
Keeping Time at NIST - oedmarap
https://www.nist.gov/blogs/taking-measure/keeping-time-nist
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sasaf5
On laser cooling:

> Very basically what they did was use a specially tuned array of lasers to
> bombard the atoms with photons from all angles. These photons are like
> pingpong balls compared to the bowling-ball-like atoms, but if you have
> enough of them, they can arrest the motion of the cesium atoms, slowing them
> from about 130 meters per second to a few centimeters per second

Such a cool idea deserves a better brief explanation: Atoms absorb light in a
specific frequency, and are pushed by it. If your light is at a slightly lower
frequency, due to doppler effect only atoms that are moving towards you will
absorb and be slowed down. Atoms going in other directions will be
undisturbed. The net effect is a reduction of movement, hence cooling.

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Ecco
Oh wow, neat explanation, thanks! For some reason I always thought laser
cooling was about “cornering” atoms in between laser beams. But then
apparently from your explanation it’s really only about finding the right
laser color! Then you can beam it in pretty much any direction and it will
result in a global reduction of the speed of atoms (aka temperature). Did I
get that right?

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sasaf5
Yes that's the general idea. Lasers are used because of their temporal
coherence (i.e. they shine in a single, stable, frequency).

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willis936
A critical part of a laser is coherency: all energy is at the same frequency
_and_ phase. This is really what sets laser light apart from other kinds of
light and what makes it such a useful science tool.

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rubidium
One other neat use of really accurate clocks: measure if the fundamental
constants drift relative to each other.

[https://onlinelibrary.wiley.com/doi/full/10.1002/andp.201800...](https://onlinelibrary.wiley.com/doi/full/10.1002/andp.201800364)

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mikewarot
I have an older friend who has always wanted an "atomic clock", and he bought
one off of e-bay, an HP-5061b.... it almost worked right. Between the two of
us, we got it working, and tuned to the right Zeeman frequency. Once we got
the hang of repairing them, two was even better.... we put each one into the
XY channels of an oscilloscope.... the pattern never shifted over hours... so
cool!

The optical clocks and optical frequency combs mean there are devices that you
can buy, with internal atomic clocks, that are effectively Frequency Counters,
for blue light... that read down to the Hertz!

I want one of those when I'm older. ;-)

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javier10e6
I remember once seeing those NIST guys one evening at Dogfish Head Alehouse
across the street talking physics and neutrinos. To measure time between two
events, something needs to move faster than those events to perceive it. After
a few beers, you'll be left with the check.

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Ecco
I really wonder how we’ll use that mega-accurate clock that takes like a
second every gazillion years. Like the article said, GPS is great but even
then you don’t need such accuracy.

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m-ee
Calibrating slightly less accurate clocks. What those clocks are used for I
have no idea, but there’s times you need a good traceable measurement for some
calibration of your own and your uncertainty stack can start to add up. The
better, smaller, and cheaper NISTs equipment is the better your local cal lab
will be and the easier your work is.

Try finding a thermometer setup that’s better than 1 degree accurate from
0-400C. It might be more than you expect. Now imagine you want to measure some
property of a sensor or material as function of temperature using your new
thermometer, add in the cost of an oil baths and precision multimeters. Now
say you want to create your own golden standard with the best precision you
can muster, your expensive oil bath is looking lacking and you’re buying
Vienna mean standard ocean water...

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TooKool4This
Well said. The ability to better realize primary standards and disseminate
that to secondary metrology labs from national standards is so critical.

It’s the same reason I think the work done in changing the SI definition to
physical constants is so critical. In a decade or so all secondary labs should
have capabilities to measure directly to the definition of a unit without
needing to do comparison measurements to national standards. To me it seems
like the metrology equivalent of “open sourcing SI units”

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m-ee
It's important work and it can be surprisingly easy to run into the limits of
your uncertainty. If you ask someone to measure a temperature in my experience
9/10 times they'll grab a type K thermocouple. Far fewer than 9/10 will tell
you that the number they gave you is only guaranteed to be +/-3 degrees.
That's good enough for a lot of applications but not all of them, and when
something is really critical it helps to understand the full stack if your
supplier/lab/coworker is making questionable claims about their accuracy.

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jiveturkey
Perhaps not Strontium, but the other clock types they talk about, Rubidium and
Caesium, are readily available commercially at quite affordable prices.
Rubidium clocks in particular are very widely available and every RF lab will
have one.

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raverbashing
I wonder how the "conversion" between cycles of different materials work. The
spec says Cesium but it seems other clocks use different materials, how do you
know X cycles in Cesium is Y cycles in Strontium for example

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fsh
It is not possible to calculate the ratio between the transition frequencies
in different atoms with anywhere near sufficient accuracy for metrological
applications. Therefore, only Cesium clocks can serve as primary standards for
the SI second. Other clocks can be calibrated to Cesium clocks in order to
serve as secondary standards, but it is not possible to measure a frequency in
Hz with an accuracy that is better than that of the best Cesium clocks.
Probably the definition of the SI second will be changed at some point, but
currently there are still too many possible candidate species and transitions
without a clear favorite.

