[adrian_b]

In a normal optical atomic clock, you use a laser whose frequency is determined mainly by the resonant cavity. The laser is made tunable with some additional device, e.g. a piezoelectric actuator, which can make small adjustments to the resonance frequency of the cavity.

In order to tune the resonant cavity, one generates a signal that is proportional with the frequency difference between the current frequency of the laser and the frequency of an absorption line from the spectrum of some reference atoms. There are various methods for the generation of such frequency difference signals, by modulating the laser signal and passing it through some space where the reference atoms or ions are held by various methods, e.g. an optical lattice of neutral atoms, an ion trap or just a cell with metal vapor.

By tuning the cavity, the frequency locked loop ensures that the integral of the frequency difference signal over a long integration time is approximately null, which guarantees that the average frequency of the laser is equal to the frequency of the spectral line of the reference atoms.

If instead of using a laser whose frequency is determined by the resonant cavity, you use one whose frequency is determined by the stimulated emission spectral line of the laser medium, like in the parent article, you no longer need a system of control of the laser frequency. The laser frequency is itself the reference frequency.

Unfortunately, such a laser will have a very low output power. This means that if you detect the output signal of the laser it will have a very low signal-to-noise ratio. Because of this you will still have to average the laser frequency, to filter the noise. Hopefully, for filtering the noise shorter integration times will be sufficient, in comparison with those needed for the existing optical clocks.

While shorter averaging times are a possible advantage, more important is that the frequency should be less sensitive to environmental factors, like vibrations and temperature. This could enable such optical atomic clocks to be used e.g. in vehicles. Nowadays the optical atomic clocks that can be used in vehicles are many orders of magnitude less accurate than those that are restricted to a well protected laboratory environment.

[WillPostForFood]

Nowadays the optical atomic clocks that can be used in vehicles are many orders of magnitude less accurate than those that are restricted to a well protected laboratory environment.

If anyone is wondering, we aren't yet to the point of having an atomic clock in the dashboard of your Toyota. But they have been reduced to ~suitcase size. Example if one being tested in a Navy ship:

https://www.geoconnexion.com/in-depth/scientists-create-new-...

[dtgriscom]

https://www.microchip.com/en-us/products/clock-and-timing/co...

[adrian_b]

The microchip miniature atomic clocks are not optical atomic clocks, but old-school microwave atomic clocks.

They are orders of magnitude less accurate than even portable optical atomic clocks and the difference is much greater in comparison with SOTA laboratory cesium clocks or hydrogen masers, which are again orders of magnitude less stable than the best laboratory optical atomic clocks.

However, these miniature atomic clocks are much smaller and cheaper than better atomic clocks and there are applications where something better than an OCXO-based quartz clock is desired.

[adrian_b]

Indeed, this clock, which uses iodine absorption cells to provide the reference frequency, is one of the kinds of already existing portable optical atomic clocks to which I was referring.

The best laboratory optical clocks, which use ion traps or optical lattices with neutral atoms, have a higher accuracy by up to 6 orders of magnitude, which makes much harder for the system that stabilizes the length of the laser cavity to keep up with it.

Minute length changes that would not matter for a less accurate iodine clock would cause unacceptable frequency shifts in a SOTA optical clock. Therefore such optical clocks are much more sensitive to their environment.