An atomic clock works by comparing the frequency of a local oscillator (a micrwave generator) with the hyperfine transition of the cesium atom at 9.2 GHz. In general, the precision of a clock is limited by the observation time. For an atomic clock, it is the time of free flight of the atoms between the two microwave pulses used to probe the atomic transition.
In an earth bound fountain clock, using laser cooling technology, the time of flight is limited by gravity to about half a second (the atoms fall). In order to increase the time of flight and the accuracy, it is necessary to operate in microgravity. The PARCS project (Primary Atomic Reference Clock in Space) aims at building such a clock which will operate in the International Space Station.
It is expected that, although the atoms will not fall under gravity, the time of observation will be limited by the thermal expension of the atomic cloud. To achieve a time of flight of several seconds, we will have to implement the most advanced laser cooling techniques, and reach unprecedented low temperatures.
I will present one the techniques that we are currently studying in our laboratory, called Raman cooling.