All-nitride superconducting qubit made on a silicon substrate
New material platform for large-scale integration of superconducting
qubits

Date:
September 20, 2021
Source:
National Institute of Information and Communications Technology
(NICT)
Summary:
Researchers 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. This qubit
uses niobium nitride (NbN) with a superconducting transition
temperature of 16 K (-257 DEGC) 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 (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.



FULL STORY ========================================================================== 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. This
qubit uses niobium nitride (NbN) with a superconducting transition
temperature of 16 K (-257 DEGC) 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 (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.


==========================================================================
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).

Background and Challenges Toward the coming future Society 5.0, there
are limits to the performance improvement of semiconductor circuits that
have supported the information society so far, and expectations for
quantum computers are rising as a new information processing paradigm
that breaks through such limits. However, 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.

Since superconducting qubits are solid-state elements, they have excellent design flexibility, integration, and scalability, but they are easily
affected by various disturbances in their surrounding environment. The challenge is how to extend the coherence time, which is the lifetime of
quantum superposition states. Various efforts are being made by research institutes around the world to overcome this problem, and most of them
use 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, and it was essential
to study materials that could solve this problem.



==========================================================================
As an alternative to aluminum and amorphous aluminum oxide with a superconducting transition temperature TC of 1 K (-272 DEGC), epitaxially
grown niobium nitride (NbN) with a TC of 16 K (-257 DEGC), NICT has
been developing superconducting qubits using NbN / AlN / NbN all-nitride junctions, focusing on aluminum nitride (AlN) as an insulating layer.

In order to realize a NbN / AlN / NbN Josephson junction (epitaxial
junction) in which the crystal orientation is aligned up to the upper electrode, it was necessary to use a magnesium oxide (MgO) substrate
whose crystal lattice constants are relatively close to those of
NbN. However, MgO has a large dielectric loss, and the coherence time
of the superconducting quantum bit using the NbN / AlN / NbN junction
on the MgO substrate was only about 0.5 microseconds.

Achievements NICT has succeeded in realizing NbN / AlN / NbN epitaxial Josephson junctions using titanium nitride (TiN) as a buffer layer on
a silicon (Si) substrate with a smaller dielectric loss. This time,
using this junction fabrication technology, we designed, fabricated,
and evaluated a superconducting qubitthat uses NbN as the electrode
material and AlN as the insulating layer of the Josephson junction.

The quantum circuit is fabricated on a silicon substrate so that the
microwave cavity and the qubit can be coupled and interact with each
other. From the transmission measurement of the microwave characteristics
of the resonator weakly coupled to the qubit under small thermal
fluctuation at the extremely low temperature of 10 mK, we achieved an
energy relaxation time (T1) of 18 microseconds and a phase relaxation
time (T2) of 23 microseconds. The mean values for 100 measurements are
T1=16 microseconds and T2= 22 microseconds.

This is an improvement of about 32 times for T1 and about 44 times for
T2 compared to the case of superconducting qubits on MgO substrates.

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.

Using this new material platform that may replace conventional aluminum,
we will accelerate research and development of quantum information
processing, which will contribute to the realization of more power-saving information processing and the realization of quantum nodes necessary
for the construction of safe and secure quantum networks.

Prospects 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.

========================================================================== Story Source: Materials provided by National_Institute_of_Information_and_Communications
Technology_(NICT). Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Sunmi Kim, Hirotaka Terai, Taro Yamashita, Wei Qiu, Tomoko Fuse,
Fumiki
Yoshihara, Sahel Ashhab, Kunihiro Inomata, Kouichi Semba. Enhanced
coherence of all-nitride superconducting qubits epitaxially grown
on silicon substrate. Communications Materials, 2021; 2 (1) DOI:
10.1038/ s43246-021-00204-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210920100915.htm

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