[time-nuts] vs Hg ion? Re: GPS clock stabilitiy, Rb vs Cs

Magnus Danielson magnus at rubidium.dyndns.org
Sun May 5 16:10:27 EDT 2013

Hi Poul-Henning,

On 05/05/2013 08:29 PM, Poul-Henning Kamp wrote:
> In message<51867DF4.4010006 at karlquist.com>, "Richard (Rick) Karlquist" writes:
>> BTW, it is important to understand that
>> the architecture is the key factor, not the flavor of atom.
> Well, somewhat.
> Some flavours of atoms don't work with some architectures, so for
> most of the stuff in reach for us, the atoms do indeed equate an
> architecture.

Well, the basic rule is that you want a single electron, preferably in 
the S state in the outermost shell. You can have that either as neutral 
atom or ionized. Depending on neutral or ionized you go into the 
"classic" or "ion" classes. This is why alkali atoms is so popular, as 
group 1 fits the description well in its neutral state.

Another aspect is how you achieve the state imbalance which can be 
achieved either by state selection magnets or by pumping. Rubidium has 
been selected traditionally because of the suitable spectrum of Rb-85 
and Rb-87 isotopes, and that these is relatively easy to come by (both 
exist in normal rubidium ore) allowing for simple filtering. Modern 
filtering and modern lasers allows a much freer selection.

> The exception seems to be fountains, which can run on pretty much
> any alkali atom you care to feed it, and some even able to use
> both Rb og Cs (Built to nail the Rb frequency firmly down, as
> I understand it).

Beam devices has been built for much more than caesium. In the original 
conception caesium was fighting with thallium, with thallium actually 
being somewhat better, but was judged a bit impractical at the time. 
Rubidium was also built as research beam units, but it has higher 
sensitivity to magnetic field pulling, which used to be an issue, which 
now largely is gone with servo capability.

The fountains is just an evolvement of the beam devices, but the "beam" 
falls backwards halfway "through". In the fountain-context rubidium now 
has an edge over caesium. What make fountains feasible is the 
laser-cooling as it not only allows cooling, but also bouncing around 
the ball of atoms.

> However, there seems to be actual differences between the flavours
> of atoms in fountains, and USNO have picked Rb over Cs because
> they get better results that way.

Indeed. Wasn't NPL in UK early out as well?

In the end, the actual atom used for a particular device is a result of 
what makes practical engineering at the time and achieving the best 

The optical clocks stretches the imagination even more than ion-traps 
ever did, but affordable ion-traps is still very interesting.


More information about the time-nuts mailing list