PhD defense by Johannes Borregaard – Niels Bohr Institute - University of Copenhagen

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PhD defense by Johannes Borregaard

During the last couple of decades, quantum mechanics has moved from being primarily a theory describing the behavior of microscopic particles in advanced experiments to being the foundation of a novel technology. Extended research is taking place to find robust quantum systems and protocols, which can move quantum technology from the specialized laboratories to practical applications. In this thesis defense, I will describe the work of my collaborators and I along these lines.

First, I will focus on the task of distributing entanglement over large distances, which is a necessity in quantum networks for, e.g., secret key sharing.  I will discuss distribution protocols, called quantum repeaters, based on both atomic ensembles and single emitters. Specifically, I will present some novel proposals on how to realize room-temperature quantum memories and near-perfect heralded entanglement swapping. Furthermore, I will discuss how, even in the presence of noise, entanglement can be used to push the stability of atomic clocks to near the so-called Heisenberg limit, which is the absolute upper limit of the stability allowed by the Heisenberg uncertainty relation. Finally I will present, a new way of operating atomic clocks, which can lead to an exponential improvement of the stability even without entanglement.

Assessment Committee:
Professor Nicolas Sangouard, University of Bases, Switzerland
Professor Klemens Hammerer, Leibniz University of Hannover, Germany
Professor Peter Lodahl, The Niels Bohr Institute, Denmarm