Time-domain measurements of an ultra-strongly coupled qubit-resonator circuit*

In cavity- and circuit QED, when the coupling energy becomes ~10 % or more of a photon and qubit energies (ultra- to deep-strong coupling regime), interesting physics are predicted [1]. So far, ultra- and deep-strong coupling [2-5] were achieved and the transition energy spectra of their circuits were studied. In this work, we have measured the coherence properties of an ultra-strongly coupled flux qubit-resonator circuit by driving the qubit while probing the resonator transition, which is also flux-bias dependent due to its very strong coupling to the qubit. The coherence times of ~1 μs were obtained for the qubit at the optimal point. In the presentation, a possible origin of the decoherence will also be discussed.
[1] S. Ashhab and Franco Nori, Phys. Rev. A 81, 042311 (2010). [2] T. Niemczyk, et al., Nature Phys. 6, 772 (2010). [3] P. Forn-Diaz, et al., Phys. Rev. Lett. 105, 237001 (2010). [4] P. Forn-Diaz, et al., Nature Phys. 13, 39 (2017). [5] F. Yoshihara, T. Fuse, et al., Nature Phys. 13, 44 (2017).