Quantum computers are fundamentally more efficient than classical computers at certain tasks, including prime factorization and the simulation of molecules. This provides an enormous potential advantage for applications such as drug design and optimization of sustainable supply chains. However, this potential can only be realized if we learn how to program a quantum computer, that is, design efficient quantum algorithms, verify and test quantum software, and compile it onto physical quantum platforms. Hence, quantum algorithms and software technology bridges the gap between quantum computers and applications.

The broad aim of this research theme is to develop all theory, techniques, and tools that are required for an efficient quantum software pipeline, which can realize the potential of quantum algorithms and demonstrate their application. We expect to adapt and apply many traditional computer science methods and technologies, e.g. algorithms, complexity theory, programming language design, compiler technology, and automated reasoning for verification, synthesis, optimization and simulation of quantum circuits.

We aim at two different time scales:

• Quantum computers today – in the NISQ era – have limited size and reliability. Optimizing quantum circuits for minimal depth, and finding optimal mappings from logical qubits to physical qubits, are essential to avoid noise. Therefore, quantum software technology is indispensable to boost performance and accuracy of quantum computers, and increase the scale of quantum experiments that can be conducted in the short-term future.

• Fault-tolerant computing, including error correction, requires the development of much larger quantum computer platforms than we can build today. On the long term, this development will enable the scale and precision required for advanced quantum algorithms, like Quantum Phase Estimation, and fully programmable quantum computers. Also on the long run, quantum algorithms and software technology will be essential to implement and optimize quantum error correction.

Quantum circuit: Double qubit-excitation, building block in the VQE algorithm [Yordanov etal.]