balls and the Dick effect

In single-pulse fountain operation, there is only one ball of atoms moving through the fountain at a time. Since the fountain cycle takes approximately one second to complete, but the atoms are being interrogated for half a second, the local oscillator receives feedback for only half the cycle time. During the time that the local oscillator is not receiving feedback, a phase error may occur on the signal. The degradation of the clock's performance during the period of time when the local oscillator is not being stabilized by the atomic frequency is known as the Dick effect.

To minimize the Dick effect, the local oscillator must receive feedback corresponding to a signal representing the phase measurement over the entire cycle time. If one ball of atoms could be trapped and launched while another is in free flight above the microwave cavity, then the local oscillator would receive feedback twice as frequently. However, light from one ball of atoms must not reach the other because stray light could affect the atoms' state and thus change their response to the detection lasers.