Johannes BorregaardJohannes Borregaard

A thesis submitted November 6, 2014 for the degree of Doctor of Philosophy and defended January 15, 2015.

The PhD School of Science
Faculty of Science, Niels Bohr Institute, Theoretical Quantum Physics, University of Copenhagen, Denmark

Academic supervisor:
Prof. Anders S. Sørensen

Download Thesis >>


Quantum repeaters and atomic ensembles
During the last couple of decades, quantum mechanics has moved from being primarily a theory describing the behaviour of microscopical particles in advanced experiments to being the foundation of a novel technology. One of the cornerstones in this new quantum technology is the strong correlations that can exist between remote quantum systems called entanglement. These correlations are exploited to detect eavesdroppers and construct unconditionally secure communication channels, enhance the sensitivity in various metrology schemes and construct powerful quantum computers, which can solve extremely hard problems. Quantum technology is, however, still premature, which is partly due to the fragile nature of these quantum correlations to noise. Extended research is therefore taking place to find robust quantum systems and protocols, which can move quantum technology from the specialized laboratories to practical applications.

In this thesis, I describe me and my collaborator’s work along these lines. The first part of the thesis describes our work on optimizing a novel protocol of how to distribute entanglement over large distances for the construction of secure communication channels. We modify a previous protocol, thereby enabling fast local processing, which greatly enhances the distribution rate. We then move on to describe our work on improving the stability of atomic clocks using entanglement. Entanglement can potentially push the stability of atomic clocks to the so-called Heisenberg limit, which is the absolute upper limit of the stability allowed by the Heisenberg uncertainty relation.

It has, however, been unclear whether entangled state’s enhanced sensitivity to noise would prevent reaching this limit. We have developed an adaptive measurement protocol, which circumvents this problem and allows for near-Heisenberg limited stability of atomic clocks. Furthermore, we describe how the operation of a clock can be altered to gain an exponential improvement of the stability even without entanglement. In the next part of the thesis, we describe our work on a novel type of heralded quantum gates with integrated error detection, which greatly enhances the performance of the gates at the expense of a finite but possible small failure probability. Such gates may facilitate fault tolerant quantum computation or high rate entanglement distribution. In the final part of the thesis, we describe our work on room temperature quantum memories and single photon sources. We have introduced a novel concept of motional averaging, which can be used in room-temperature systems, where fluctuations due to thermal motion is an issue. In particular, we have considered a system based on microcells filled with Cs-atoms, which can facilitate efficient quantum memories and coherent single photon sources at room temperature. Finally, we describe our work on optimizing entanglement distribution protocols based on optical cavitites and single emitters. We have compared entanglement generation schemes based on single - and two-photon detection and implemented the heralded gate described above together with a similar deterministic gate in order to swap the entanglement to large distances. We have then found the combination resulting in the highest distribution rate, which is shown to outperform one of the fastest distribution protocols based on atomic ensembles.

Download Thesis >>