Mémoire

Abstract

Time measurement has evolved from astronomical definitions, such as Universal Time (UT), to atomic standards that define Coordinated Universal Time (UTC). The current SI second is based on the microwave hyperfine transition of cesium-133 and is realized through atomic fountain clocks. These clocks achieve high accuracy and stability, which can be statistically assessed using Allan variance. However, recent developments in atomic and optical physics have led to the emergence of optical atomic clocks, which now surpass microwave standards in both short-term stability and long-term accuracy. These clocks operate on narrow optical transitions in neutral atoms trapped in optical lattices or in single ions confined by electromagnetic fields. Their higher transition frequencies and extremely low systematic uncertainties make them prime candidates for the redefinition of the second. This thesis investigates the physical principles, operational techniques, and metrological advantages of these new systems, and places them in the broader context of international efforts to redefine the SI second within the coming decade.