This is important since impurities in the crystals used lead to all kinds of fluorescence that could be mistaken for a signal from the Thorium ions. Now two groups have seen exactly the same signal in different Thorium-doped crystals which is very covincing that they have found the actual nuclear transition.

I am always in awe of folks who come into the lab every day and work on figuring out the one thing. I envy that level of focus.

So what's the wavelength? I felt like the article left me hanging.

The answer is: 148.3821 nm

Yes, I admit that it's meaningless to me. It's sort of like a big news story announcing that Malaysia Airlines MH-370 has been located somewhere in the world's oceans, but not saying where because a number like 148.3821 km SSE of the Cocos Islands is going to be meaningless to most people.

Else there's Octarine from the Discworld books, it's the colour of magic.

Another one in that same SO thread is err, quantifying synesthesia in the study of "chromophonics", where sound is assigned a color and vice-versa, that is, one could name a colour after a sound, which matches up with the earlier "octave" analogy.

Missing out on the magic number does seem like a bit of a problem, but really the expectations on the audience are already quite low. That number could easily turn out to be worth more than a trillion dollars to humanity at large, but I'd bet most readers just think of it as a party factoid.

While in the best atomic clocks one must use single ions held in electromagnetic traps or a small number of neutral atoms held in an optical lattice with lasers, in both cases in vacuum, because the ions or neutral atoms must not be close to each other, to avoid influences, with thorium 229 it is hoped that a simple solid crystal can be used, because the nuclei will not influence each other.

The ability to use a solid crystal not only simplifies a lot the construction of the atomic clock, but it should enable the use of a greater number of nuclei than the number of ions or atoms used in the current atomic clocks, which would increase the signal to noise ratio, which would require shorter averaging times than today, when the best atomic clocks require averaging over many hours or days for reaching their limits in accuracy, making them useless for the measurement of short time intervals (except for removing the drift caused by aging of whatever clocks are used for short times).

The deeper you go into a gravitational field, the slower time goes. Therefore comparing clocks in different places gives a way to measure gravity. These clocks could be sufficiently precise to find mineral deposits underground from their gravity signature.

A practical constraint is mass density, which has maximum and minimum values. We can make a crude approximation that the planet's density is constant, evaluate the field on the surface from the planet's shape and compare it with measurement. This would be more useful, but still, it wouldn't tell us whether there is a combo of water reservoir and a large massive deposit below it.

Sure this isnt going to be a star trek scanner but for practical purposes theres a bunch of other techniques to constrain the results

But now that you know it is there, you can use other techniques, like seismic measurements, to nail that down.

I'd be interested to know how much more accurate a nuclear-state-transition clock might be than a conventional Caesium or Rubidium clock.

TFA seems to make the point that a nuclear clock would be more resistant to external influences, such as EM radiation, than an atomic clock, and so could be used in experiments where such influences might introduce unwanted uncertainty. But I'd like to know what the claim for greater accuracy is based on, rather than simply greater reliability.

There are some real challenges in realization: this will take optical combs and all sorts of other stuff to really take advantage of.

Generally you want to use American football fields for this because American football fields have a standard size, 100 yards x 160 feet (91.44 x 53.3 meters). That size field is used in professional, college, and high school football.

Soccer fields on the other hand not only vary from country to country, they aren't even always all the same size within a league. The English Premier League for example is trying to standardize on 105 x 68 meters but several clubs are not yet there: Brentford (105 x 65), Chelsea (103 x 67), Crystal Palace (100 x 67), Everton (103 x 70), Fullham (100 x 65), Liverpool (101 x 68), and Nottingham Forest (105 x 70).

For international play the standard is a range. 100-110 meters length and 64-70 meters width.

There are parts of soccer fields that are standardized to specific values rather than ranges so would be good for unambiguous length or area comparisons. The amusing thing is that those all have fractional values in metric but integer values in Imperial/US units:

• Radius of circle around center mark: 10 yards.

• Penalty area: 44 x 18 yards.

• Distance from penalty mark to goal: 12 yards.

• Goal area: 20 x 6 yards.

• Distance between goal posts: 8 yards.

• Height of crossbar: 8 feet.

I appreciate your valiant efforts but to my mind this is extra confusing because "soccer" is short for "association football"

Time to rename American Football to "handegg" once and for all. Ok, ok, I'll settle for "American Rugby"

Of the English Premier League fields Brentford is pretty close: 105/1.618 = 64.89; close enough to their 105x65m field.

Honestly I'd settle on a 100m length though. Thus a 100x61.8 field.

Throw in Australian Rules Football fields if you're looking for a maximum, particularly if orginal marn-grook is in the mix.

The FIFA standard (https://downloads.theifab.com/downloads/laws-of-the-game-202...) leaves a lot of leeway:

“3. Dimensions

The touchline must be longer than the goal line.

• Length (touchline): minimum 90 m (100 yds), maximum 120m (130 yds)

• Length (goal line): minimum 45 m (50 yds), maximum 90m (100 yds)”

So, a field can be almost square at 90m × 89m or approaching thrice as long as wide, at 120m × 45m.

Reason for this is prior art that can be hard to change (if there’s a stadium around your field, and it’s deemed too small, you’d have to demolish it to make the field fit the standard)

Various competitions restrict this, though.

https://en.wikipedia.org/wiki/Solar_irradiance#Absorption_an...

and the light that is emitted is absorbed by the atmosphere:

https://en.wikipedia.org/wiki/Ultraviolet#Solar_ultraviolet

It's useful to be able to see a little UV-A, perhaps, and very useful for predators to see 'heat' into the IR range, but if your eyes were sensitive to 148nm, the world would be pretty dark.

Maybe after a few million years, in the grinding dust in the back of my shop, something will evolve that has a symbiotic relationship to arc welders...

The ethene double bond absorbs at ~165 nm, a benzene ring at ~180 nm, and building things out of those tends to increase the wavelength, not decrease it. 148 nm is single bond territory - could you have a chromophore which uses photons of the right wavelength to break a bond, and then somehow react to the presence of free radicals?!

I'm sure there would be some value in seeing others parts of UV. Some minerals fluoresce from one type of UV light but not another, so they'd be dark in the bands that cause them to fluoresce. Mantis shrimp can apparently see into UV-B, but I'm not aware of anything living that can see UV-C.

Presumably wouldn't apply anywhere near as far as 148nm since as you note that light doesn't make it to earth.

Based on just the frequency, I dunno what makes the thorium nuclear transition much better than optical transitions. Unless the excitement (as it were) is about scaling up to even higher frequencies.

Cs or Rb clocks give you a line width of a few hundred Hz at 9 GHz (Q=roughly 100 million), while quantum transitions in optical clocks can achieve line widths on the order of 1 Hz in the PHz region (equivalent Q in the quintillions.) There is a lot more to building a good clock than high Q, but it's a very important consideration ( http://www.leapsecond.com/pages/Q/ ).

What caught my eye is the ringdown time of the stimulated optical resonance, apparently in the hundreds of seconds. They talk about line widths in the GHz range, but that seems to refer to the laser rather than the underlying resonance being probed. It would have been interesting to hear more about what they expected regarding the actual transition line width. Probably the information is there but not in a form that I grokked, given insufficient background in that field.

It's the curse of "probing" with massive energies. No one's a hundred percent certain of whether they're detecting something that's actually there - like there there - or whether they're looking at by-product of enormous collision energies.

Physicists are smart people! I could never do what they do. But there's a limit to certainty, and inside the proton especially there's unknown first principles at work. Bringing the precision of photons and lasers into this nucleon party is going to be huge. I can't wait!

This fact is used in Mössbauer spectroscopy (recoilless gamma emission in solids). The peak is so sharp that it was famously used by Pound and Rebka to detect the gravitational red shift in the lab at Harvard in 1960, reaching 1% accuracy by 1964.

https://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment

The gamma rays normally have energies per photon many orders of magnitude greater than for visible light and also much greater than for X-rays (which are produced by electrons accelerated by very high voltages when hitting a target).

The thorium 229 nucleus is the only one that can emit gamma rays that are so low in energy that their energy is not only lower than for X-rays, but it is also lower than for many sources of ultraviolet light. For instance the ultraviolet light used in state-of-the-art lithography for semiconductor manufacturing has much higher frequency (shorter wavelength), by about ten times.

These gamma rays of the Th229 have a wavelength that is not much shorter than the 184-nm ultraviolet light that can be obtained with a mercury-vapor lamp.

What is important is that for such a frequency/wavelength it is possible to build laser sources, which enables the design of an atomic clock that will use thorium 229 nuclei instead of neutral atoms or ions of other elements (like ytterbium, lutetium, strontium, aluminum).

Hold on how does that work?

I have had a sort of sci-fi idea that sufficiently sensitive gravitational field measurements coukd detect the passing of submarines (I am not sure on the maths tbh) - which would render a lot of nuclear strategy moot.

Just need to get a grasp on the maths

The Eotvos pendulum (an instrument aka. Eotvos torsion balance) designed in 1888 started this kind of measurement. It was used commonly by the 1920s by geophysicist for mapping underground deposits by measuring the gradient of the gravitational field very precisely.

This instrument was deprecated later by even better tools for surveying.

The instrument was initially constructed for the experiment showing that inertial and gravitational mass are the same (well, linearly correlated) to a great precision: https://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_experiment

https://www.nature.com/articles/118406a0 (pretty useless link, but a famed periodical)

Detecting submarines is way harder, practically impossible. as others have already pointed out.

Are you looking for density variation between the parts and airspaces of a submarine?

(IIRC) Royal Navy trialed it (officially) for the first time last year.

This one's never going to happen.

Geologic mass concentrations are an entirely different story: you get a gravitational monopole, which is a more reasonable inverse square law. (No monopoles for a submarine, because by design they have a mean density equal to water—as the paper explains).

Range is pretty short but still large enough that you can do it from an airplane flying over.

Finding the thorium line is one of the most important open problems in precision/fundamental measurement.

2) is there any significance to the units of the wave length? Like they’ve narrowed it down to a number. Does that granularity map to anything? Some sort of discrete scale? Or is there going to be a range of values that work +/- a super tiny value.

This achievement is a step (the most important one) towards the goal of making an atomic clock that uses thorium 229 (which has important advantages mentioned in another posting).

I was reading up on this (now outdated) wiki page: https://en.wikipedia.org/wiki/Isotopes_of_thorium#Thorium-22...

And it mentions the application as qubit for quantum computers. If the state change is relatively simple, cheap and stable, what could this do for quantum computing? I picture a crystalline processor holding Thorium nuclei as the brains of a new supercomputer? Would that be viable?

Is quantum physics now considered part of classical physics? If so then man, time flies!

> It is consistent with both the principles of quantum mechanics and the theory of special relativity, and was the first theory to account fully for special relativity in the context of quantum mechanics.

What we don't have is a grand unified theory (a single set of rules that generates both theories), but we can consider relativistic effects in QM theories, and (I assume) vice versa.

https://en.wikipedia.org/wiki/Dirac_equation

IIRC nuclear physics was largely phenomenological with a lot of observations that had simple models fit to them without being able to reduce those to the particle physics models. This might be about establishing a link between the phenomenological nuclear models and the fundamental QM models.

Perhaps doable with a free electron laser, but probably not with traditional lasers, due to the energies involved.

But, yeah, not sure what the use would be. Maybe a form of lidar that allowed measuring speed of objects to extreme precision, by measuring the dopler shift of reflected light? (Assuming light at such a short wavelength is reflected sufficiently, which it probably wouldn't be).

If there happened to be one atom/matrix that could be tuned to the transition energy of another atom's fission transition energy, then you could use it to burn nuclear material / waste. But you could just do that with the pump laser directly.

I suppose you could maybe pump such a laser with a very high temperature plasma (like fusion temperatures hot), rather than with a free electron laser. Then maybe it might make more sense.

Nobel in 2022 for Zeilinger

Nobel in 2023 for Kraus, who did his work at TU Wien

Now this. Giving a lot of other unis a run for their money.

Like the other poster mentions, CaF2 is an ionic crystal, but I don’t think that’s an important detail because you wouldn’t expect a nuclear transition to be affected by the bonding state of the electrons. My guess is it’s just a convenient way to get a very dilute collection of thorium atoms without using an ion trap

The details of how the nucleus manifests that extra energy are complicated, but you can imagine it as like, picking up a certain vibrational frequency.

The half life concept seems to be standard over much of physics.

That a Markov assumption could hold might suggest some new physics.

Now in this case they use lasers. I suspect if you choose the right wavelenght (=frequency) of light there is some sort of resonance phenomenom.

> This nucleus has two very closely adjacent energy states – so closely adjacent that a laser should in principle be sufficient to change the state of the atomic nucleus.

> the correct energy of the thorium transition was hit exactly, the thorium nuclei delivered a clear signal for the first time. The laser beam had actually switched their state.

I don't know enough to explain any further.

The laser is used to transition the nucleus from the ground state to the excited isometric state.

https://en.wikipedia.org/wiki/Hafnium_controversy

This would be Iron Man and Star Wars tech if it worked. Unfortunately experiments went dark after 2009, probably because it worked haha, but maybe because Hf is too rare to make a practical battery. So it looks like they tried spalling element 73 Tantalum (Ta), 74 Tungsten (W) and 75 Rhenium (Re) with protons at 90-650 MeV to create 72 Hf with atomic masses 178, 179 and high spin 178m2, 179m2 isomers if I read this right:

https://publications.jinr.ru/record/151982/files/071%28E6-20...

https://apps.dtic.mil/sti/tr/pdf/ADA525435.pdf

There's a lot here though, so I can't really get a clear picture of what the yields are, or simply how many joules it takes to store one joule in an excited isomer. Which is of course all that matters, but papers often leave off the one part we're curious about, forcing us to learn nearly the entirety of the subject matter to derive it ourselves. Although on the bright side, maybe that protects us from nuclear armageddon and stuff.

Maybe someone can fill us in?

Edit: dangit _Microft beat me by 17 minutes, please answer there :-)

So they hit their thorium with a laser, and then instead of the laser passing through, it gets absorbed, and then they get a flash of radiation back, letting them know the thorium was excited. The delay between the laser pulse and the flash of radiation is a property of the particular thorium nucleus, and is not affected by environmental circumstances like temperature or electric/magnetic fields, so can be relied on as a very precise measurement of time.

> The decay back to ground state happens at a very precise rate that is not influenced by effectively anything

That sounds contradictory to me.

Resources companies are salivating

The latter is important in physics to determine if these constants are truly constant in space and time. Which is a large assumption we have about the universe.

Existing atomic clocks based on electrical interactions are extremely sensitive to the surrounding magnetic and electrical environment-- so for example accuracy is limited by collisions with other atoms, so state of the art atomic clocks have optically trapped clouds in high vacuums. Beyond limiting their accuracy generally makes the instruments very complex.

One could imagine an optical-nuclear atomic clock in entirely solid state form on a single chip with minimal support equipment achieving superior stability to a room sized instrument.

The cool applications usually come later (or they're more esoteric). The researchers were more excited to determine the actual frequency than think about clocks

Are today's atomic clocks really so imprecise? Without further explanation of this, it reminds me of this comic (which is alas showing its age both by mentioning flash, and by implying that 1024 is already a uselessly high number of cpus to support):

https://xkcd.com/619/

We've known about photon-atom interactions for well over 100 years, with excitation of electrons which are either released or drop back to the original orbit, right?

So, ok, the Nucleus is smaller and the energies to alter the quantum state are probably higher, but - why is this so special, and why Thorium in particular rather than any old nuclei?

Disclaimer: I'm not a physicist.

This means that to exicte to this nuclear state is possible using an ultraviolet laser

It has important applications for nuclear theory, nuclear atomic clocks and fundemental constant metrology.

.. and given that it decays through gamma emission, does this mean we could now build an optically pumped gamma ray laser?

The development of dedicated VUV lasers with narrow linewidth will make it possible to access a new regime of resolution and accuracy in laser M¨ossbauer spectroscopy and to perform coherent control of a nuclear excitation"

Previously, if you wanted to manipulate nuclear states, you needed a synchrotron. Now, you need an infinitely less expensive instrument. I suppose the idea is that that will generate a lot of interest in improving the less-expensive instrument.

A "wavelength converter" might be possible.

PS: Are you sure it's gamma emission? That takes more energy than the exciting UV photon.

Apparently it is neither:

Decay of the 229Th isomeric state of the neutral thorium atom occurs predominantly by internal conversion (IC) with emission of an electron

https://www.nature.com/articles/nature17669

https://en.wikipedia.org/wiki/Internal_conversion

This is pretty weird. You shine UV light (with exactly the right wavelength) on 229Th, and it spits out electrons. But not like the photoelectric effect, where the electrons stop as soon as you turn off the light. No no. The Thorium keeps spitting out an exponentially-decaying stream of electrons for hours after you stop illuminating it.

Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar").

> Almost like an exponentially-discharging solar-powered current source (for a very specific wavelength of "solar").

If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.

When the atom ejects an electron, the hole left behind gets filled by an electron from a neighboring atom. Then the same thing happens to the neighbor -- and so on. This is electrical current flowing. Eventually the loop closes and some hole somewhere in the universe gets filled by the original ejected electron.

The hole in one atom can get filled by an electron from a higher orbital in a neighboring atom. In that case the energy gained will be greater than the energy lost by the original electron ejection. This is the situation where you get a photon (x-ray) with a higher energy (= shorter wavelength) than the original incident photon (ultraviolet).

Of course there's no free lunch. The way this happens is that N thorium atoms eject electrons from some orbital with energy X, the electrons shuffle around, and those N holes get filled by donors from orbitals whose total energy is N*X even though some of the donors are at higher levels and some are at lower levels.

If one could make the UV source highly efficient perhaps it could be used as a battery with extremely good energy density.

Yeah I've been thinking that if we had really tiny VLSI-integrated UV lasers (which we absolutely don't, not even close) that a bunch of these 229Th atoms embedded in a silicon chip would be a device with totally fascinating properties.

We can build waveguides in silicon wafer processes pretty easily but I'm not sure we can do that at UV wavelengths. You could imagine a single, big, off-chip laser whose beam can be steered by waveguides to illuminate any of a few billion 229Th deposits. These could act like the configuration memory bits of an FPGA. They would be "almost nonvolatile" -- you'd have to refresh them every hour or so, instead of several thousand times per second (dram) or never (sram). At such a low refresh rate the steering doesn't need to be particularly fast, and having to share one laser across all the deposits would not be a problem.

Unfortunately 229Th is mildly radioactive, but so are household smoke detectors so hopefully people wouldn't freak out about this.