[time-nuts] Thinking outside the box a super reference
attila at kinali.ch
Sat Nov 5 10:16:25 EDT 2016
On Sat, 05 Nov 2016 12:25:35 +0000
"Poul-Henning Kamp" <phk at phk.freebsd.dk> wrote:
> >Active maser like the hydrogen would be possible naturally, but would
> >require the resonator.
> I don't think they are.
They are. It took a while, but they have been a thing since '64.
Though all of them have been using vapor cells.
> As I understand it not all excited modes of all atoms and molecules
> have the not-quite-pinned-down quantum-thaumagic property to do that.
> And I remember reading somewhere that the alkali atoms have been
> poked and prodded to no end about this, in the hope of creating
> active Cs, Rb or Sr frequency standards, but the very reluctant
> (and expensive) conclusion was that hydrogen is the only one in the
> family which knows the trick.
Nope, the problem, as far as I understand it, is not that you cannot
get the atoms to emit, but to keep them in one place without perturbing
them. For hydrogen, a teflon coating does a very good job and the atom
can go for many wall collisions without losing its state/phase. Even the
early hydrogen maser got to >10^4 collisions and modern coatings offer
something like 10^6 IIRC, ie the life time is measured in seconds
Until recently, there didn't exist such a coating for Rb or Cs  and
relaxation time was measured in milliseconds. For comparison, have a look at
page 37 of Bandi's dissertation, the broadening due to wall collisions
is stated as 300Hz for the wall coated, evacuated cell, while the buffer
gas collision broadening is a mere 10Hz (yes, I am cheating here, the
correct comparison would be the total broadening which is 350Hz vs 600Hz).
Yes, the wall shift is smaller than the buffer gas shift (4000Hz vs 300Hz)
but this is in first order approximation a constant.
Additionally, all the coatings have rather low temperature limits
(the hydrocarbon coatings are very similar to parafin wax) and the
high temperatures that were needed for the Rb lamps didn't work togheter.
And even for modern laser pumped systems, you still need a rather high
temperature (40-90°C are usual) to get a high enough amount or Rb in the cell.
I guess you can understand that people have not been using vapor cell
active masers because they do not offer the long term stability one
seeks (due to buffer gas changes) and their short term stability is
not any better than the passive masers (limited by buffer gas relaxation).
Probably, with the new coatings and laser pumping one could build a usable
Rb active maser with evacuated cells instead of buffer gas. But sofar
nobody has tried that yet (as far as I am aware of). I guess it's more
sexy to investigate optical standards than something as old as Rb.
Especially as there are already Rb fountains which offer quite a
high stability already.
 "Polarized Alkali-Metal Vapor with Minute-Long Transverse
Spin-Relaxation Time", by Balabas, Kraulanov, Ledbetter, Budker, 2010
 "Double-Resonance Studies on Compact, High-performance Rubidium Cell
Frequency Standards", by Thejesh Bandi, 2013
 "The USNO rubidium fountains", by Peil et al. 2015
Any simple idea will be worded in the most complicated way.
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