Tokyo, March 24, 2023 - A consortium of joint research partners including RIKEN, the National Institute of Advanced Industrial Science and Technology (AIST), the National Institute of Information and Communications Technology (NICT), Osaka University, Fujitsu Limited (Fujitsu) and Nippon Telegraph and Telephone Corporation (NTT) announced the successful development of Japan’s first superconducting quantum computer. Starting March 27, 2023, the partners will provide the newly developed technology to users in Japan as a cloud service for non-commercial use under a joint research agreement with RIKEN.
The new technology represents a significant step toward the wider use of quantum computing in Japan.
The joint research group is comprised of: Dr. Yasunobu Nakamura, Director of the RIKEN Center for Quantum Computing (RQC), Dr. Katsuya Kikuchi, Group Leader of the 3D Integration System Group of the Device Technology Research Institute at AIST, Dr. Hirotaka Terai, Director of the Superconductive ICT Device Laboratory at the Kobe Frontier Research Center of the Advanced ICT Research Institute of NICT, Dr. Masahiro Kitagawa, Director of the Center for Quantum Information and Quantum Biology at Osaka University, Dr. Shintaro Sato, Head of the Quantum Laboratory at Fujitsu Research of Fujitsu, and Dr. Yuuki Tokunaga, Distinguished Researcher at NTT Computer & Data Science Laboratories.
Superconducting quantum computer developed at RIKEN
Dawn of the Quantum Age: a new frontier in computing technology
Since the early twentieth century, quantum mechanics has been attracting attention as a fundamental theory of physics, laying the foundation for the development of various scientific fields. In particular, phenomena including quantum superposition and quantum entanglement have contributed significantly to the development of today’s science and technology. From the perspective of quantum information science, however, the potential of quantum mechanics has not yet been fully exploited. Thus, R&D to apply the fundamental principles of quantum mechanics to the technological fields of computation, communications, and measurement have been gaining momentum worldwide.
To further promote research in quantum information science, RIKEN established the Macroscopic Quantum Coherence Research Team under the lead of Dr. Jaw-Shen Tsai in 2001, and the RIKEN Center for Quantum Computing under the lead of Dr. Yasunobu Nakamura in 2021, focusing on R&D in quantum computing. The Center for Quantum Computing conducts R&D not only on superconducting quantum computer hardware, but also on various physical systems including photonics research (led by Dr. Akira Furusawa), semiconductor methods (led by Dr. Seigo Tarucha), and methods using atoms in a vacuum. RIKEN is further promoting R&D in quantum software including quantum computing theory, quantum algorithms and quantum architecture, thus covering the entire spectrum of R&D in quantum computing.
In 2021, RIKEN and Fujitsu established the "RIKEN RQC-Fujitsu Collaboration Center" within the RQC. Research results were also utilized within the newly developed superconducting quantum computer cloud service.
RIKEN and Fujitsu will further leverage their expertise in computing technologies and applied quantum technology to provide a superconducting quantum computer for industrial use by the end of FY 2023.
Research Methods and Results
The newly developed superconducting quantum computer uses integrated circuits with 64 qubits with two special features: two-dimensional integrated circuits and perpendicular wiring packages.
In the two-dimensional integrated circuits, four qubits are arrayed in a square, with each connected to its neighbors by inter-qubit connections (upper right of Fig. 1). In addition, readout resonators and filter circuits for multiple readout are arranged in the square. This basic unit consisting of four qubits can be arranged in two dimensions to form a qubit integrated circuit. The 64 qubit integrated circuit consists of 16 of these basic units formed on a two centimeter square silicon chip (Fig. 1).
Figure 1: 64 qubit integrated circuit chip
Left: 64 qubit two-dimensional integrated circuit chip performs quantum computation. The design has 16 basic units, each comprised of 4 qubits. The titanium nitride film, which is a superconductor, gives off a gold shine.
Upper right: schematic diagram of a basic unit of 4 qubits. Qubits are arranged at the four corners of a square, and a readout circuit designed in the middle.
Lower right: electron micrograph of a Josephson junction forming a qubit.
Upper right: schematic diagram of a basic unit of 4 qubits. Qubits are arranged at the four corners of a square, and a readout circuit designed in the middle.
Lower right: electron micrograph of a Josephson junction forming a qubit.
Furthermore, if the wiring is on the same planar surface as the qubits in the system, the space of the wiring to the outside is insufficient for the number of qubits arrayed in the chip, so it is necessary to devise methods to control individual qubits and wiring for readout. Therefore, the joint research group adopted a design based on perpendicular wiring packages, in which the wiring to the chip for the qubits arrayed on a two-dimensional planar surface is perpendicularly connected. The research team is further developing a wiring package that enables the wiring to the qubit integrated circuit chip at once (Fig. 2).
Taking advantage of the features of the two-dimensional integrated circuits and perpendicular wiring packages, a highly scalable system is formed in which the number of qubits can be easily increased. This enables the large-scale of the newly developed system without altering the basic design.
Figure 2: Perpendicular wiring packages
Left: schematic diagram of perpendicular wiring. The control and readout wiring for the qubits is perpendicularly connected to the chip via a contact probe for signals. Microwave signals are transmitted and received through the wiring.
Right: wiring package for the qubit integrated circuit chip.
Right: wiring package for the qubit integrated circuit chip.
The signals controlling the qubits use voltage pulses that oscillate at microwave frequencies (8-9 GHz). However, as each qubit requires a different microwave frequency, the joint research group developed a controller that can generate stable microwave phases with high precision (Fig. 3). The team also developed software for the controller that controls the qubits.
Figure 3: Qubit controller
Qubit controller consisting of an oscillator and receiver for microwave signals. The new 64 qubit quantum computer uses 96 input wires and 16 output wires for control and readouts to perform quantum calculations.
Starting March 27, 2023, RIKEN is providing the newly developed superconducting quantum computing technology as a “quantum computing cloud service” (Fig. 4) for non-commercial use to researchers and engineers in Japan under joint research agreements. Users will be able to access the new superconducting quantum computer for jobs within the scope of the joint research agreement and can send data/receive results via the cloud.
Figure 4: Image of user access to superconducting quantum computer
Authentication of registered users and job transmission/reception on a web interface.
Moving forward, the joint research group will further enhance the new system to enable quantum computing operations with a higher number of qubits and increase the density of wiring inside the dilution refrigerator (Fig. 5). The research team will further provide access to the superconducting quantum computer as a testbed for noisy intermediate-scale quantum (NISQ) computers.
Based on the newly developed services, the research partners will further accelerate R&D in quantum computing through deeper collaboration with quantum software developers, quantum computing researchers, and corporate developers.
Figure 5: Wiring inside the dilution refrigerator of the 64 qubit superconducting quantum computer
A 64 qubit integrated circuit chip is placed in a central cylindrical magnetic shield to connect control and readout wiring. In operations, it is necessary to cool the periphery of the chip to about 10 mK (about -273 °C), so the entire chip is placed in a vacuum insulated container and cooled by a dilution refrigerator.
Future plans
Using the highly scalable integrated circuit (Fig. 6) as a core technology, the joint research team will continue to work toward the realization of quantum computers with 100 and 1,000 qubits, and promote further R&D toward the integration of 1 million qubits and the realization of fault-tolerant quantum computing for real world application.
Figure 6: Future image of qubit integrated circuits
By periodically arranging basic units composed of four qubits on a planar surface, the number of integrated qubits can be increased. The figure above shows the future image of 1,024 qubits by periodically arranging 64 qubits in a 4 x 4 array.
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