All-Nitride Superconducting Qubit For Quantum Computers


Researchers developed an all-nitride superconducting qubit on a silicon substrate, making it possible to create superconducting quantum circuits that operate more stably.
With existing semiconductor technology, there are limits to the performance improvement of semiconductor circuits. And therefore, the expectations for quantum computers are rising as a new information processing paradigm that breaks through such limits. However, the qubits and the quantum superposition state, the form in which the energy is stored in quantum information, is indispensable for the operation of a quantum computer, and is easily destroyed by various disturbances (noise), and it is necessary to properly eliminate these effects.
The superconducting qubits have excellent design flexibility, integration, and scalability, but they are easily affected by various disturbances in their surrounding environment. Therefore, it is now a challenge to extend the lifetimes of quantum superposition states.
Various researchers have used aluminum (Al) and aluminum oxide film (AlOx) as superconducting qubit materials. However, amorphous aluminum oxide, which is often used as an insulating layer, is a concern as a noise source.
As an alternative, epitaxially grown niobium nitride (NbN) with a superconducting transition temperature of 16 K can be used. 
Researchers at the National Institute of Information and Communications Technology, in collaboration with researchers at the National Institute of Advanced Industrial Science and Technology and the Tokai National Higher Education and Research System Nagoya University have developed an all-nitride superconducting qubit using epitaxial growth on a silicon substrate that does not use aluminum.
By realizing this new material 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.
“For this result, we did not use conventional aluminum and aluminum oxide for the Josephson junction, which is the heart of superconducting qubits. We have succeeded in developing a nitride superconducting qubit that has a high superconducting critical temperature TC and excellent crystallinity due to epitaxial growth. These two points have great significance. In particular, it is the 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.”
Researchers believe that 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.
They say, “We plan to work on optimizing the circuit structure and fabrication process with the aim of further extending the coherence time and improving the uniformity of device characteristics in anticipation of future large-scale integration. In this way, we aim to build a new platform for quantum hardware that surpasses the performance of conventional aluminum-based qubits.”
The research appeared in the journal Communication Materials.