Atom-Photon Entanglement

Entanglement is the key element for many experiments in quantum communication and information. Especially for future applications like quantum networks or the quantum repeater it is mandatory to achieve entanglement also between separated quantum processors. For this purpose, entanglement between different quantum objects like atoms and photons forms the interface between atomic quantum memories and photonic quantum communication channels, finally allowing the distribution of quantum information over arbitrary distances. Furthermore , atom-photon entanglement is also the key element to give the final answer to Einstein's question wether a local and realistic description of physical reality is possible or not. Until now, the results of many experiments testing Bell's inequality indicate that local realistic theories are not a valid description of nature. However, all these tests were subject to loopholes. In this context, atom-photon entanglement represents a crucial step towards the realization of entanglement between distant atoms that would allow a final loophole-free test of Bell's inequality. This thesis describes the generation and verification of an entangled state between a single neutral atom and a single photon at a wavelength suitable for long distance information transport. For this purpose we store a single 87 Rb atom in an optical dipole trap. The atom is prepared in an excited state, that together with its two decay channels forms a Λ-type transition. In the following spontaneous decay, conservation of angular momentum leads to the formation of an entangled state between the angular momentum of the atom and the polarization of the emitted photon. To verify the entanglement we introduce an atomic state-analysis, based on a state-selective adiabatic population transfer between atomic hyperfine levels. This allows the direct analysis of the internal state of the atom in arbitrary measurement bases without the necessity of additional state manipulations. Using this method together with a polarization measurement of the emitted photon, we performed correlation measurements as well as a full state tomography of the combined atom-photon system. From the experimental results we obtain an entanglement fidelity of 87%, which clearly shows that the generated state is entangled. The degree of entanglement observed in our experiment is high enough to allow the generation of entanglement between distant atoms via entanglement swapping, which would allow a final, loophole-free test of Bell's inequality. Furthermore, it opens up a variety of applications in quantum communication and information science.