Lasers, Cold Atoms and Atomic Clocks: Realizing the Second Today

Abstract

Abstract. The time is the physical quantity that mankind could measure with the best accuracy, thanks to the properties of the atomic physics, as the present definition of time is based on atomic energy transitions. This short review gives some basic information on the heart of the measurement of time in the contemporary world, i.e. the atomic clocks, and some trends related. The atomic secondThe idea to refer the units of measurements to fundamental physics and in particular to quantum physics found its first, and presently the most fruitful application in time and frequency metrology.Considering the quantization of the atomic energy levels and the fundamental Bohr equivalence E = h stating that the difference of two atomic energy level E is proportional to a frequency  by the Planck's constant h, it is straightforward to think of the atom as a possible frequency standard. (AFS) is basically a device composed of a macroscopic oscillator, hereafter Local Oscillator (LO), e.g. a quartz oscillator for a microwave clocks, or a laser for an optical clock, which frequency is directly compared to the frequency of an atomic transition taken as standard, called clock transition. The Cesium iperfine ground transition has been chosen for the definition of the second, and Cesium clocks are also called primary frequency standards. Other kind of atomic clocks exist, based on different atoms and different atomic transitions: there are clocks based on Hydrogen, Rubidium, Mercury, Ytterbium, Strontium, Indium, Aluminum… Once compared to the atomic transition, the frequency of the LO is corrected to match as better as possible the frequency of the reference. EPJ Web of Conferences