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.

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

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.

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.

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.

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.

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

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".

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.

> 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.

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.