8 ms·

Keeping Time at NIST

87 points by oedmarap 6y ago

sasaf5 6y ago
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.
- Ecco 6y ago
  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?
  - sasaf5 6y ago
    Yes that's the general idea. Lasers are used because of their temporal coherence (i.e. they shine in a single, stable, frequency).
    - willis936 6y ago
      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.
  - fsh 6y ago
    Laser cooling only reduces the velocity component that is directed against the direction of the laser beam. For free atoms you therefore need three pairs of counter-propagating beams to achieve overall cooling. If the atoms are somehow confined (for example ions in a trap), their motion has to reverse periodically and a single cooling beam can be enough.
- stan_rogers 6y ago
  Bill Phillips (a NIST type who's also a Nobel laureate) gave a lecture called "Time, Einstein, and the Coolest Stuff in the Universe" at several locations a few years ago, and there are various videos of it online. It explains things rather thoroughly, and is a great example of an enthusiastic scientist going full nerd, including several classes of really bad jokes, etc. Worth a Google and a watch. If you can find the one he did in Toronto, that's probably the best one, but TVO (our provincial public broadcaster in Ontario) doesn't seem to have it up on their YouTube channel anymore.
Ecco 6y ago
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.
- garmaine 6y ago
  All the time in certain kinds of lab work.
- wahern 6y ago
  Improved gravimetric geodesy? https://arxiv.org/pdf/1803.01585.pdf https://arxiv.org/pdf/1803.01585.pdf
  - jhayward 6y ago
    That's a nice overview paper. There is much value in the gravimetric mapping realm and we're not even close to the limits of what people are imagining. For instance, by combining multiple ultra-precise orbiting clocks with quantum teleportation networks linking them for time transfer one could gravimetrically map the planet in 3D at millimeter resolutions. E.g.: these clocks would be able to carefully map the shape of Earth’s terrain down to the millimeter. “You’d be able to see someone digging a tunnel under the U.S.-Mexico border from space” [1] One other capability of such a system: whoever possessed it would be able to locate every single significant concentration of dense matter, e.g., plutonium on the planet no matter where it was stored. They would know exactly how many nuclear weapons, etc. everyone had and exactly where they were keeping them. [1] https://www.sciencenews.org/article/quantum-timekeeping https://www.sciencenews.org/article/quantum-timekeeping (paywall)
- m-ee 6y ago
  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...
rubidium 6y ago
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.201800364 https://onlinelibrary.wiley.com/doi/full/10.1002/andp.201800...
raverbashing 6y ago
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
- fsh 6y ago
  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.
javier10e6 6y ago
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.
jiveturkey 6y ago
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.
mikewarot 6y ago
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. ;-)

87 points by oedmarap 6y ago

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.

Ecco 6y ago
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?
- sasaf5 6y ago
  Yes that's the general idea. Lasers are used because of their temporal coherence (i.e. they shine in a single, stable, frequency).
  - willis936 6y ago
    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.
- fsh 6y ago
  Laser cooling only reduces the velocity component that is directed against the direction of the laser beam. For free atoms you therefore need three pairs of counter-propagating beams to achieve overall cooling. If the atoms are somehow confined (for example ions in a trap), their motion has to reverse periodically and a single cooling beam can be enough.
stan_rogers 6y ago
Bill Phillips (a NIST type who's also a Nobel laureate) gave a lecture called "Time, Einstein, and the Coolest Stuff in the Universe" at several locations a few years ago, and there are various videos of it online. It explains things rather thoroughly, and is a great example of an enthusiastic scientist going full nerd, including several classes of really bad jokes, etc. Worth a Google and a watch. If you can find the one he did in Toronto, that's probably the best one, but TVO (our provincial public broadcaster in Ontario) doesn't seem to have it up on their YouTube channel anymore.

Ecco 6y ago

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.

garmaine 6y ago
All the time in certain kinds of lab work.
wahern 6y ago
Improved gravimetric geodesy? https://arxiv.org/pdf/1803.01585.pdf https://arxiv.org/pdf/1803.01585.pdf
- jhayward 6y ago
  That's a nice overview paper. There is much value in the gravimetric mapping realm and we're not even close to the limits of what people are imagining. For instance, by combining multiple ultra-precise orbiting clocks with quantum teleportation networks linking them for time transfer one could gravimetrically map the planet in 3D at millimeter resolutions. E.g.: these clocks would be able to carefully map the shape of Earth’s terrain down to the millimeter. “You’d be able to see someone digging a tunnel under the U.S.-Mexico border from space” [1] One other capability of such a system: whoever possessed it would be able to locate every single significant concentration of dense matter, e.g., plutonium on the planet no matter where it was stored. They would know exactly how many nuclear weapons, etc. everyone had and exactly where they were keeping them. [1] https://www.sciencenews.org/article/quantum-timekeeping https://www.sciencenews.org/article/quantum-timekeeping (paywall)
m-ee 6y ago
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...

rubidium 6y ago

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.201800364 https://onlinelibrary.wiley.com/doi/full/10.1002/andp.201800...

raverbashing 6y ago

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

fsh 6y ago
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.

javier10e6 6y ago

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.

jiveturkey 6y ago

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.

mikewarot 6y ago

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. ;-)