
Energy of the 229 Th nuclear clock transition - Tomte
https://www.nature.com/articles/s41586-019-1533-4
======
avsteele
Alright!

I did research for my PhD working towards this. We trapped and laser cooled
Th3+ with the intent to isolate and measure this transition optically that
way.

[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.10...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.233004)

~~~
cyanoacry
Neat work! Question from someone who's familiar with RF atomic clocks but not
optical -- how do you steer a laser in frequency? For RF, it all comes down to
VCOs: if you want a different tune range or different frequency, you just pick
a different VCO.

But for lasers, I thought the frequency was driven by the energy difference
between electron states for a given species? How do you synthesize an
arbitrary frequency for a laser without something crazy like a FEL?

~~~
avsteele
Atomic physicists use mostly semiconductor diode lasers.

You need to narrow their emission wavelength to something ~ the atomic
transition (MHz) and then carefully tune it as you say.

Typically diffraction grating is used to carefully feedback some of the
laser's output (<10%) back into it, and this can be used to to narrow the
emission from several nm to MHz, and to coarsely tune the frequency to few
10-100 Ghz.

Fine tuning is done by changing the current flowing through the laser or the
temperature of the junction. These both cause a frequency shift on the MHz
scale scale.

You might fine it interesting, you can also use VCOs for fine tuning, but you
take the generated RF and send it into a crystal called an Austo-optic
modulator. The laser light refracts from the RF phonons propagating through
the crystal and this can be used to shift the laser light by the RF frequency.

~~~
fsh
Typical optical clock transitions have ~Hz linewidths, not MHz.

The intrinsic frequency stability of almost all lasers is by far not good
enough to be able to probe such transitions. Therefore, ultra-stable optical
cavities are used as a frequency reference, and the laser is constantly
steered to stay on the cavity resonance by a fast electronic feedback system.
In this way, laser linewidths in the sub-Hz range can be achieved. Then an
acousto-optic modulator is used to scan the laser frequency across the clock
transition.

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thatcherc
This is suuuuper interesting! All atomic clocks since the first alkali metal
ones operate by syncing a an oscillator (either a radio or optical source) to
precisely known transitions in the electronic structure of specific atoms, and
then counting how many cycles elapse on the source oscillator to measure time.
A clock using this thorium transition has the potential to be even more
precise than contemporary clocks since it syncs to a transition in _nucleus_
of the atom: it responds to rearrangements of the protons and neutrons instead
of the electrons. This kind of transition is less affected by external factors
like fields and temperatures[0] and so should be a more stable frequency
reference.

[0] -
[https://en.wikipedia.org/wiki/Nuclear_clock#Principle_of_Ope...](https://en.wikipedia.org/wiki/Nuclear_clock#Principle_of_Operation)

~~~
xgk

       precisely known transitions 
    

There is an interesting philosophical conundrum here, in that one could argue
it's the other way around: the precision you mention is a _consequence_ of the
fact that we currently _define_ time relative to atomic transitions (the
Caesium standard [1]). So if atomic transitions fluctuated (relative to some
abstract standard that we are currently not having access to), then this would
_not_ affect the precision you mentioned. Wittgenstein famously made a similar
argument about length in the _Philosophical Investigations_ §50: _" There is
one thing of which one can say neither that it is one metre long, nor that it
is not one metre long, and that is the standard metre in Paris. – But this is,
of course, not to ascribe any extraordinary property to it, but only to mark
its peculiar role in the language-game of measuring with a metre-rule."_

[1]
[https://en.wikipedia.org/wiki/Caesium_standard](https://en.wikipedia.org/wiki/Caesium_standard)

~~~
tialaramex
So, the thing about all the definitions now is there are no prototypes (the
kilogram was the last one). So Wittgenstein's observation now becomes an
actual claim about how our universe works.

It feels intuitively obvious/ redundant that a prototype metre is one metre
long, but a statement that light in a vacuum travels a fixed distance in one
second is not so obviously redundant. It doesn't matter what colour the light
is? Nope. It doesn't matter when I measure? Apparently not.

~~~
xgk
You cannot measure how fast light travels in a fixed time unit without
reference to time: you need to define length! Length is currently defined with
reference to caesium time: the 2019 SI definition of metre takes "the fixed
numerical value of the speed of light in vacuum c to be 299792458 when
expressed in the unit m⋅s−1, where the second is defined in terms of the
caesium frequency ΔνCs." (From [1].)

As far as I can see, measuring time is the foundation of all definitions of
other units. And the core reason why time is used to define everything else,
is pragmatic: it's just technically easier to count (photon absorption) than
to do anything else.

[1]
[https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...](https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Metre)

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wolfram74
Nuclear isomers are a fascinating subject. I like to fantasize about batteries
with energy storage of KeV stored per element as opposed to chemical batteries
which would be limited to a few eV per element. This is of course limited by
our ability to effectively turn x rays and gamma rays into useful energy and
preparing meta-stable samples.

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eloff
Well I wouldn't keep my cellphone in my pocket in that case, but joking aside,
that would solve the energy density issue for electrical planes.

~~~
hwillis
That's an understatement. Li-air batteries would solve the energy density
problems for electric planes. Storing energy in nuclear isomers would be more
suited to like... single stage to orbit electric scramjets. Hovershoes and
laser guns, that kind of thing.

~~~
datenwolf
Well among other things, one problem to solve is, how to control the output
power. If you were to just charge a bunch of nuclear isomers, they'd afterward
release the energy by a regular decay process with a given half life (or call
it nuclear "flourescence" lifetime, or whatever). This puts them on the same
conceptual stage as RTGs.

It should be possible to stimulate the emission of radiation, i.e. build the
nuclear isomer equivalent to a chemical laser. That'd would allow to control
the release of power and not depend on random decay.

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NGC404
Full preprint/paper:

[https://arxiv.org/abs/1710.11398](https://arxiv.org/abs/1710.11398)

~~~
est31
That's a 2017 paper. This one looks more like it:
[https://arxiv.org/abs/1905.06308](https://arxiv.org/abs/1905.06308)

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Robotbeat
Interesting. In principle this could be used as a battery, then, emitting UV
(instead of harmful penetrating gamma radiation).

7.9eV worth of energy for each 229 atomic mass units gives a specific energy
of: 925 Wh/kg.

...unfortunately, it's only stable for long periods of time in an ionized
state. In the neutral state, it decays within microseconds.

But fantastic for a clock application. They mention a solid state nuclear
clock. Imagine an extremely precise clock able to measure _altitude_ (well,
gravitational altitude) using relativistic time delay... You could use this
for mapping mineral deposits. Could enhance GPS precision on both the
satellite side and the receiver side. Very interesting.

~~~
fsh
Unfortunately, the stated transition wavelength of 149.7 ± 3.1 nm is well
outside the range where practical laser sources exist. There is a proposal for
driving the transition with a vacuum ultra-violet frequency comb generated by
high harmonic generation [1], but such systems easily fill a lab with no clear
path for miniaturization.

[1] [https://arxiv.org/abs/1905.08060](https://arxiv.org/abs/1905.08060)

~~~
Robotbeat
There are some lasers near many range (Ar2* Excimer), but not many (as you
mention) that are tunable to that exact wavelength.

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jakeogh
Thorium-229 Nuclear Clock - Benedict Seiferle @ ThEC2018
[https://www.youtube.com/watch?v=nrbGhAy-
fXI](https://www.youtube.com/watch?v=nrbGhAy-fXI)

