In the military calibration sections, we used to use equipment...

In the military calibration sections, we used to use equipment called caesium beams and rubidium beams, as frequency standards to calibrate other frequency equipment. Due to the stability of the natural frequency, or it vibrates the least of any substance, its used as an atomic clock; Caesium was the most stable element and rubidium was second most stable element. We used to use the caesium beam to calibrate the rubidium beam, in theory, or link both together for similar.

by numbers 6 834 682 608 Hz

A rubidium standard, or rubidium atomic clock, is a high accuracy frequency and time standard, usually accurate to within a few parts in 10¹¹. ... But the Rb standard is comparatively inexpensive, compact, and widely used in TV stations, Mobile phone base stations and GPS systems and is considered as a secondary standard.27 May 2015

The caesium clock will not gain or loose a second in 20 million years.

https://en.wikipedia.org/wiki/Atomic_clock

In April 2015, NASA announced that it planned to deploy a Deep Space Atomic Clock (DSAC), a miniaturized, ultra-precise mercury-ion atomic clock, into outer space. NASA said that the DSAC would be much more stable than other navigational clocks.^[13]

In March 2008, physicists at NIST described a quantum logic clock based on individual ions of beryllium and aluminium. This clock was compared to NIST's mercury ion clock. These were the most accurate clocks that had been constructed, with neither clock gaining nor losing time at a rate that would exceed a second in over a billion years.^[42] In February 2010, NIST physicists described a second, enhanced version of the quantum logic clock based on individual ions of magnesium and aluminium. Considered the world's most precise clock in 2010 with a fractional frequency inaccuracy of 8.6 × 10⁻¹⁸, it offers more than twice the precision of the original.^[43][44] In July 2019, NIST scientists demonstrated such an Al+ quantum logic clock with total uncertainty of 9.4 × 10⁻¹⁹, which is the first demonstration of such a clock with uncertainty below 10⁻¹⁸.^{[45][46][47] [48]}

The accuracy of experimental quantum clocks has since been superseded by experimental optical lattice clocks based on strontium-87 and ytterbium-171.

The rare-earth element ytterbium (Yb) is valued not so much for its mechanical properties but for its complement of internal energy levels. "A particular transition in Yb atoms, at a wavelength of 578 nm, currently provides one of the world's most accurate optical atomic frequency standards," said Marianna Safronova.^[55] The estimated amount of uncertainty achieved corresponds to a Yb clock uncertainty of about one second over the lifetime of the universe so far, 15 billion years, according to scientists at the Joint Quantum Institute (JQI) and the University of Delaware in December 2012.


Applications......gps and internet;

Each Galileo satellite has two passive hydrogen maser and two rubidium atomic clocks for onboard timing.

The cost of rubidium frequency standards is less than that of other atomic standards.
The rubidium frequency standard is prized for its low cost.