Caroline Champenois

Laser cooling of trapped ions

When the laser cooling looses the race against RF-heating : possibility to detect single heavy molecules

The thermodynamic equilibrium of an ion cloud depends on the competition between laser Doppler cooling and radio frequency heating, which is inherent to these traps. By choosing to confine clouds of several thousand ions with RF amplitudes corresponding to high Mathieu parameters (greater than 0.5), the ion cloud equilibrium is made susceptible to a small perturbation. When this perturbation comes from the passage of a very heavy single charged molecule, the cloud is made a single molecule detector, with no upper mass limit. This is the core of the GiantMol project, where the detection signal is based on the variation, upon passage of the molecular ion, of the laser-induced fluorescence emitted by the ion cloud during its laser cooling. This project aims to provide a new solution to the field of mass spectrometry of very large molecules (mass of the order of or greater than 1 MegaDalton) in particular for biological applications (direct detection of viruses, DNA, proteins, etc.). Indeed, this spectrometry is limited in practice to masses of the order of 1 MegaDalton due to the inefficiency of detectors of charged particles at very large masses.(voir ici).

Thanks to the financial support of SATT-Sud-Est, a prototype was built and it is now entering its second stage prematuration with the support of CNRS-Innovation.

Cooling beyond the Doppler limit without resolving the motional side-bands

For calcium-like ions, an alternative cooling scheme using the same laser configuration than side-band cooling can be used with single ions or ion strings, but with no requirement of side-band resolution and no need for Lamb-Dicke regim. One of the motivation is to cool ions without using UV laser sources, like usual Doppler cooling do. The excitation scheme implies the quadrupolar transition and most photons are scattered on the UV transition, which results in a detection with no background. On the signal over noise ratio point of view, this compensates for the lower number of scattered photons than in the conventional cooling scheme. A major difference with side-band cooling is that the photon recoil from the two lasers are involved in the cooling. It was confirmed experimentally by comparing two configurations where the recoils add up or cancel (see here).

This work, both theoretical and experimental, involved the CIML group and the group of Michael Drewsen