## NIST Ytterbium Atomic Clocks Set Record For Stability 85

New submitter bryanandaimee writes

*"An optical lattice clock like the one discussed earlier on Slashdot has broken the stability record. Comparing two OLC's using trapped atoms of Ytterbium, the stability of the clocks was measured to 2 parts per quintillion (10^18). While the previously reported OLC used strontium, these clocks, built by another group, use Ytterbium. Interestingly, while the stability of the clocks is now the best in the world, the accuracy has yet to be measured."*
## Re:Weird choice of measurements (Score:4, Informative)

I guess accuracy here refers to the cycle-to-cycle jitter. That is, how much the clock edge position varies when looking at single cycles.

I guess stability here refers to wandering. That is, how much the single cycle error accumulates when looking at e.g. 1 million cycles.

## Re:Weird choice of measurements (Score:5, Informative)

Accuracy measures how close the frequency is to the target, on average. Stability measures how the frequency drifts over time (and temperature, etc.). Accuracy is more of an absolute measurement while stability is more of a relative measurement. From the article:

and

So it sounds like accuracy is defined in terms of how well the clock reproduces the ideal frequency of the physical process it's based on. Hopefully there's a physicist or two around who can give us the exciting details.

## Re:Weird choice of measurements (Score:5, Informative)

Imagine a mechanical clock that has such a heavy minute hand that it goes much faster down than up. But it returns after exactly one hour, and even after ten years, it shows the full hour accurate to the second. This is a clock with very low stability, but quite high accuracy (for a mechanical clock, for an atomic clock that would of course still be terrible accuracy).

On the other hand, imagine a mechanic clock which doesn't have this stability problem, but the pendulum is not compensated for temperature changes. That is, when it gets warmer, the pendulum gets slightly longer and the clock goes slightly slower. Now temperature doesn't change very fast, so you'll not notice the effect in a short time span. However over time, the clock will drift away from the correct time, unless you manually correct it. This is clock with good stability, but not so good accuracy.

## Re:Weird choice of measurements (Score:5, Informative)

I work with atomic clocks for a living. You are on the right track.

Stability is the amount of variation over time. Clock frequency changes as parts age, temperature varies and so forth. Sometimes the variation is predictable and can be compensated for, sometimes not.

Accuracy is how close the frequency is to the specified one. A cheap ebay atomic clock can do 1 second per century.

There is also jitter which is the cycle to cycle variation of the output, which could be a square or sine wave. For some applications it matters.

## Re:Weird choice of measurements (Score:5, Informative)

What is the reference clock against which other clocks are measured?Good question. For things like drift you can compare a new and an old clock, or compare one in a cold environment to one in a cool environment. Actually, most atomic clocks have a heater built in that maintains a stable temperature internally.

Even just comparing two clocks of the same age under the same conditions is useful, because they will never be at

exactlythe same frequency so will eventually drift apart.Beyond that it's a question of determining what the uncertainties are using physics. In other words it's a mathematical proof, rather than the result of an actual measurement. Obviously measurements are made to make sure the clock is close to other known accurate clocks, but the ultimate determination of frequency accuracy is a paper exercise.