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

Chris Caudle chris at chriscaudle.org
Sat Dec 9 12:54:30 EST 2017


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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