[time-nuts] IEEE Spectrum - Dec 2017 - article on chip-scale atomic frequency reference

[Bob kb8tq]

Hi

If you dig back into the various papers on the subject (and the proceedings
that log the post paper questions) the issue of “can we trust the implementation?”
does indeed come up. It’s come up for at least the last 50 years that I’m aware of.
The basic argument runs that for fundamental standards, you need to approach
the process in different ways. You then compare the results from those different 
methods. Only after you have done that, do you build confidence in the accuracy
of the various processes. Indeed once you have built confidence in a single approach, 
you may decide to all go with that one approach or implementation. 

Bob

> On Dec 9, 2017, at 12:54 PM, Chris Caudle <chris at chriscaudle.org> wrote:
> 
> There is a piece missing for me in the articles I have found on new atomic
> standards.
> 
> This is what I (think I) do understand:
> Quantum properties of the atoms can be interrogated using various RF or
> optical means to servo the frequency of an oscillator (which could be a
> laser based optical  oscillator).
> 
> The international standard for frequency (based on time) is defined in
> terms of a theoretical condition of cesium atoms which cannot be perfectly
> achieved in practice, needing absolute zero temperature,
> gravity/acceleration equivalent exactly to mean sea level of earth, no
> magnetic perturbation, no interaction such as bouncing off of cavity
> walls, etc.
> 
> New optical standards can achieve "accuracies" of parts in 10^16, verified
> by comparing multiple instances of the standards with each other, and if
> the standards are built correctly and the theory of operation is correct,
> the multiple separate pieces of equipment should agree in frequency output
> to within some parts in 10^x, where x has historically been around 15, but
> is now reaching toward 17.
> 
> So far so good, but here is where I have a gap:
> I put "accuracies" in quotations above because as far as I understand you
> can actually compare consistency of center frequency or stability over
> periods of time between two instances of a particular type of atomic
> oscillator, but accuracy in the sense of comparing how closely the the
> output frequency matches the calculated theoretical output frequency
> (assuming that the operating mechanism is fully understood) is going to
> depend on having a reference for comparison that is as good or better than
> the new standard to be measured.  That implies that the reference has
> systematic offset that is known to better than parts in 10^17, but that
> would require knowing the quantum properties of the atoms in use to that
> level, knowing the gravitational potential at your location to that level,
> knowing that the temperature dependence of the equipment was below that
> level, etc.
> 
> How can anyone ever talk about accuracy in the terms of SI second
> definition for these new oscillators?  Are they really using layman's
> shorthand, and they mean stability and consistency?  Or are they really
> able to measure all the other factors well enough that they can actually
> mean accuracy in the sense of how the SI second definition calls out
> absolute zero, gravitational potential, etc.?
> 
> -- 
> Chris Caudle
> 
> 
> 
> 
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