**Study of the Electromagnetically Induced Transparency in a cold atomic ensemble and application to quantum memories** - Lambert Giner

In order to implement quantum communications, such as quantum cryptography,
over distances larger than one hundred of kilometers using optical fibers,
it is required to develop new tools know as "quantum repeaters". The way
those devices work is based on the development of quantum memories which
that store the quantum state of light and to retrieve it on demand.
This talk presents the realization of a quantum memory based on a cold
atomic ensemble of cesium using a protocol based on Electromagnetically
Induced Transparency (EIT). The first part of this talk describes the
realization of a magneto - optical trap producing a cold atomic cloud of
cesium exhibiting a large optical depth. The second part explains the study
of the transparency of the media. Indeed, in a Ã‹ energy scheme, the
application of a control field on the nearby transition of the signal field
results in opening a transparency window. Two phenomena can explain this
observation: the EIT which corresponds to destructive interferences between
various excitation paths and Autler - Townes Splitting (ATS) corresponding
to the separation of the excited state into two dressed states. A
quantitative and detailed study has been carried out. The last part shows
the demonstration of the storage of a coherent state in the single photon
regime with an efficiency of 24% and the storage of a quantum information
bit encoded into the orbital momentum of light results in the single photon
regime with a fidelity larger than 92%