Kibble balance

The Kibble balance is so precise and reproducible that it was responsible for the 2019 redefinition of the Kilogram.

It relies rely on not one, but three macroscopic quantum mechanical effects:

atomic spectra: basis for the caesium standard which produces precise time and frequency
Josephson effect: basis for the Josephson voltage standard, which produces precise voltage
quantum Hall effect: basis for the quantum Hall effect, which produces precise electrical resistance

How cool is that! As usual, the advantage of those effects is that they are discrete, and have very fixed values that don't depend either:

on the physical dimensions of any apparatus (otherwise fabrication precision would be an issue)
small variations of temperature, magnetic field and so on

One downside of using some quantum mechanical effects is that you have to cool everything down to 5K. But that's OK, we've got liquid helium!

The operating principle is something along:

generate a precise frequency with a signal generator, ultimately calibrated by the Caesium standard
use that precise frequency to generate a precise voltage with a Josephson voltage standard
convert that precise voltage into a precise electric current by using the quantum Hall effect, which produces a very precise electrical resistance
use that precise current to generate a precise force on the object your weighing, pushing it against gravity
then you precisely measure both:
- local gravity with a gravimeter
- the displacement acceleration of the object with a laser setup

Then, based on all this, you can determine how much the object weights.

How We're Redefining the kg by Veritasium

The Kibble Balance, realizing the Kilogram from fundamental constants of nature by Richard Green

. Source. Presented in 2022 for a CENAM seminar, the Mexican metrology institute. The speaker is from the Canadian metrology institute

youtu.be/ZfNygYuuVAE?t=854: they don't actually use the Quantum Hall effect device during operation, they only use it to calibrate other non-quantum resistors

The Watt balance and redefining the kilogram by National Physical Laboratory

. Source. Nothing much, but fun to hear Kibble talking about his balance in beautiful English before he passed.

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