Atomic oscillations in atomic clocks as the basis for the second
In an atomic clock, free atoms assume one of two possible energy states. They are stimulated to change their state by an external frequency. The better the frequency matches the maximum possible oscillations of the respective atoms, the more measurable is the different resonance frequency that the atoms emit in their different states. The applied frequency can thus be optimised to the vibration of the atoms. The high stability and accuracy of the change of state of the atoms thus generates a high stability and accuracy of the applied frequency.
The oscillations of the cesium atom have defined the basis for a second since 1967:
"The second is 9 192 631 770 times the period of the radiation corresponding to the transition between the two hyperfine structure levels of the ground state of atoms of the nuclide Cs-133."
As there are now even more stable and precise atomic clocks, the International Bureau of Weights and Measures recommends a new definition of the second on a new basis. A working group of the Consultative Committee for Time and Frequency (CCTF) has drawn up an action plan in co-operation with the Physikalisch-Technische Bundesanstalt PTB. A possible date for a new definition is 2030 (source: PTB).
What types of atomic clocks exist?
There are different types of atomic clocks, which can be differentiated according to the atom used and the specific technology. Some of these clocks are the size of several 19-inch cabinets and are so expensive to buy and operate that only government institutions specialising in precise time operate these primary atomic clocks. They contribute to world time and serve as the primary reference time for sophisticated synchronisation tasks.
There are also atomic clocks in smaller sizes that are used as time references in data centres, satellites, laboratories, communication networks or for synchronising power grids. It is also possible to synchronise clocks precisely to the time of the primary atomic clocks, for example via the time of GNSS navigation satellites or via fibre optic connections.
Conventional cesium atomic clocks
In the cesium atomic clocks, which have been in widespread use for several decades, cesium atoms are heated in a furnace and made to oscillate via a microwave signal. The frequency optimised for the oscillations serves as the basis for defining seconds, and therefore minutes, hours and days. In Germany, two of these clocks at the PTB (Physikalisch Technische Bundesanstalt) in Braunschweig contribute to world time.
Casesium fountain
The PTB in Braunschweig also has two cesium fountains. In these newer cesium atomic clocks, the free cesium atoms are cooled and thus slowed down considerably. A laser brings the atoms onto a ballistic track, where they are made to oscillate. As they react more slowly when cooled, they can form the basis for an even more accurate frequency.
Optical atomic clocks
Optical atomic clocks excite atoms to oscillate at a frequency in the optical spectral range. This clock frequency of 100 THz to 1000 THz is many times higher than that of conventional cesium atomic clocks. A second is thus broken down into much smaller units, increasing precision and stability. Examples of atoms used in optical atomic clocks are strontium, ytterbium and aluminium.
Strontium lattice clock
In the optical lattice clock, strontium atoms are captured in the interference pattern of two laser beams. In this so-called "optical lattice", the atomic "pendulum", i.e. the absorption frequency of the atoms, can then be determined very precisely - currently with an accuracy of 17 digits. (Source and picture above: PTB)
Hydrogen maser
MASER stands for Microwave Amplification by Stimulated Emission of Radation (amplification of microwaves by stimulated emission of radiation). In a hydrogen maser clock, hydrogen molecules are split into hydrogen atoms and excited to oscillate, which amplify the energy of the applied microwave signal through their radiation.
Rubidium oscillator
In a rubidium oscillator, rubidium is vaporised and irradiated with microwaves by an adjustable quartz oscillator. The rubidium vapour is contained in a gas discharge lamp whose brightness oscillates with the rubidium atoms. The applied frequency of the quartz oscillator is regulated by the oscillations of the rubidium atoms.
A rubidium oscillator has a size that allows it to be used in laboratory clocks. The accuracy is somewhat lower than that of the atomic clocks mentioned above, but still has a relative deviation of 3x10-15, which means that the time measurement can deviate from its average value by around 94.62 nanoseconds per year.
Further links and sources
In the picture above:
The most stable optical atomic clock in the world: PTB's optical strontium lattice clock. (Fig.: PTB)
How does an atomic clock work?
https://www.ptb.de/cms/ptb/fachabteilungen/abt4/fb-44/fragenzurzeit/fragenzurzeit13.html
How is a second defined?
https://www.ptb.de/cms/ptb/fachabteilungen/abt4/fb-44/fragenzurzeit/fragenzurzeit14.html
What is a strontium lattice clock?
https://www.ptb.de/cms/de/ptb/fachabteilungen/abt4/fb-43/ag-432/strontium-gitteruhr.html
