A. Tomonaga

Efficient spectrum analysis for multi-junction nonlinear superconducting circuit

  1. A. Tomonaga,
  2. H. Mukai,
  3. K. Mizuno,
  4. and J. S. Tsai
The extraction of transition frequencies from a spectrum has conventionally relied on empirical methods, and particularly in complex systems it is a time-consuming and cumbersome process.
To address this challenge, we establish an semi-automated efficient and precise spectrum analysis method. It, at first, employs image processing methods to extract transition frequencies, subsequently estimates Hamiltonians of superconducting quantum circuit containing multiple Josephson junctions. Additionally, we determine the suitable range of approximations in simulation methods, evaluating the physical reliability of analyses.
arxiv pdf

Ultrastrong tunable coupler between superconducting LC resonators

  1. T. Miyanaga,
  2. A. Tomonaga,
  3. H. Ito,
  4. H. Mukai,
  5. and J. S. Tsai
We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the wide range tunability is achieved both antiferromagnetics
and ferromagnetics from -1086 MHz to 604 MHz. Ultrastrong coupler is composed of rf-SQUID and dc-SQUID as tunable junctions, which connected to resonators via shared aluminum thin film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or NISQ devices with dense connections between qubits.
arxiv pdf