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Towards Long-Distance Quantum Communication

Marc Almendros Parra
Universitat Politècnica de Catalunya, September 30th, 2010
This dissertation reports on an experimental quantum optics project performed in the group of Prof. Jürgen Eschner at ICFO - The Institute of Photonic Sciences in Spain, to advance in the research of long-distance quantum communications. It describes in detail several crucial engineering contributions to the project and puts them in the framework of the physical background and of the first successful proof-of-principle experiments.
The document starts with an introduction about long-distance quantum communication, discussing some of its applications and its state of the art, and introducing the different scenarios to create distant entanglement with an apparatus like the one presented here. Then, it follows the technical description of the setup, which is the main part of the document. This is consistent with the percentage of time of the PhD dedicated to its design and construction, because the experiment was started from scratch. This technical description has a small part which explains all the work the group has done as a team, e.g. the vacuum setup and the laser system, and then it focuses in my individual contribution to this technical work, with the Cavity Locker and the pulse sequencer called Hydra. The latter is a highly complex project which took two years of development, and consequently it occupies the biggest portion of this document. After the technical description, the first operations with single trapped ions are explained, finishing with an implementation of a bandwidth-tunable single-photon source for quantum networking applications.
The experimental setup consists of two ions traps -so far the only ones in Spain, and one out of a handful similar setups worldwide- located in two different vacuum chambers which are separated by approximately one meter of distance. In each apparatus, single trapped ions are manipulated with laser light, allowing us to control the energy level of their valence electron to perform quantum information processing. One main goal of the experiment is to create entanglement between two distant ions -one in each trap-, a mandatory resource to perform quantum communications between them. Therefore, each vacuum chamber can be understood as a transceiver, or node, in a quantum network scenario. Such a network could be used, for example, to create more secure communication links using quantum cryptography; or to extend the power of two quantum processors located at a distance by means of the interchange of quantum information between them, e.g. using quantum teleportation.
The quantum transceiver technology studied in this thesis will also help establishing the basis for space-based quantum communication links using satellites, enabling the creation of global quantum networks in a not too far future.