Gijs de Lange

Controlling the charge dispersion of a nearly-open superconducting island

  1. Arno Bargerbos,
  2. Willemijn Uilhoorn,
  3. Chung-Kai Yang,
  4. Peter Krogstrup,
  5. Leo P. Kouwenhoven,
  6. Gijs de Lange,
  7. Bernard van Heck,
  8. and Angela Kou
Isolation from the environment determines the extent to which charge is confined on an island. This confinement leads to an energy cost for adding an electron onto the island, which
manifests experimentally through Coulomb oscillations such as charge dispersion. In superconducting circuits, the link to the environment has typically been formed from tunnel junctions where the charge dispersion can be tuned by changing the ratio between the Josephson energy EJ and the charging energy Ec. If, instead, a transparent ballistic junction forms the link between the superconducting island and the environment, the charge dispersion is predicted to be suppressed far beyond the dependence on the EJ/Ec ratio due to imaginary-time Landau-Zener tunneling between Andreev bound states. Here we investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion consistent with the predicted scaling law incorporating diabatic transitions between Andreev bound states. We also observe greatly improved qubit coherence times at the point of highest charge dispersion suppression. Our observations further our fundamental understanding of charging effects in superconductors and suggest novel approaches for building charge-insensitive qubits.
arxiv pdf

A gate-tunable, field-compatible fluxonium

  1. Marta Pita-Vidal,
  2. Arno Bargerbos,
  3. Chung-Kai Yang,
  4. David J. van Woerkom,
  5. Wolfgang Pfaff,
  6. Nadia Haider,
  7. Peter Krogstrup,
  8. Leo P. Kouwenhoven,
  9. Gijs de Lange,
  10. and Angela Kou
Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic
quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconductivity. Extending the capabilities of hybrid flux-based circuits to work in magnetic fields would be especially useful both as a probe of spin-polarized Andreev bound states and as a possible platform for topological qubits. The fluxonium is particularly suitable as a readout circuit for topological qubits due to its unique persistent-current based eigenstates. In this Letter, we present a magnetic-field compatible hybrid fluxonium with an electrostatically-tuned semiconducting nanowire as its non-linear element. We operate the fluxonium in magnetic fields up to 1T and use it to observe the φ0-Josephson effect. This combination of gate-tunability and field-compatibility opens avenues for the exploration and control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits.
arxiv pdf