Fredrik Nilsson

Project title: Accurate many-body descriptions of crystal defects for applications in quantum technology

Host Institution: Technical University of Denmark (DTU)

Host Supervisor: Prof. Kristian S. Thygesen

Co-host Institution: École Polytechnique (l’X)

Co-host Supervisor: Prof. Silke Biermann

Summary project: The first quantum revolution, with inventions such as lasers and transistors, formed the basis of the current information technology which has profoundly shaped our society. Although this technology exploits quantum mechanical properties it does not require the control and manipulation of individual quantum states. Indeed, the control of individual quantum states was long thought to be impossible. The experimental techniques to measure and manipulate individual quantum systems were developed in the 1980:s and resulted in the award of the Nobel Prize in physics 2012 to David J. Wineland and Serge Haroche. This paved the way towards the second quantum revolution with entirely new technological possibilities, where quantum mechanical properties are exploited to perform certain tasks and functions much more efficiently than what is possible with classical technologies.

Point-like crystal defects in wide band gap insulators and semiconductors, also called color centers, have in recent years emerged as leading candidates for a wide range of quantum technologies. In these systems the defects embedded in the crystal host give rise to highly localized electronic quantum mechanical states with long coherence times (order of milli-seconds) while still being controllable and readable by optical excitation and detection. However, the theoretical description of these systems is currently hampered by the difficulties in describing the strongly correlated defect states embedded in the crystal host. State of the art quantum chemistry methods that can provide accurate descriptions of the defect states are too computationally heavy to apply to the large supercells needed to also account for the relevant low-energy states of the crystal host. Density functional theory, on the other hand which is well suited for the description of the host states cannot account for the strong local correlations among the defect states.

In this project we aim at developing new methods that combines the strengths of the two approaches. Our goal is to develop an efficient computational method that can predict the energy levels of the defects within 0.1 eV. This research will advance the understanding of known defects as well as pave the way towards high-throughput calculations and machine learning protocols for the discovery and design of new defect systems with tailored properties. We will also explore how the similar methods can be used to explore other physical systems, such as e.g. 2D excitonic insulators.

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Fredrik Nilsson