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Dynamics and organisation of large laser-cooled clouds

par Caroline CHAMPENOIS - publié le , mis à jour le

One of the purpose of the TADOTI project is the understanding of the interplay between Coulomb interactions and confinement potential in the self-organization of trapped ions. Along with the experimental study, a numerical simulation of the experiments allow us to identify the impact of each parameter on phase transition and to analyze finite size effects. Technically, the simulation of a large number of particles with longue range interaction is a challenge in itself as every interaction needs to be accounted for to reproduce experimental observations. As for the experimental set-up, which lies on linear radiofrequency traps, it was essential to make sure that their relevant dimensions make possible the stable trapping of cold clouds, large enough to observe finite-size free effect in their center.
In this multi-zone set-up, we can design ion transport protocoles between the different trapping zones. The transport of a single ion without exciting its motion is one of the major issue in trapped ion based quantum computer development. We wish to expend these protocoles to a large cloud in a centimeter size trap in the purpose of, for example, improving microwave ion clock performances. The large distance between electrodes and the Coulomb interaction makes it difficult because of the coupling between different degrees of freedom, during the transport.

Piège sur TADOTI

Left : picture of the multi-zone trap through one of the window of the TADOTI experiment. The electrodes are in the center of the picture whereas the three bars over and under them are holding the structure together. The double quadrupole is on the right and the octupole is on the left.
Right : picture on a CCD camera of a small crystal made of laser cooled calcium ions, trapped in the quadrupole part of the trap.

Financial support :
the TADOTI project was initiated thanks to the financial support of region PACA and ANR (JC/JC 2008). It is now supported by CNES.

Results :
Cold clouds made of hundreds of thousands of ions are trapped in the quadrupole part of the trap. Thanks to laser cooling, the phase transition to the liquide phase is observed and the onion-like concentric layers appearing for still colder ions is the signature of a Coulomb crystal seeing its harmonic potential.
Ion clouds are transported in the gas phase within the four electrodes trapping zone with an efficiency cloase to 100% one way (+/- 5%). The time evolution of the emitted fluorescence on arrival tells us about the energy gain induced by the transport. This is an important issue for CNES, which is involved in the project, within its micro-wave atomic clock program. Indeed, ion transport is at the core of the micro-wave clock prototype developed by JPL-NASA (

We are now able to :
- run a molecular dynamic simulation of 100 000 laser cooled ions during 1 ms, thanks to the code parallelisation.
- load the second part of the quadrupole trap from the first one, without any observation of signal emitted from the atoms or ions.
- load the octupole part of the trap from the quadrupole part and observe their fluorescence.