All-nitride superconducting qubit on a silicon substrate

September 20, 2021 // By Nick Flaherty
All-nitride superconducting qubit on a silicon substrate
NICT in Japan has developed a platform for large-scale integration of superconducting qubits using an all-nitride qubit on a silicon substrate

Researchers at the National Institute of Information and Communications Technology (NICT) in Japan have developed an all-nitride superconducting qubit using epitaxial growth on a silicon substrate that can scale for large quantum computers.

The niobium nitride (NbN) device 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.

The key to the qubit is that it does not use aluminium as the conductive material. It has a superconducting transition temperature of 16 K (-257 °C) with aluminium nitride (AlN) for the insulating layer of the Josephson junction. This is 15 degrees warmer than required for other qubit structures.

By building this qubit on a silicon substrate, long coherence times have been obtained: an energy relaxation time (T1) of 16 microseconds and a phase relaxation time (T2) of 22 microseconds as the mean values. This is about 32 times T1 and about 44 times T2 of nitride superconducting qubits grown on a conventional magnesium oxide substrate.

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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 Communications Materials.

The quantum superposition state, which is indispensable for the operation of a quantum computer, is easily destroyed by various disturbances (noise), and it is necessary to properly eliminate these effects. The challenge is how to extend the coherence time, which is the lifetime of quantum superposition states.

This is  first time that anyone in the world has succeeded in observing coherence times in the tens of microseconds from nitride superconducting qubits by reducing dielectric loss by epitaxially growing them on a Si substrate. The superconducting qubit of this nitride is still in the early stages of development, and we believe that it is possible to further improve the coherence time by optimizing the design and fabrication process of the qubit.

Using this new material platform that may replace conventional aluminium and can accelerate research and development of quantum information processing.

The next step is to optimise the circuit structure and fabrication process to extend coherence time and improve the uniformity of device characteristics for large-scale integration.

10.1038/s43246-021-00204-4

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