The exponential performance improvement of modern computers approaches technological and economic limits of the semiconductor microfabrication. We are no longer able to enjoy the "free lunch" of performance improvement by the increase in the number of transistors. Meanwhile, computational demands, such as big data analytics, media processing, deep learning, combinatorial optimization, secure cloud computing, are expected to increase, thus great social expectation is given to the improvement of the capability of modern computer systems. It is urgent to continually improve the performance without increasing the number of transistors by using a new computing paradigm, novel programming models, new algorithms and software, nonconventional architectures, devices and materials, and so on.

Reflecting such a research trend headed to the post-Moore's-law era, "quantum computer" is attracting academia and industry in recent years. If quantum computer operates according to the theory, it is possible to perform essentially faster computation than the modern computer (or so-called "classical" computer). However, this quantum speedup has never been proven by experiment at the present time. There is still a big gap between the situation of real machines and the number of the qubit (quantum bit) and fidelity of the control gate required by typical quantum algorithms, such as Shor's factoring and Grover's search algorithms.

In order to fill this gap, it is necessary to enhance the research and development of quantum software and architecture and strengthen the whole quantum computing research from quantum information theory to quantum hardwares. The research targets are (1) development, implementation, and demonstration of quantum-classical hybrid algorithms, (2) preparation of quantum software development environment including language, compiler, debugger, and simulator, and (3) quantum computer architecture design based on quantum error correction code.

In particular, for NISQ² computers, it is essential to explore and discover new algorithms and killer applications as to what kind of problems can take benefit from quantum computing. To that end, it is necessary to construct a software development platform packaged with various tools and simulators that make the quantum programs executable in a trial-and-error manner. It is also important to tackle research topics that span hardware and software. More specifically, this includes the quantum computer architectures that implement quantum error-correcting code, the development of middleware and firmware to support the implementation of error correction codes, electrical engineering on control electronics for precise control and measurement of qubits.

The field of quantum information processing has been growing as a part of physics, but the trend of worldwide R&D on the quantum computing is gradually shifting to the engineering challenge of "How to build quantum computers". This may be fully answered only by neither physics nor computer science.

From now on, "quantum computer science" will play an important role as a guiding principle for building the quantum computer. The various players are needed in each aspect of interdisciplinary integration and collaboration, the participation from industry and developers community, and partnership with international collaborators. Because it is difficult for any player to prepare the necessary technology and personnel in full stack, it is key to exchanging everything necessary for the realization of the quantum computer such as knowledge, technology, human resources, over barriers of disciplines and affiliations.

Although the great economic and social impact of realization of the quantum computer is expected, its R&D requires a long-term perspective and thus it is still too uncertainty and risky for many private companies. Therefore, the investment on the quantum computer from the market will be insufficient. Thus, the government should take action on the promotion of inevitable multidisciplinary research on quantum hardware and software, establish new R&D centers as a hub for the network, the provision of a quantum software development environment, and fostering quantum computing community and business ecosystem.

In addition to the promotion of R&D by "quantum-native" researchers and engineers, it is necessary to provide education and training programs to people who are not so familiar with quantum mechanics. The quantum computing community has a responsibility to provide accurate and aggressive outreach and engagement to the society. Supporting the quantum start-up companies to spin out is also important to the formation of quantum computing ecosystem.

In the next 50 years, we will enter the era of "quantum ICT", where we can freely use and combine various quantum technologies such as quantum sensors, quantum net-work, quantum clock, quantum cryptography, and quantum computers. In this era, we should not only see the great advancement of quantum chemical calculation and quantum machine learning as discussed in this proposal but also the creation of new quantum ap-plications, services, and industries. At that time, scientific research and discovery using quantum computers will not be science fiction anymore.

We should strongly accelerate and promote quantum computer science towards the quantum ICT era beyond the NISQ era and advance a solid step to the realization of the scalable fault-tolerant quantum computer.