Time from a fountain of cold atoms
In many respects the atomic fountain clock is the logical further development of atomic clocks with a thermal atomic beam. In a conventional atomic clock, a thermal caesium atomic beam is formed which contains only atoms in a single hyperfine level. In two separate zones, this beam is irradiated by microwave fields which oscillate in phase, with a frequency close to the caesium resonance frequency. In the first zone, a coherent superposition of the two hyperfine states is created. The resulting superposition state then oscillates at its characteristic eigenfrequency. In the second microwave field the phase of this oscillation is compared with that of the generator field: The resulting population of hyperfine states after the second interaction is determined by this phase relation. The measured quantity is the population of the state which was initially unpopulated.
The phase comparison is the more accurate the lager the time interval between the two microwave excitations. A resonance-like reaction of the atoms is registered. The spectral "linewidth" W of this signal is given by <nobr>W = 1/(2.T),</nobr> where the interaction time T denotes the time of flight between the two excitations.
As early as in the 1950s, Jerrold Zacharias of the Massachusetts Institute of Technology tried to prolong the time of flight compared to that achievable with a thermal atomic beam. To reach this aim he tried to set up a "caesium fountain", in which the slow atoms from a vertically emitting thermal source were to turn back under the effect of the gravitational force. He wanted to demonstrate that their state would change after they had flown twice through the same microwave field while rising and falling down again. Zacharias was not successful: As a result of collisions in the area of the atomic beam source, the number of sufficiently slow atoms in his thermal beam was reduced to such an extent that no signal could be detected. The atoms were too "hot", their relative velocities too high.
The situation is quite different today: By laser cooling about 107 cold caesium atoms are collected in a cloud within fractions of a second, the relative velocities of these atoms lying in the range of a few cm/s. This provides a source of cold atoms. If the lasers which influence the vertical motion of the atoms are detuned relative to each other for a short time in a well-defined way, an upward "push" can be given to the cold atoms: They fly up at an initial speed of a few m/s, rise until the gravitational force has consumed their kinetic energy, and fall down again on the same path. Indeed, an atomic fountain.
In a similar way as in conventional atomic clocks, the energy state of the atoms is manipulated and measured. Initially, the atoms are prepared in a single energy state; during their up- and down-movement they fly through the same microwave field. The time of interaction can become considerably longer than in thermal-beam clocks: Any object thrown up by one metre needs little less than one second to return to the origin. This is also the length of the interaction time T which determines the linewidth of the resonance signal.