Improved atomic clocks

Articles written by US Government entities like NIST cannot be copyrighted, and all such articles are available gratis on the NIST web site somewhere.

Joe

As far as I know, NIST publications were planned to be publicly available, but are not currently available to the great unwashed masses for free.

"The NIST Plan for Providing Public Access to Results of Federally Funded Research 2023 edition"

"Public Access to NIST Research"

It's always been hidden in plain sight, well preceeding 2023. The pdf files have names of the form "<integer>.pdf", with no hint as to what's inside.

Joe

Given a narrow-bandwidth optical source, how does one divide it down to electronic frequencies?

Rubidium clocks use some optical-microwave interaction in the rubidium vapor itself.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

That is more a feature of the cursed content management system. You can usually find stuff published in the open literature on arXiv. Many academic journals encourage researchers to publish there (even ones that have a paywall on the main site).

You heterodyne it with one line of a modelocked Ti:sapphire laser. The Hall-Haensch frequency comb works by broadening the pulse spectrum to an octave (by narrowing the pulses) and then locking a line on the high end to the second harmonic of one at the low end.

That gives all the lines the same absolute stability (i.e. in hertz) as the RF reference. That bit of extreme cleverness got them the 2005 Nobel prize, and well deserved it was. (I know both of them slightly.)

Cheers

Phil Hobbs

This question has been asked here before. The 2005 Nobel prize in physics went to the people who developed this technique (amongst others).

Whatever.

Optical Frequecy Combs, Tara Fortier et al, NIST, 2019 and subsequent.

See

Joe

Yes, and the Nobel was well deserved. But Ti:sapphire lasers are still boat anchors even today.

What has happened since 2005 is that the Telecom industry has developed and miniaturized the needed components, developing robust and simple solutions.

Joe

All true, but NIST is a special case. By US law, US Government publications cannot be copyrighted (because taxpayers already paid for them), so NIST always has an archive where one can get a copy, even if the article is also published in a formal journal behind a paywall.

So a typical approach is to search for an article in the usual ways, and when the exact title is found, search for the title alone. This will also bring up the relevant NIST archive URL.

Joe

OK, that's cool. A modelocked laser runs roughly 100 MHz but makes such narrow pulses that it has harmonics in the optical range.

John Larkin Highland Tech Glen Canyon Design Center Lunatic Fringe Electronics

Nice tip, thanks. One can also often obtain a copy of journal articles by contacting the lead author, which can work for papers not paid for by taxpayers, which is the case for some of the papers on optical lattice clocks and hyperfine transitions, of which there are at least 2 for 87Sr and at least 1 for Yt and Al. I decided I did not to try to figure out how they get millihertz linewidth at visible light frequencies when I got to the part about interaction with the nucleus allowing a doubly- prohibited state. The part about keeping an array of atoms at ~2 uK is easier to understand :-).

Glen

Welcome.

And Thorium clocks are coming soon.

Joe

I suppose if you chop up a laser beam into short pulses and squirt them into a cloud of Rubidium vapour, the exactly 6.834 682 610. 904 GHz harmonic component will get scattered by the rubidium vapour, and you will be able to pick up the scattered photons emitted at right angle to the beam.

Presumably there would be a phase shift on the detected pulses, which you might be able to use to lock the harmonic number precisely.

That would be the 68th harmonic of a laser beam pulsed at just above

Discharge lamps are crude beasts - the rubidium vapour doing the emission is hot, so the atoms are moving fast enough to Doppler shift the emissions. The description has a Rb-85 buffer gas cell between the discharge lamp and the Rb-85 gas detector cell which may tame that.

Trapping the atom in optical wells so that they aren't moving at all is clearly a superior (and much more complicated) system.