Joshua Y. Mutus

Alternating Bias Assisted Annealing of Amorphous Oxide Tunnel Junctions

  1. David P. Pappas,
  2. Mark Field,
  3. Cameron Kopas,
  4. Joel A. Howard,
  5. Xiqiao Wang,
  6. Ella Lachman,
  7. Lin Zhou,
  8. Jinsu Oh,
  9. Kameshwar Yadavalli,
  10. Eyob A. Sete,
  11. Andrew Bestwick,
  12. Matthew J. Kramer,
  13. and Joshua Y. Mutus
We demonstrate a transformational technique for controllably tuning the electrical properties of fabricated thermally oxidized amorphous aluminum-oxide tunnel junctions. Using conventional
test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature-dependent, and is independent of junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of ≈2×10−6, along with a significant reduction in resonant- and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, as well as the many other metal-insulator-metal structures used in modern electronics.
arxiv pdf

Modular Superconducting Qubit Architecture with a Multi-chip Tunable Coupler

  1. Mark Field,
  2. Angela Q. Chen,
  3. Ben Scharmann,
  4. Eyob A. Sete,
  5. Feyza Oruc,
  6. Kim Vu,
  7. Valentin Kosenko,
  8. Joshua Y. Mutus,
  9. Stefano Poletto,
  10. and Andrew Bestwick
We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multi-chip tunable
couplers using vacuum gap capacitors or superconducting indium bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.
arxiv pdf