Taro Yamashita | Ferromagnetic Josephson junctions and its application to quantum bits

Taro Yamashita1,2

1Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

2 JST-PRESTO, 4-1-8, Honcho, Kawaguchi, Saitama, 332-0012 Japan e-mail: yamashita@nuee.nagoya-u.ac.jp

Recently, superconductor/ferromagnet (SC/FM) hybrid systems have been studied actively because of their novel physics emerged by the interaction between the superconductivity and magnetism [1]. Especially, the π state, which appears in ferromagnetic Josephson junctions (SC/FM/SC junctions) due to the spatial oscillation of the superconducting order parameter in FM, is attractive as a phase shifter for superconducting devices. We are developing novel flux quantum bits (qubits) with a π junction [2,3]. Contrary to the conventional flux qubits, which require the external magnetic field corresponding to half flux quantum in the superconducting loop, the π-junction flux qubits can form the coherent two level states and operate without an external magnetic field due to the intrinsic phase shift of the π junction. Because the magnetic coil for applying the magnetic field is one possible noise source, it is expected that the coherence time is improved in the π-junction flux qubits. Furthermore, zero magnetic field operation provides merits for realizing a highly-integrated quantum computing system with many qubits. We adopted niobium nitride (NbN) which has a relatively smooth surface due to its epitaxial growth on a magnesium oxide or silicon substrate, as the superconducting material of the junction. Regarding the ferromagnetic and insulating barriers, we used copper nickel (CuNi) which is a diluted weak ferromagnet and aluminum nitride (AlN), respectively. We fabricated the NbN/CuNi/NbN and NbN/AlN/CuNi/NbN junctions and measured the temperature and CuNi thickness dependences of the Josephson critical current systematically. As a result, we observed distinct behaviors of the critical current around the 0-π transition CuNi thickness in the ferromagnetic Josephson junctions [4]. In the talk, we show the recent results on the π-junction flux qubits with the nitride-based ferromagnetic Josephson junctions.

[1] J. Linder and J.W.A Robinson, Nat. Phys. 11, 307 (2015).

[2] T. Yamashita et al., Phys. Rev. Lett. 95, 097001 (2005).

[3] T. Yamashita et al., Appl. Phys. Lett. 88, 132501 (2006).

[4] T. Yamashita et al., Phys. Rev. Appl. 8, 054028 (2017).