenhoven
Superconducting electronic devices have re-emerged as contenders for both classical and quantum computing due to their fast operation speeds, low dissipation and long coherence times.
An ultimate demonstration of coherence is lasing. We use one of the fundamental aspects of superconductivity, the ac Josephson effect, to demonstrate a laser made from a Josephson junction strongly coupled to a multi-mode superconducting cavity. A dc voltage bias to the junction provides a source of microwave photons, while the circuit’s nonlinearity allows for efficient down-conversion of higher order Josephson frequencies down to the cavity’s fundamental mode. The simple fabrication and operation allows for easy integration with a range of quantum devices, allowing for efficient on-chip generation of coherent microwave photons at low temperatures.
arxiv pdf