The master’s student will join Laboratoire de Physique des Lasers (LPL) to develop a compact microwave (MW) spectrometer operating from 2–20 GHz. This instrument is designed both as a sensitive detector of internal quantum states in polyatomic molecules and as a precision tool for molecular frequency metrology. Proof-of-principle measurements have already shown free induction decay signals on the OCS J = 0 → 1 transition at 12.163 GHz with excellent signal-to-noise ratios. The next objective is sub-Hz accuracy, made possible by ultrastable, SI-traceable frequency references distributed by the REFIMEVE network. Within the ANR Ultiμos project, the spectrometer will enable cross-checks between MW rotational frequencies and mid-infrared (MIR) rovibrational data measured at the 100 Hz level. These comparisons are motivated by the search for variations of the proton-to-electron mass ratio μ, a key constant of the Standard Model. Methanol and ammonia, with transitions highly sensitive to μ, are particularly powerful probes. MIR and MW results, obtained via independent experimental chains, will provide robust cross-validation of frequencies and uncertainty budgets. The student will contribute to optimizing waveguide and cavity configurations, performing precision spectroscopy on benchmark species, developing MIR–MW double-resonance schemes to enhance sensitivity, and preparing integration with a cryogenic buffer-gas cooling source (~3 K).