Martin Spiecker

Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency

  1. Andre Schneider,
  2. Tim Wolz,
  3. Marco Pfirrmann,
  4. Martin Spiecker,
  5. Hannes Rotzinger,
  6. Alexey V. Ustinov,
  7. and Martin Weides
We report on spectroscopic and time-domain measurements on a fixed-frequency concentric transmon qubit in an applied in-plane magnetic field to explore its limits of magnetic field
compatibility. We demonstrate quantum coherence of the qubit up to field values of B=40mT, even without an optimized chip design or material combination of the qubit. The dephasing rate Γφ is shown to be not affected by the magnetic field in a broad range of the qubit transition frequency. For the evolution of the qubit transition frequency, we find the unintended second junction created in the shadow angle evaporation process to be non-negligible and deduce an analytic formula for the field-dependent qubit energies. We discuss the relevant field-dependent loss channels, which can not be distinguished by our measurements, inviting further theoretical and experimental investigation. Using well-known and well-studied standard components of the superconducting quantum architecture, we are able to reach a field regime relevant for quantum sensing and hybrid applications of magnetic spins and spin systems.
arxiv pdf

Granular aluminum: A superconducting material for high impedance quantum circuits

  1. Lukas Grünhaupt,
  2. Martin Spiecker,
  3. Daria Gusenkova,
  4. Nataliya Maleeva,
  5. Sebastian T. Skacel,
  6. Ivan Takmakov,
  7. Francesco Valenti,
  8. Patrick Winkel,
  9. Hannes Rotzinger,
  10. Alexey V. Ustinov,
  11. and Ioan M. Pop
Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, of about 100 Ohm, and small anharmonicity,
which can limit their coherence and logic gate fidelity. A promising alternative are circuits based on so-called superinductors, with characteristic impedances exceeding the resistance quantum RQ=6.4 kΩ. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here we present a fluxonium qubit design using a granular aluminum (grAl) superinductor strip. Granular aluminum is a particularly attractive material, as it self-assembles into an effective junction array with a remarkably high kinetic inductance, and its fabrication can be in-situ integrated with standard aluminum circuit processing. The measured qubit coherence time TR2 up to 30 μs illustrates the potential of grAl for applications ranging from protected qubit designs to quantum limited amplifiers and detectors.
arxiv pdf