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 Rb-87 atom in an
optical dipole trap. The atom is prepared in an excited state, that
together with its two decay channels forms a lambda-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.