These atoms are ideally suited for matter-field coupling experiments, since they combine an extremely long lifetime with a very strong coupling to the millimetre-wave field.
Circular states characteristics
Circular Rydberg atoms combine a high principal quantum number $n$ (51 or 50 in our experiments) and maximum orbital and magnetic quantum numbers $\ell= ; m ; =n-1$. In classical terms, the orbit of the electron around the core is a circle. The quantum wavefunction is a very thin torus (...)
Velocity selection is essential to control the timing of the experiment. A combination of velocity-selective optical pumping techniques and time-of-flight selection allows us to select the atomic velocity with a 0.5 m/s uncertainty around any mean value between 100 and 600 m/s. The preparation time of the circular Rydberg atoms being precisely known, the position of all atoms is determined within a 1 mm uncertainty at any time. This allows us to apply selected transformations on the atoms. (...)
Field ionization provides an efficient and state-selective detection of the circular Rydberg atoms. It combines a high efficiency (up to 80%) and a high selectivity (level assignation errors of a few %).
A moderate electric field (about 150 V/cm) is enough to ionize the circular Rydberg atoms. The electron can be easily accelerated and counted by an electron multiplier. Since the ionization electric field varies rapidly with the principal quantum number (as $n^4$), it is possible to detect (...)
Ramsey interferometry plays a central role in all our experiments. It allows us to probe in a sensitive way the atom-field interaction in the cavity.
Before they enter the cavity, the atoms can be prepared in an arbitrary superposition of $e$ and $g$ by a resonant classical microwave pulse in zone $R_1$ (see the general scheme). A second pulse in zone $R_2$, at the exit of the cavity $C$ maps two arbitrary orthogonal states on the Bloch sphere onto the energy eigenstates $e$ and $g$. The (...)