An article discussing the progression of atomic clock technology, and also relating to something I posted earlier, a discussion of what happens when the next generation of atomic clocks is deployed: the clocks won’t be the limiting factor in determining the time.
To tell the time consistently, all clocks need to be at a known height relative to Earth’s “geoid”, an imaginary surface that links points at which the gravitational field has the same strength. But the height of this geoid varies over time at any given place by up to 20 centimetres, because of effects such as tectonic movements, glacial melting and changes in ocean levels, and varying atmospheric pressure. Changes of that magnitude could wreak havoc with any attempt to establish a global time standard at an accuracy of 1 part in 1018 or better
One of the things that always glossed over in these discussions is that almost everything that is called a clock is actually a frequency standard, which is part of a clock. Clocks run continuously, because you are measuring a phase, and frequency standards don’t. Now, there’s a caveat here in “running continuously,” because even commercially-available clocks will skip measurement cycles to do self-diagnostics. During those skips in measurement, as with the time between measurements, the frequency is maintained with some oscillator. Usually this is a quartz crystal, which typically has excellent short-term stability. The important point becomes how long the clock is running on the “flywheel” oscillator and what kind of degradation that introduces. The latest generation of frequency standards run for several hours, but then are shut down for extended periods of time, which is not surprising for a cutting-edge kind of experiment. But while the frequency standard is not running, the clock’s performance approaches whatever the flywheel performance is, whether that’s cesium beam clocks or hydrogen masers, etc., or some ensemble made up of several clocks.
So when these stories appear touting the great performance of cutting-edge clocks, there’s an unwritten implication that we will be getting an improvement in the flywheel operation as well, to be able to leverage this improved frequency-standard performance.