The 1920's saw the development of quantum theory to explain distinctly non-classical (non-Newtonian) physical phenomena. Since then, experimental breakthroughs have greatly increased our control over microscopic systems. Not only has this led to increasingly precise confirmation of quantum physics as our best description of the world at small energy scales, it has also revealed potential practical advantages in the emerging field of quantum technology.

Under this framework, quantum technologies encounter a total new way of dealing with information, leading to improvements beyond what was possible within the classical formalism. New technologies and devices are starting to be designed to push the limits of computation (quantum computing), to rigorously simulate Nature’s processes (quantum simulation), to allow for secure communication (quantum networks and internet) or to enhance the precision of physical measurements (quantum sensing). At the Global Research Center for Quantum Information Science we work actively to collaborate in the construction of this huge quantum puzzle whose extension is so vast that may well end up impacting science and technology in forms that we have yet not been able to imagine.

# RESEARCH

##### NII

###### national institute of informatics

###### 2004-2020

# QUANTUM CENTER

## Research Topics

### Quantum Computing Architectures

The particular architecture underlying a quantum computer will depend both on the physical platforms in which qubits are implemented and on the methods of encoding employed for error suppression. Near-term implementations are of necessity restricted to homogeneous, single-qubit, nearest-neighbor embeddings in 2D lattices. However, as quantum advantage draws nearer to reality with each passing year, we observe a corresponding increase in the variety of proposed computer architectures. Here we have a particular interest in the interplay between architecture, error correction and computational cost, with a focus on the relative merits of distributed and inhomogeneous, modular schemes for quantum computing.

### Quantum Compilation

Even though global efforts are pushing towards the design and implementation of a fault-tolerant large-scale quantum computer, its hardware is still far from reality due to the technical challenge of achieving large qubit redundancies. However an efficient compilation and optimization of quantum circuits can bring one step forward this regime of quantum practicality. In our group we tackle the general question of parsing a quantum problem from a high-level description into machine language for large-scale applications. We develop techniques from the software side that allow us to reduce the resources in terms of number of qubits and time complexity associated to fault-tolerant quantum circuits as well as envisage effective translation methods between languages that are capable of representing quantum processes.

### Quantum Simulation

The simulation of Nature’s processes and their translation into algorithmic procedures has been one of the main applications of computation. This not only has an impact in the sense of basic research but also presents a wide applicability in a range of technological areas. Yet the fact that Nature is quantum sets a practical limitation on its complete simulation. The number of variables needed to tackle problems related to quantum phenomena grows exponentially with the size of the system and no classical computational paradigm has yet been postulated for an efficient and accurate complete description of such problems. In our group we use quantum mechanics in our favor and propose the design of quantum devices as computers or single-purpose simulators able to mimic specific large and complex problems, either classical or quantum, that would otherwise be untreatable by their classical counterparts.

### Quantum Networks and the Internet

Quantum networks will be a fundamental step towards the implementation, in the near future, of several quantum technologies, such as quantum key distribution, distributed quantum computing, sensing and more. In addition to the difficulties in distributing quantum states over long distances, quantum networks will have to connect many different nodes and allocate quantum resources over a complex network. Building efficient and scalable quantum networks remains a significant theoretical and experimental challenge. In our group we developed a promising technique, called quantum multiplexing, that allows to drastically reduce the number of physical resources required to distribute information encoded with photons over small and large distances. We do believe that with such a method the feasibility of quantum technologies that rely on sharing information between nodes of a quantum network system can be dramatically improved.

### Quantum Sensing

When the technology becomes more and more precise, we would eventually face the fundamental limit where the nature does not allow us to go further. At this limit, it looks as if quantum nature gives noise which cannot be eliminated. This noise is called quantum noise, and the fundamental limitation given by this noise is called the quantum standard limit. Quantum sensing is to break this limit and allows us to achieve higher sensitivity. In the last two decades, researchers have revealed how such sensing can be done in an ideal situation, however it is still a question how we can achieve it in the real world. Quantum sensing can be extremely sensitive, but is also fragile. It is a big question how we can achieve both the high sensitivity and robustness. In our group, we design new quantum sensing schemes and evaluate their performance and feasibility. Considering quantum sensing as one element in the quantum internet, there would be a large number of applications not yet even considered. At the implementation side, quantum sensing tends to be smaller in the system size and hence can be implemented in the near future. In fact, quantum sensing could be the first application that shows the quantum advantage of quantum information processing.