[time-nuts] Re: Project Great

[Lux, Jim]

On 11/29/21 1:57 AM, Hal Murray wrote:
> jim at luxfamily.com said:
>> And a lot ofsources may have a low flat spot in the curve, but it
>> eventually trends up. Except for primary standards like Cs beam.
> What's magic about "primary standard" or "Cs beam" that keeps the ADEV from
> trending up?
>
Their ultimate accuracy is dependent only on a invariant physical 
property that is independent of time.

A quartz crystal ages. No matter how good your oven is, the frequency 
will change over a long time.

Mercury ions, Cs ions (perhaps all ions?) have frequencies that are a 
fundamental property of the ion.  There might be practical 
implementation limits or side effects that limit the lowest achievable 
uncertainty - hot atoms will have higher variance than cold atoms, for 
instance. So not all Cs primary standards have the same ultimate 
performance.