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## Dispositif expérimental.

Cette section décrit brièvement le dispositif expérimental.

### Cryostat

The superconducting cavity and the control of the blackbody radiation field require a very low temperature environment (below 1K). We achieve this in a $^4$He-$^3$He cryostat.
This picture presents a cut through the cyrostat. The outer dimensions are about 60 cm in diameter, for a height of about 150 cm. Successive cryogenic stages are used to reach a base temperature between 0.6 and 0.8 K.
A shield cooled to liquid nitrogen temperature protects first all the inner parts from the room (...)

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### Un dispositif à deux cavités

The set-up has been designed to hold two cavities and three Ramsey zones. This configuration is ideal for the generation of non-local mesoscopic states or for quantum information experiments. At the present date, only one of the two cavities is operational.
This figure presents an artist’s view of the set-up. It includes two identical superconducting cavities $C_1$ and $C_2$ and three Ramsey zones for the manipulation of the atomic states, $R_1$, $R_2$ and $R_3$.
This is a picture of the (...)

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### Contrôle des champs parasites

To preserve the atomic coherences, it is essential to control the stray electric and magnetic fields experienced by the atoms. The experimental core must also be screened from the room temperature blackbody photons.
Magnetic field control
The $e\rightarrow g$ transition is shifted by 1.4 MHz in a 1 Gauss field. It is thus essential for a good contrast of the Ramsey interferometer to control the stray magnetic fields.
All the experimental core is made of non-magnetic materials (copper, (...)

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### Technologie micro-onde

The generation and control of microwave fields plays an essential role in our experiments, both for cavity characterization and for the operation of the Ramsey interferometer. We have developed original microwave techniques, in close collaboration with P. Goy and the ABmm company.
These experiments require extremely monochromatic millimeter-wave sources for the Ramsey interferometry and the control of tiny field intensities, at the single photon levels, when we prepare the cavity mode in a (...)

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### Contrôle informatique

The experiments may involve comples sequences, with a few thousand atomic samples, each undergoing selective state transformations in the Ramsey zones and in the cavity. The whole experiment is controlled by a dedicated computer software.
This software has been primarily developed by S. Kuhr (now at Mainz university) during his post doctoral stay in our group. It is written in C in the LabWindows National Instruments development environment. The interface with the experiment is realized (...)

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