Thomas Fösel

Contacting Josephson Junctions via Airbridges in Superconducting Circuits

  1. Prakiran Baidya,
  2. Momčilo Milosavljević,
  3. Murali Krishna Kurmapu,
  4. Thomas Fösel,
  5. Harshanth Ram Murugesan,
  6. Victor Kemme,
  7. Mojahed Jaber,
  8. Markus Sondermann,
  9. and Christopher Eichler
Superconducting circuit devices require electrical interconnects between different circuit elements on the chip, for which conventional device architectures use a combination of two
structural elements: \textit{airbridges} to connect non-adjacent elements in the base layer, and \textit{bandages} to connect the electrodes forming the Josephson junctions to the base layer. Bandages introduce unwanted parasitic material interfaces and increase the manufacturing complexity. Here, we overcome the limitations imposed by \emph{bandages} by establishing \textit{all} electrical interconnects with airbridges of varying size fabricated in a single step. The airbridges show a high yield and mechanical stability over a wide range of sizes from 0.5μm to 4μm in width and from 5μm to 40μm in length, and show low loss when integrated in coplanar waveguide resonators and transmon qubits. Measured relaxation times up to more than 250μs in standard transmon geometries show that the process achieves high coherence while substantially easing and accelerating device fabrication.
arxiv pdf

Efficient cavity control with SNAP gates

  1. Thomas Fösel,
  2. Stefan Krastanov,
  3. Florian Marquardt,
  4. and Liang Jiang
Microwave cavities coupled to superconducting qubits have been demonstrated to be a promising platform for quantum information processing. A major challenge in this setup is to realize
universal control over the cavity. A promising approach are selective number-dependent arbitrary phase (SNAP) gates combined with cavity displacements. It has been proven that this is a universal gate set, but a central question remained open so far: how can a given target operation be realized efficiently with a sequence of these operations. In this work, we present a practical scheme to address this problem. It involves a hierarchical strategy to insert new gates into a sequence, followed by a co-optimization of the control parameters, which generates short high-fidelity sequences. For a broad range of experimentally relevant applications, we find that they can be implemented with 3 to 4 SNAP gates, compared to up to 50 with previously known techniques.
arxiv pdf