Why are atomic clocks so accurate?

[iVenky]

For any clock, the way I define accuracy of the clock is based on the Q factor (the ability to reject noise) and the inherent noise in the system. In that case, can you explain me why atomic clocks are so accurate? Do they have high Q factor? (if so, how)? or low noise to begin with?

 

[davenn]

For any clock, the way I define accuracy of the clock is based on the Q factor (the ability to reject noise) and the inherent noise in the system. In that case, can you explain me why atomic clocks are so accurate? Do they have high Q factor? (if so, how)? or low noise to begin with?

Have you googled how an atomic clock works ?
It is likely to answer your questions

have a go and then come back with anything you don't understand
provide a link to the site so that we can see what you were readingDave

 

[iVenky]

Hi,

I checked everywhere online and they all talk about how accurate the hyperfine transition of cesium atom is. Is it because there is not much loss mechanism involved here in this natural oscillation process for atoms?

 

[DaveE]

High Q is certainly important for accuracy, but, in general, I'm not sure it's the only factor. Think about a trombone (a slightly less accurate oscillator) but still high Q. If you move the slide the note changes, but the Q doesn't necessary change much. Or, consider a quartz crystal oscillator. The most stable ones have a temperature controlled oven for the crystal to stabilize the frequency, even though the Q isn't effected much.

 

[davenn]

Hi,

I checked everywhere online and they all talk about how accurate the hyperfine transition of cesium atom is. Is it because there is not much loss mechanism involved here in this natural oscillation process for atoms?

Because the oscillations per second is very well known and it is very stable over very long time periods,
as good as 1 second in 100 million years. That stability makes for a very accurate clock

I use a rubidium standard clock oscillator ( similar to a caesium one) in several of my microwave radio transceivers on
10 GHz and 24 GHz.
The unit produces a very stable 10MHz output that I use as a reference oscillator for the transceiver local oscillatorDave

 

[iVenky]

Because the oscillations per second is very well known and it is very stable over very long time periods,
as good as 1 second in 100 million years. That stability makes for a very accurate clock

like I said, my question is every more fundamental, why is it stable over very long time periods? I read atomic clocks have Q factor of > 10¹¹ or something. Is the decay process (energy lost) involved in these oscillations super small? then I can understand why atomic clocks have high Q factor since Q=2*pi*energy stored/energy dissipated per cycle

 

[davenn]

why is it stable over very long time periods?

that is just the nature of the ceasium atom

"why" questions can't really be answered as they always just lead to another "why" question

 

[Nugatory]

like I said, my question is every more fundamental, why is it stable over very long time periods

Try attacking this question from the other direction: what things cause instability in other clocks? Are these factors present in a cesium clock?

 

[davenn]

Try attacking this question from the other direction: what things cause instability in other clocks? Are these factors present in a cesium clock?

thanks ... good angle of attack

 

[f95toli]

. Is the decay process (energy lost) involved in these oscillations super small? then I can understand why atomic clocks have high Q factor since Q=2*pi*energy stored/energy dissipated per cycle

Sort of. The energy levels involved are very long-lived making them very "sharp" in frequency.
However, this is only part of the story. The fact that the transition is stable in the long term also has to do with a variety of technical factors. When you add up the budget for a clock there are many, many issues. Some of them have to do with sensitivity to external factors (collisions with other atoms, sensitivity to external magnetic or electric fields, Doppler shifts etc) and some of them have to with how easy (relatively speaking) to manipulate the atoms. The latter will depend on which transitions are available, how easy it is to cool etc.

Note that there is no such thing as a "best" clock. What we call an "atomic clocks" are really a combination of two clocks: hydrogen masers have good short term accuracy and are used to generate the actual times signal (say 10 MHz). However, masers drift over longer times (say seconds) and are therefore "disciplined" using a cesium fountain (which by its very nature does not give a continuous time-signal out). Hence, the long-term accuracy is completely given by the accuracy of the Cs clock.

 

[sophiecentaur]

Note that there is no such thing as a "best" clock.

Perhaps not "best" but the most accurate clocks do have something in common and that is the very low level of interaction between the individual atoms and the bulk of the substance. That implies the active atoms need to be in a gas.

 

[f95toli]

Perhaps not "best" but the most accurate clocks do have something in common and that is the very low level of interaction between the individual atoms and the bulk of the substance. That implies the active atoms need to be in a gas.

sort of.
However, note that for short-term stability there are other options. Cryogenic dielectric resonators can have a short-term (up to tens of seconds) accuracy similar to or better than hydrogen masers. I believe an ADEV better than 1e-15 at about 10s has been reported for sapphire resonators which is something like an order of magnitude better than a hydrogen maser.

 

[sophiecentaur]

sapphire resonators

That's interesting. How does the interactions in such a condensed material not affect the resonance bandwidth? Why is there no line broadening?