[time-nuts] Current state of optical clocks and the definition of the second

[Attila Kinali]

On Mon, 12 Jan 2015 20:09:45 +0000
Gregory Maxwell <gmaxwell at gmail.com> wrote:

> On Mon, Jan 12, 2015 at 12:34 PM, Attila Kinali <attila at kinali.ch> wrote:
> > I just stumbled over this [1] nice article by Fritz Riehle that might be
> > of interest to others as well.
> 
> I've seen less discussion of non-atomic stable optical oscillators.
> Most (all?) of these optical atomic standards are passive atomic
> clocks and need a free running oscillator.

The local oscillator is considered a solved problem in research.
If you need something low noise and stable you either go to Oscilloquartz
or to Wenzel and get one of ther OCXO. The only place where you have
to be carefull is the Dick Effect, but that's well understood and
people usually acount for it.

One exception here is space qualified oscillators. For those you
go to the JPL and ask them to help you.
 
> Seems that the state of the art in stabilized lasers has improved a
> lot lately, e.g. there are commercial available 1550nm devices which
> have a <=3Hz line-width: http://stablelasers.com/products.html (well
> on a short term basis, the medium term performance is not so
> impressive)

Laser stabilization, especially for quantum metrology is still 
an actively researched field. Current state of the art is IIRC
0.3Hz linewidth (sorry, cannot find the reference at the moment).
Mid- and long term stability depends highly on the reference
used. Current research is fucused mainly on special, low vibration
structures made out of low expansion glass or silicon. And these
cavities are usually put into a temperature controlled chamber in
vacuum.

> Considering the rarity and extreme cost of H-masers, or just really
> exceptional quarts oscillators; might it be the case that optical LOs
> start looking interesting for applications which just need stability
> (or being steered by other sources; e.g. GPSDL)?

Well, an 8607 costs more than a Rb-standard. Yes, the 8607 has lower
close in phase noise and up to several 1000s it rivals the Rb, but
handling it is much more difficult than handling an Rb.
Also, if you want to buy one of those exceptionally low noise/high stable
8607's (those that go down into the 10^-14 range) you'd have to sell your car.

But, if you buy a H-maser from SpectraTime, you get a 8607 for free ;-)

There used to be quite some literature on how to build low noise
quartz oscillators. Most of those books are out of print today.
With two notable exceptions:

"Discrete Oscillator Design: Linear, Nonlinear, Transient, and Noise Domains"
by Randall Rhea, 2010

and

"Understanding Quartz Crystals and Oscillators", by Ramon Cerda, 2014

I had a look at the book by Rhea, it looks quite well written and contains
a lot of real world information, but is a bit weak on the more theoretical
part (description of oscillation, noise sources,...) and thus on the
on the why things are done that way.
I didn't had the chance to buy Cerdas book yet.

The UFFC has some of the older books online. You need to be registered
to access them, though.

There is also a lot of knowledge on quartz crystalls hidden in old papers,
but going trough them is some serious work.

On the topic of opto-electronic oscillators, those are technologically
nice, but they are rather bulky. That's why they are mostly used in 
research projects for atomic clocks. Also getting them to do low phase
noise is not that easy, and unlike quartz oscillators, there is not
much literature about that.

> (They can be down-converted to microwave frequencies using an optical
> comb; a mode-locked laser whos pulses are phase locked to an incoming
> beam.)

That is actually the current trend. There was a paper by NIST last year
on downconverting the beat frequency of an optical comb down to RF using
a frequency divider chain. They managed to get noise measures that rival
that of a good quartz oscillator at 5MHz. Ie at higher frequencies, it is
actually better than what a quartz oscillator can deliver.
(for some reason i have not archived that paper and google fails me)
 
> Certainly just the local oscillator is _closer_ to something a
> time-nut might experiment with than a complete optical atomic standard
> (if still not quite in reach).

Well, building a CPT based Rb vapor cell frequency standard should be feasible.
Yes, it's not a primary standard, but should do the job for most :-)

>From what i've read, using one of the MOT cells like those of
Sachser Laser [1] one might even be able to build a primary standard.
But my understanding of MOT is relatively weak and i cannot say how
difficult it actually would be. But it would be definitly a fun project
to try :-)

But i agree, building one of those ion or neutral atomic standards is
pretty much out of question on a hobby budget. Heck, even an optical
frequency comb is difficult to build, at best. And buying them.. i think
buying a good Rb is still cheaper.

			Attila Kinali


[1] http://www.sacher-laser.com/home/lab-equipment/spectroscopy/reference_gas_and_vapor_cells/mot_cells.html

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