Atomic clocks how does it work

The transitions between energy states can take place by releasing or absorbing energy at optical or microwave frequencies. An atomic second corresponds to 9 counts of the frequency of the energy detected in the transition of the Cesium isotope when exposed to suitable excitation.

The Galileo rubidium clock is made of an atomic resonator and its associated control electronics. Inside the atomic resonator there is a rubidium vapour cell.

The atoms are kept in a gaseous state at high temperature. In order to initiate the resonance, the atoms of the cell are excited to a higher state by the light of a rubidium discharge lamp located in one end of the atomic resonator. At the other end of the resonator there is a photodiode which detects the amount of light that passes through the cell. Following the excitation, the atoms decay to a lower state. From this state, the atoms are excited back to an intermediate level by injecting microwave energy in the resonator at a given frequency.

Transition to the intermediate level only occurs if the frequency corresponds exactly to the one associated with this transition. When the atoms are in the intermediate state, the absorption of light is at a maximum. The efficiency of exciting the atom is constantly monitored and the frequency of the light is constantly corrected to yield the highest efficiency.

Strictly speaking, the energy difference between the ground state and excited state is constant only for individual, completely isolated atoms. External influences, such as electromagnetic fields, can influence the energy levels of the states and thus also the transition frequency. Timekeeping can be as simple as counting "one Mississippi, two Mississippi. In both cases, the trick is counting the intervals of something that occurs repeatedly with as little variation as possible.

A pendulum swing, say, or a 'Mississippi' just about equates to a second, the unit of timekeeping that as we know comprises minutes and hours. But even the best mechanical pendulums and quartz crystal-based clocks develop discrepancies. Far better for timekeeping is the natural and exact "vibration" in an energized atom. When exposed to certain frequencies of radiation, such as radio waves, the subatomic particles called electrons that orbit an atom's nucleus will "jump" back and forth between energy states.

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