23 October 2015
Since 1967, the second of the international system of units is defined from the resonance of the cesium 133 atom corresponding to the transition between the two hyperfine levels |F = 3> and |F = 4> of the electronic state 6S1/2. The frequency of this transition has been fixed to the conventional value ν0 = 9 192 631 770 Hz, for an isolated atom.
The performances of clocks are characterized by their frequency stability and frequency accuracy :
stability is the capacity of the device to reproduce the same frequency along time. This quantity is associated to the various sources of noise of the clock: technical noise associated to the electronic or to the detection lasers, phase noise of the Ramsey interrogation oscillator, quantum projection noise at the detection, which constitutes the irreducible ultimate noise. The SYRTE fountains operate at this quantum projection noise limit.
accuracy is the uncertainty of our knowledge of all the systematic effects which shift the clock transition frequency with respect to the definition. Indeed, in practice the atoms are not strictly isolated, but they are manipulated in a device whose parameters need to be well controlled : effects of magnetic and thermal environments, collisions between atoms, technical effects associated to microwave leakages or to phase perturbations on the interrogation signal synchronous with the clock cycle, phase distribution inside the interrogation resonator, producing a residual Doppler effect, which has been one of the effect the most recently studied, relativistic effects… Note that the symmetry of the interrogation in the fountain geometry is advantageous for the control of some systematic effects.