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

When Feb 22, 2019
from 02:00 PM to 03:00 PM
Where Goldsmiths 2
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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:

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).

The study of functional materials has underpinned the enormous changes in information technology and electronic systems seen in the past decades. Research in the Department on device materials spans many of the most exciting areas in which the functional properties of new materials are being understood and developed.

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