Presumably two different instances of this clock would have different tick rates due to the specific crystal used and it's very difficult to produce two identical crystals.

An atomic clock is very consistent across devices as it exploits the properties of an element which is much more repeatable (just have to have a quantity of the element).

Reminds of the Anne McCaffrey Crystal Singer series where crystals were mined for use in their space-faring technology. Each crystal was unique and would be tuned to a particular use.

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

There could be issues with creating accurate copies of this. You could still get very precisely spaced ticks but not get an accurate measure that could be replicated around the world. A standard doesn’t work well if there’s one extant copy you can compare against. That’s why they’re working so hard to create a new standard kilogram where it could in theory be replicated without reference to the original.

> That’s why they’re working so hard to create a new standard kilogram

That's done, for now: https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...

Ironically in terms of this conversation, the new definition of a Kg requires a time measurement.

>The kilogram is the mass of a body at rest whose equivalent energy equals the energy of a collection of photons whose frequencies sum to [1.356392489652×1050] hertz.

Now the units for almost all physical quantities (the few exceptions are those that are not dependent on time, length or force, e.g. angles and amount of substance) are derived from the unit of frequency.

The frequency of an atomic clock with cesium has a conventional value, and the units for the other physical quantities, e.g. length, force, voltage, electrical current, temperature and so on are derived from the unit of frequency by conversion factors that are functions of various universal constants, all of which have conventional values chosen so that the units derived in this way are approximately equal with the units that have been used in the past.

Neat, I just remembered the quasi competition between the Si-28 sphere and the torsion balance (which I think is what they've gone with).

What they meant by accuracy and precision was actually about which is the average value and which is the variance of the duration of a clock period.

What you want is that both the average duration of a clock period is exactly a certain fraction of a second and also its variance is very small.

However, you cannot have both in the same clock device.

For atomic clocks, you know very well which is the average value, but the duration of the clock periods has a large variance.

For sapphire clocks, the variance of the duration of the clock periods is very small but the average value is known only approximately.

According to the terminology used in the article, someone who shoots accurately spreads the shots over a large area, but the center of the area is the same as the center of the shooting target.

On the other hand, someone who shoots precisely sends all the shots through the same hole in the shooting target, but that hole is far from the center of the target.

The result of the difficulty of having both high accuracy and high precision in the same clock device is that whenever you measure short times, you must use either quartz clocks for a cheaper solution or sapphire clocks for higher precision (or also hydrogen masers in the past, before the development of sapphire clocks).

Because the average frequency of the precise clock is not known, you must either measure it periodically with an atomic clock and correct digitally the measurement results or you must use a PLL loop to adjust continuously the frequency to be in a certain ratio with the average frequency of the atomic clock.