# Edward Pollack Distinguished Lecture Series

Prof.
Claude
Cohen-Tannoudji

Collège de France and Laboratoire Kastler Brossel,
École Normale Supérieure (Paris)

**Measuring Time with Ultracold Atoms: Achievements and Perspectives**

Time measurement is based on the use of periodic phenomena like the rotation of the earth, the oscillation of a pendulum or the vibration of a quartz crystal. The oscillation frequency of the radiation emitted or absorbed by an atom undergoing a transition between 2 energy levels has the advantage of being universal since it is the same for all atoms of the same type. An atomic clock is an oscillator whose frequency is locked on the frequency of an atomic transition.

Ultracold atoms, which move with very low velocities, allow longer observation times which increase considerably the precision of the measurement of the atomic frequency. Recent spectacular progress in the realization of cesium atomic clocks using microwave transitions of laser cooled cesium atoms will be described, leading to relative frequency stability in the range of 10^{-16}, which corresponds to an error less than one second in 3×10^{8} years. The perspective of putting these clocks in a microgravity environment is very attractive. In the optical domain, atomic clocks using optical transitions of trapped ions or of atoms trapped in optical lattices are developed by several groups. Several applications of these ultra precise atomic clocks are also being investigated and will be briefly reviewed.

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