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All-nitride superconducting qubit made on a silicon substrate

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Researchers at the National Institute of Information and Communications Technology (NICT, President: Tokuda Hideyuki, Ph.D.), in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology (AIST, President: Dr. Ishimura Kazuhiko) and the Tokai National Higher Education and Research System Nagoya University (President: Dr. Matsuo Seiichi) have succeeded in developing an all-nitride superconducting qubit using epitaxial growth on a silicon substrate that does not use aluminum as the conductive material.
(a) Conceptual diagram of microwave cavity and qubit (b) Optical micrograph of nitride superconducting qubit circuit (c) Electron micrograph of nitride superconducting qubit (part) and cross-sectional view of the device (d) Transmission electron micrograph of epitaxially grown nitride Josephson junction. Credit: National Institute of Information and Communications Technology, National Institute of Advanced Industrial Science and Technology, and Nagoya University

This qubit uses niobium nitride (NbN) with a superconducting transition temperature of 16 K (-257 °C) as the electrode material, and aluminum nitride (AlN) for the insulating layer of the Josephson junction. It is a new type of qubit made of all-nitride materials grown epitaxially on a silicon substrate and free of any amorphous oxides, which are a major noise source. By realizing this new material qubit on a silicon substrate, long coherence times have been obtained: an energy relaxation time (T 1 ) of 16 microseconds and a phase relaxation time (T 2 ) of 22 microseconds as the mean values. This is about 32 times T 1 and about 44 times T 2 of nitride superconducting qubits grown on a conventional magnesium oxide substrate.

By using niobium nitride as a superconductor, it is possible to construct a superconducting quantum circuit that operates more stably, and it is expected to contribute to the development of quantum computers and quantum nodes as basic elements of quantum computation. We will continue to work on optimizing the circuit structure and fabrication process, and we will proceed with research and development to further extend the coherence time and realize large-scale integration.

These results were published in the British scientific journal Communications Materials on September 20 2021 at 18:00 (Japan standard time).

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