Shunsuke Kamimura

Fluxon Time-Delay Readout of a Superconducting Qubit Protected by a Spectral Gap in a Josephson Transmission Line

  1. Shunsuke Kamimura,
  2. Aree Taguchi,
  3. Masamitsu Tanaka,
  4. and Tsuyoshi Yamamoto
We theoretically investigate a readout scheme of the quantum state of a superconducting qubit based on time delay of a single flux quantum (SFQ), also known as a fluxon, propagating
in a Josephson transmission line (JTL). We concretely study the time-delay readout based on capacitive coupling between a transmon qubit and a JTL, and we evaluate the time delay depending on the qubit state. We also reveal a feature of the absence of fluxon pinning and exponential suppression of nonadiabatic transitions caused by the propagating fluxon, which is advantageous for the time-delay readout. We extend the analysis to a multi-level transmon as well. Owing to the spectral gap in the JTL, the radiative decay of the qubit mediated by the JTL is exponentially suppressed, and thus the transmission line itself also serves as a filter protecting the qubit. The readout scheme requires neither complicated wiring to low-temperature stages nor bulky microwave components, which are bottlenecks for integration of a large-scale superconducting quantum computer.
arxiv pdf

Stabilization of Kerr-cat qubits with quantum circuit refrigerator

  1. Shumpei Masuda,
  2. Shunsuke Kamimura,
  3. Tsuyoshi Yamamoto,
  4. Takaaki Aoki,
  5. and Akiyoshi Tomonaga
A periodically-driven superconducting nonlinear resonator can implement a Kerr-cat qubit, which provides a promising route to a quantum computer with a long lifetime. However, the system
is vulnerable to pure dephasing, which causes unwanted excitations outside the qubit subspace. Therefore, we require a refrigeration technology which confines the system in the qubit subspace. We theoretically study on-chip refrigeration for Kerr-cat qubits based on photon-assisted electron tunneling at tunneling junctions, called quantum circuit refrigerator (QCR). Rates of QCR-induced deexcitations of the system can be changed by more than four orders of magnitude by tuning a bias voltage across the tunneling junctions. Unwanted QCR-induced bit flips are greatly suppressed due to quantum interference in the tunneling process, and thus the long lifetime is preserved. The QCR can serve as a tunable dissipation source which stabilizes Kerr-cat qubits.
arxiv pdf