Quantum optical experiments towards atom-photon entanglement

In 1935 Einstein, Podolsky and Rosen used the assumption of local
realism to conclude in a Gedankenexperiment with two entangled
particles that quantum mechanics is not complete. For this reason
EPR motivated an extension of quantum mechanics by so-called local
hidden variables. Based on this idea in 1964 Bell constructed a
mathematical inequality whereby experimental tests could
distinguish between quantum mechanics and local-realistic theories.
Many experiments have since been done that are consistent with
quantum mechanics, disproving the concept of local realism. But all
these tests suffered from loopholes allowing a local-realistic
explanation of the experimental observations by exploiting either
the low detector efficiency or the fact that the detected particles
were not observed space-like separated. In this context, of special
interest is entanglement between different quantum objects like
atoms and photons, because it allows one to entangle distant atoms
by the interference of photons. The resulting space-like separation
together with the almost perfect detection efficiency of the atoms
allows a first loophole-free test of Bell's inequality. The primary
goal of the present thesis is the experimental realization of
entanglement between a single localized atom and a single
spontaneously emitted photon at a wavelength suitable for the
transport over long distances. In the experiment a single optically
trapped Rb87 atom is excited to a state which has two selected
decay channels. In the following spontaneous decay a photon is
emitted coherently with equal probability into both decay channels.
This accounts for perfect correlations between the polarization
state of the emitted photon and the Zeeman state of the atom after
spontaneous decay. Because these decay channels are spectrally and
in all other degrees of freedom indistinguishable, the spin state
of the atom is entangled with the polarization state of the photon.
To verify entanglement, appropriate correlation measurements in
complementary bases of the photon polarization and the internal
quantum state of the atom are performed. It is shown, that the
generated atom-photon state yields an entanglement fidelity of
0.82. The experimental results of this work mark an important step
towards the generation of entanglement between space-like separated
atoms for a first loophole-free test of Bell's inequality.
Furthermore entanglement between a single atom and a single photon
is an important tool for new quantum communication and information
applications, e.g. the remote state preparation of a single atom
over large distances.