Deep strong coupling in a circuit QED system (2) - experiment -

Among a variety of cavity/circuit-QED systems, the superconducting flux qubit is a promising candidate for increasing the coupling strength further because of its huge magnetic moment. Using a flux qubit, $g / \omega_r = 0.12$ ($g$: coupling strength, $\omega_r$: bare resonator frequency) has been reported [1]. However, $g / \omega_r$ is still lower than 1. Here, instead of the widely used coplanar waveguide (CPW) resonators, we use a lumped-element resonator consisting of an inductor (L) and a capacitor (C). While CPW resonators are distributed-element circuits and are therefore restricted by impedance matching constraints, one can freely choose the ratio, $L/C$, of a lumped-element resonator. This allows us to design a much smaller inductance and to make the zero-point current fluctuation much larger. Using a flux qubit and a lumped-element resonator, we have achieved $g / \omega_r$ comparable to or larger than 1, which is the deep strong coupling regime, where a variety of interesting physics is expected [2]. In this presentation, the sample design and spectroscopy data will be shown. [1] T. Niemczyk et al., nature physics 6, 772 (2010). [2] S. Ashhab and F. Nori, PRA 81, 042311 (2010).