Phase-shift flux qubit with a ferromagnetic π junction*

A flux qubit with high anharmonicity is an attractive choice for large-scale superconducting quantum circuits. However, one of the biggest challenges is the necessity of an external flux bias corresponding to half flux quantum to achieve flux-insensitive point with the longest coherence time for each flux qubit.
In light of this, we developed phase-shift flux qubits by integrating a ferromagnetic π junction in the qubit loop providing spontaneous π phase shift, enabling flux-bias-free operations [1]. The developed qubit consists of a superconducting loop with three Al/AlO_x/Al junctions and an NbN/CuNi/NbN π junction [2] on an Si substrate. The qubit is shunted to a large capacitor and is capacitively coupled to a coplanar waveguide resonator made of NbN/TiN. Our results indicate that the phase-shift qubits are indeed at their flux-insensitive point without any external flux, with a relatively long coherence. In this talk, we will discuss the NbN-Al hybrid fabrication process and the results of the spectroscopy and time-domain measurements.
[1] T. Yamashita et al., Phys. Rev. Lett. 95, 097001 (2005).
[2] T. Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017).