For several millennia, the measurement of time has been at the heart of social and economical life. Initially based on the observation of periodic natural phenomena like the Earth rotation or lunar cycles, with the invention of the pendulum Galileo and Huygens have opened the era of man-made clocks. Today quantum technologies and cold atoms form the core of ultra-stable modern clocks which display an error of less than a second over the age of the Universe, 14 billion years.

In this presentation, we will discuss the principles of atomic clocks and present a few tests of fundamental physical laws such as precision measurements of the Einstein effect (clock gravitational shift) or searches for the variability of fundamental constants of Physics. We will finish with a few perspectives for clocks based on quantum degenerate gases or quantum correlated atoms.