Douillet works on precision spectroscopy of cold trapped molecular hydrogen ions, developing laser-cooling and rotational-state-selection techniques to improve the accuracy of tests of molecular QED alongside Laurent Hilico.

Hilico develops high-resolution laser and two-photon spectroscopy of trapped, sympathetically-cooled molecular hydrogen ions (H2+, HD+) to test molecular QED and extract fundamental constants (proton/electron mass ratio), part of LKB's broader precision-metrology and fundamental-interaction-testing programme.