Johannes Fink

A gate tunable transmon qubit in planar Ge

  1. Oliver Sagi,
  2. Alessandro Crippa,
  3. Marco Valentini,
  4. Marian Janik,
  5. Levon Baghumyan,
  6. Giorgio Fabris,
  7. Lucky Kapoor,
  8. Farid Hassani,
  9. Johannes Fink,
  10. Stefano Calcaterra,
  11. Daniel Chrastina,
  12. Giovanni Isella,
  13. and Georgios Katsaros
Gate-tunable transmons (gatemons) employing semiconductor Josephson junctions have recently emerged as building blocks for hybrid quantum circuits. In this study, we present a gatemon
fabricated in planar Germanium. We induce superconductivity in a two-dimensional hole gas by evaporating aluminum atop a thin spacer, which separates the superconductor from the Ge quantum well. The Josephson junction is then integrated into an Xmon circuit and capacitively coupled to a transmission line resonator. We showcase the qubit tunability in a broad frequency range with resonator and two-tone spectroscopy. Time-domain characterizations reveal energy relaxation and coherence times up to 75 ns. Our results, combined with the recent advances in the spin qubit field, pave the way towards novel hybrid and protected qubits in a group IV, CMOS-compatible material.
arxiv pdf

Geometric superinductance qubits: Controlling phase delocalization across a single Josephson junction

  1. Matilda Peruzzo,
  2. Farid Hassani,
  3. Gregory Szep,
  4. Andrea Trioni,
  5. Elena Redchenko,
  6. Martin Žemlička,
  7. and Johannes Fink
There are two elementary superconducting qubit types that derive directly from the quantum harmonic oscillator. In one the inductor is replaced by a nonlinear Josephson junction to
realize the widely used charge qubits with a compact phase variable and a discrete charge wavefunction. In the other the junction is added in parallel, which gives rise to an extended phase variable, continuous wavefunctions and a rich energy level structure due to the loop topology. While the corresponding rf-SQUID Hamiltonian was introduced as a quadratic, quasi-1D potential approximation to describe the fluxonium qubit implemented with long Josephson junction arrays, in this work we implement it directly using a linear superinductor formed by a single uninterrupted aluminum wire. We present a large variety of qubits all stemming from the same circuit but with drastically different characteristic energy scales. This includes flux and fluxonium qubits but also the recently introduced quasi-charge qubit with strongly enhanced zero point phase fluctuations and a heavily suppressed flux dispersion. The use of a geometric inductor results in high precision of the inductive and capacitive energy as guaranteed by top-down lithography – a key ingredient for intrinsically protected superconducting qubits. The geometric fluxonium also exhibits a large magnetic dipole, which renders it an interesting new candidate for quantum sensing applications.
arxiv pdf