2 April 2015
The atomic fountain operation is sequential (cycle duration 1 - 1,6 s). At each cycle, a few 106 up to 107 atoms are captured in an optical molasses, at the intersection of six laser beams, then cooled down to a temperature of the order of one µK, from an atomic vapour or a slow beam issued from a two dimensional magneto-optical trap. Then launched vertically inside the vacuum chamber at a velocity of a few m/s, they perform a ballistic flight up to an apogee of the order of 1 m, before falling down, hence the denomination of this type of clock.
Just after launch, the atoms are prepared in one state of the clock transition by interacting with a microwave pulse in a first resonator and thereafter with a “pusher” laser beam. The atom thus selected cross a second microwave cavity, once on their way up and a second time on their way down, thus undergoing the so-called Ramsey interrogation. The atomic response in frequency is an interference fringe pattern, where the fringe width, of the order of 1 Hz, is inversely proportional to the time spent above the Ramsey resonator. The populations of the two states of the clock transition are finally measured by detecting the fluorescence induced by a set of laser beams, from which the transition probability can be calculated. This quantity is used as frequency discriminator to realize the clock.