Capacitively shunted flux qubits embeded in a 3D cavity*

Signigicant worldwide efforts have been undertaken to improve the coherence time of superconducting qubits. Recently a 3D cavity architecture has been developed to drastically elongate the coherence time of transmon qubits to 92 μs [1]. On the other hand, flux qubits were also improved using high quality shunt capacitors [2]. A coherence time of 85 μs has been oberved. In this case the C-shunt flux qubit was coupled to a 2D coplanar waveguide resonator to perform a dispersive qubit readout. A 3D cavity is expected to provide a cleaner electromagnetic environment for qubits compared to the typical coplanar resonator. Therefore we put a C-shunt flux qubit into a 3D cavity to improve its coherence. We have fabricated a C-shunt flux qubit on a sapphire substrate. The third junction of the qubit is 0.45 times smaller than the other two. From spectroscopy measurements, we obtain energy of a typical junction E_J/h = 192 GHz, charging energy E_C/h = 2 GHz, and a shunt capacitor C_S = 50 fF. These are close to the designed values. In this talk I will discuss about coherence properties of the C-shunt flux qubit.
[1] H. Paik, et al., PRL 107, 240501 (2011), H. Paik, et al., PRL 107, 240501 (2011).
[2] F. Yan, et al., Nat. Commun. 7, 12964 (2017).