Takaaki Aoki

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

Control of the ZZ coupling between Kerr-cat qubits via transmon couplers

  1. Takaaki Aoki,
  2. Taro Kanao,
  3. Hayato Goto,
  4. Shiro Kawabata,
  5. and Shumpei Masuda
Kerr-cat qubits are a promising candidate for fault-tolerant quantum computers owing to the biased nature of errors. The ZZ coupling between the qubits can be utilized for a two-qubit
entangling gate, but the residual coupling causes unnecessary always-on gates and crosstalk. In order to resolve this problem, we propose a tunable ZZ-coupling scheme using two transmon couplers. By setting the detunings of the two couplers at opposite values, the residual ZZ couplings via the two couplers cancel each other out. We also apply our scheme to the Rzz(Θ) gate (ZZ rotation with angle Θ), one of the two-qubit entangling gates. We numerically show that the fidelity of the Rzz(−π/2) gate is higher than 99.9% in a case of 16 ns gate time and without decoherence.
arxiv pdf