This request concerns the continuation of the project n. 200020_130381/1 in the group led by G. Mileti at the Laboratoire Temps – Fréquence (LTF) of the Institute of Physics, Faculty of Sciences, University of Neuchâtel. The main research topic is precision laser-based spectroscopy (double resonance and coherent population trapping) of alkali atoms in, respectively; wall-coated and micro-fabricated buffer gas cells. The application of these two configuration in compact and miniature atomic frequency standards is also evaluated from a metrological point of view. The requested subsidy will be almost exclusively devoted to co-fund two PhD works, one of which will be concluded and the second started. Their subjects coincide with the research plan of project. Comparative studies on Double Resonance and CPT in Rubidium wall-coated cells Thanks to a separate FNS-R’Equip subsidy the LTF alkali atoms cell-filling system has been upgraded and now also allows wall-coating of the resonance cells. The produced cells will be characterised using both Double Resonance and Coherent Population Trapping spectroscopy, after purely optical investigations. The objective consists in obtaining narrower spectral features thanks to longer “longitudinal” and “transverse” coherence times resulting from the non-relaxing atoms-walls collisions. The role of the size of the cell and of the overall geometrical parameters (laser beam, microwave field spatial distribution, static magnetic field geometry, etc.) will be studied and the application in future high performance atomic clocks (10-14 fractional frequency instability between 100 and 10’000 s) investigated. The AC-Stark shift with monochromatic and non-monochromatic excitation will be evaluated in detail, as well as the possibility to suppress it. Coherent Population Trapping (CPT) spectroscopy of Caesium atoms in micro-fabricated cells CPT resonances of Cs atoms confined in a micro-fabricated cell constitute in principle a relatively accurate and relatively stable compact reference to frequency-stabilise a quartz oscillator in an atomic clock. A number of spectroscopic studies are however still required to identify the main sources of frequency instability if one desires to reach the target performances, which are typically a fractional frequency drift below 10-11 and a reduced environmental sensitivity. The research will therefore focus on the physical processes that affect the CPT resonance at an averaging time ranging from 1000 to 100’000 seconds and on the origin of their long-term drift. In particular, the AC-Stark shift due to the laser excitation; the temperature dependence of the resonance displacements due to atoms-buffer gas collisions; the pressure changes in the micro-fabricated cell and the effects of the static magnetic field. This second research line will find a direct application in a novel low-consumption atomic clock for telecommunications.