Counting Down to the New Ampere (2016) | Hacker News Reader

Counting Down to the New Ampere (2016)

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Does anyone know why these apparatuses are usually "inverted", i.e. hang from the ceiling. I've seen the same thing for quantum computers. Is it related to the way they are cooled? Or is it easier to work with somehow?

Swinging wild-ass guess - it might because it makes it easier to damp the equipment against vibrations.

The STM in a surface chemistry lab I worked in for a while was inside a box, and hanging from the roof of the box. By choosing appropriate springs to hang the sensitive bits from and sound proofing the box, it could adequately isolate the scanning table from its environment.

I'm sharing this prior to reading it:

https://en.m.wikipedia.org/wiki/Dilution_refrigerator

The Germans working on the same problem are running at 100 mK aka 0.1 K, so pretty chilly yes

http://iopscience.iop.org/article/10.1088/1681-7575/54/1/S1/...

Thank you for sharing.

I've started searching for "dilution refrigerator" and found this video, explaining a little bit how ³He–⁴He mixture cooling works and what the applications are. I think the video also helps putting the Wikipedia article in a perspective and vice versa.

Quantum Cooling to (Near) Absolute Zero (2013): https://www.youtube.com/watch?v=7jT5rbE69ho

One thing that I know is that it is easier to dump vibrations by hanging your device with springs than by supporting it above springs.

But I don't think this is relevant here.

EDIT: Yeah, pohl answered it. Vibrations are not relevant.

If you read the article he posted the cooling mechanism is very susceptible to vibrations, which is why they suspend it.

The entire apparatus sits in a cryostat (a fancy Thermos bottle) which is filled with liquid helium. In general, one supports things in liquid baths from above.

It's because in a dilution refrigerator, the bottom of the dilution unit gets cold. So its easier to put the coldest part at the bottom.

More exciting is that the mol and Avogadro's constant will be pushed off to the side on their own. Hopefully an even more future update will remove them entirely. They're really quite redundant and pretty much only exist to facilitate the needless presence of the non-SI mass unit, the unified atomic mass unit, which thankfully will now be defined in terms of the kg instead of having two independent mass units like we have now.

The mole and amu's (similar concepts) will never go away. It's a fundamental concept in chemistry. Reactions happen between individual particles but counting out individual atoms or molecules to supply for a reactions is impossible or ridiculously impractical at best. So chemicals have to be measured by mass (which we can measure) and then converted into number of particles. Until a molecular counting device exists, the mole will remain.

It won't go away for the same reason pounds and miles won't go away - people who know it can't be bothered learning something new, and people who are learning it aren't influential enough to cause change. Avogadro's constant is not in any way fundamental. It exists to reconcile the two different mass units that chemists use - gram and amu.

With the 2018 SI change, Avogadro's constant will be defined as an arbitrary number without any physical basis, and the amu will be a constant multiple of the kg. No more 1/12 the mass of carbon 12.

Of course we'll still need a way to represent large numbers, but there's no fundamental reason it has to be such a complicated number. It could be exactly 10^24, for instance. Again, I agree this isn't going to happen because of legacy inertia.

Why is an ampere a fundamental unit but,not a coulomb?

I suspect it's just random, based on what was convenient when we first found out about electric charges and current.

If you go back in history to when the SI units were defined, there would have been no way to measure a Coulomb of electrons, since in nature you'd somehow have to distinguish the electrons you are counting from all the other electrons sitting around. With the invention of single electron counting, we could do it now, but we aren't going to change the SI system a century in. And, the idea was that the seven primary units are both independent (you can not derive one from any of the others) and sufficient to derive all of the other units of measure. Once you have the Ampere, you can define a Coulomb as a relative quantity.

A mostly unrelated question, in case any metrology geeks are around: why is the Kelvin an SI unit?

Naively, there seem to be multiple approaches to derive temperature from other, more fundamental units. Like using the thermodynamic definition, 1/T = dS/dE, or using Boltzmann's law to approach temperature from the mean kinetic energy of gas particles. Are none of them suitable for precise measurement?

It's not really possible to derive temperature from other more fundamental units. For example, you can't define entropy as an absolute number; it needs to be assigned a unit, and the standard way of doing this is to multiply it by the Boltzmann constant, which depends on a unit of temperature. The mean kinetic energy of gas particles (3/2 kT) also depends on the Boltzmann constant.

Currently the SI is making the triple point of water fixed. I think his point is to make the Boltzmann constant fixed, like the SI is doing for the speed of light, and derive the Kelvin from there. For example 1K could be the temperature at which an atom of (insert some gas here) has a mean kinetic energy of 1.3806488 * 3/2 * 10^-23 J.

However, you have to make sure the definition can be made into an experiment, unlike the old Ampere definition from the article. A definition that mentions perfect gases wouldn't work, if no actual gas exists that can provide a better precision than the triple-point experiment.

Edit: the Kelvin is changing too: https://www.eurekalert.org/pub_releases/2017-04/pb-rft040517...

> For example, you can't define entropy as an absolute number; it needs to be assigned a unit,

You can define entropy directly without reference to other units, although it's a bit awkward. Entropy is the log of the number of microstates that correspond to a system's macrostate. Concretely, if you put n mols of ideal gas molecules in a box of volume V at a pressure P and temperature T, there is some large number of microstates corresponding to all those parameters. Entropy is the log of this number.

In classical mechanics, there's a normalization problem if you try to get an actual number out of this type of problem -- the microstates and all the macroscopic parameters are continuous. In quantum mechanics, though, this issue is solvable, although it's still awkward.

I can imaging a different type of system in which entropy really can be calculated, though. Imagine a particle that can be in exactly one of two states that are macroscopically identical. Now try to cool the system so that the particle is in one of those states of your choice. To do so, you will need to dump exactly 1 bit of entropy.

1 bit of entropy is tiny, but adiabatic demagnetization refrigerators work kind of like this, albeit in reverse, and I could imagine an experiment that would use a device like an adiabatic demagnetization fridge to remove a calibrated number of bits of entropy from some object. From this, you could, in principle, define entropy directly.

While you are right that this is the statistical definition of entropy, there are two issues: first, entropy is a unitless quantity, so defining it exactly doesn't actually move the ball forward in terms of defining units of measure. Second, outside of its theoretical underpinnings, physicists and chemists hardly ever talk about absolute values of entropy, they almost always use differences in entropy -- which factors out the need to define the exact number of microstates present before and after.

> first, entropy is a unitless quantity

The parent post asked about defining temperature in terms of more fundamental units. Entropy is typically written in J/K. If you treat it as dimensionless, then you get a definition of temperature in terms of energy for free. My point is that you can, in principle, actually do an experiment to make this useful.

One answer is that there are SI units for a lot of derived units. A Watt is Joule/second, for example. But that's not what you're really asking.

Kelvin is considered one of the 7 base SI units, as described here: http://physics.nist.gov/cuu/Units/units.html

I think the mean kinetic energy of gas particles is impossible to measure directly. You can compute it by measuring the temperature though :)

Maybe we will someday, the kilogram and ampere are a bit higher on the priority list right now. Also remember that S is not a quantity that we can directly measure, so 1/T = dS/dE isn't so useful for measuring T.

The effort to redefine the Kelvin would probably use an acoustic thermometer to measure the speed of sound in a gas. This would fix the Boltzmann constant. It seems a bit silly to do this when the definition of the kilogram is expected to change, though.

In the meantime, triple-point cells are pretty convenient, much more convenient than the international prototype kilogram, or the infinite length of wire used to define the ampere. Also remember that laboratory temperature measurements tend to be much less accurate than voltage or mass measurements. ITS-90 is apparently good to within 100ppm, but 1ppm or better is no problem when you're measuring voltage, mass, or length.

Here's a presentation on thermometry, the acoustic thermometer described is what's used to accurately measure the Boltzmann constant: https://www.youtube.com/watch?v=Irr8fOLtiWc

Why is the kilogram the base unit instead of gram?

The kilogram has a suitable scale for physical events that matter in common engineering situations.

Acceleration due to gravity is a nice number between 0 and 10 if measured in "kilogram-meters per second squared".

So does this mean that quantum computing will become more viable? Since in quantum computing, calculations are accomplished by measuring the spin of an electron, I would imagine this would increase the throughput to a measurement instrument since this is allowing one electron to pass at a faster pace. While electron measurement instruments still need to be advanced significantly, I would imagine an innovation like this would further advance the reality of true quantum computing. Is this an accurate assessment?

I think you'd also need to redefine the meter and second to speed up light. Unfortunately, the footnote specifies that we can expect only "major changes in the kilogram, ampere, kelvin, and mole" in the new SI 2018, so it seems we'll have to wait for the release after that.

On the bright side, the changes to the kilogram might begin to address the obesity crisis.

Meter is already specified by fixing the speed of light. Seconds are defined in terms of Cesium atom vibrations.

> Seconds are defined in terms of Cesium atom vibrations.

Not vibrations of the atoms themselves. The second is defined in terms of the period of the radiation corresponding to a particular hyperfine transition of the Cesium atom.

What does "hyperfine" actually mean, in this context?

It's a reference to a particular kind of splitting of the energy levels of electrons in atoms, due to interactions between the electrons and the nucleus.

Basically, as more and more precise measurements of the energy levels of electrons in atoms were made in the 1920s, 30s, and 40s, physicists kept finding that energy levels that were thought to be degenerate (i.e., multiple states with the same energy) were actually split into multiple, closely spaced levels. The original quantum model was the non-relativistic Schrodinger equation as applied to the atom. Then it was found that electron energy levels that were degenerate in that model were actually split into multiple levels because of the effects of electron spin and certain relativistic corrections; this splitting was called "fine structure". Then it was found that there was even further splitting, of energy levels that were degenerate in the fine structure model, due to interactions between the electron and the nucleus; this further splitting was called "hyperfine structure".

More here:

https://en.wikipedia.org/wiki/Hyperfine_structure

> So does this mean that quantum computing will become more viable?

Not really. Detecting the voltage or current differences that you need for detecting an electron spin, that's a very different thing than doing the quantitative measures you need to redefine the ampere.

For example for measuring a single electron, you can use avalanche effects, which are relatively simple to realize, but would not be suitable for the quantitative approach you need to define an SI unit.

I'm not very well-informed regard quantum computing, but the problem with it seems to be more in realm of keeping the quantum states stable and controlled, and less on the measurement side.

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