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.
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?
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.
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.
*acousto-optic, of course. (I swear, auto-incorrect is getting worse.)
To expound slightly on your (correct) explanation: The phonons impart (or remove) energy to (from) the laser light, the same way a moving mirror does: via doppler shift.
I made a differential heterodyne interferometer in undergrad using the method you describe.
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.
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."
Look, I think philosophy is important and fundamental, and yes philosophy is a language game. What I don't like is how most philosophers don't actually contribute, in other words don't actually do philosophy.
The most philosophical thing I have seen is formal verification software. Like metamath.
Philosophers could easily gain a lot of my respect if they switched from natural language philosophy, to slowly formalizing physics, law, norms and values, natural language dictionaries into actual formal concepts, in a collaborative way, so that all philosophy can actually be integrated into a theory.
Back to the clocks, what you claim is patently false. It is perfectly possible given 2 types of clock A and B to assess which type of clock is more precise:
Let's model an imprecise clock as one that reports as time passed: the actual time passed plus an error term. The error term undergoes a random walk, or diffusion.
This means all you have to do is make an ensemble of 2 (or more) clocks of type A: A1 and A2, and similarily 2 (or more) clocks of type B: B1 and B2, reset all clocks and then observe for which type of clock X we have a smaller difference between X1 and X2, as time passes.
if the difference in reported time between A1 and A2 wanders away from 0 slower than the difference in reported time betwween B1 and B2 then you know clock type A is more precise.
It's mostly a philosophical conundrum, not a practical one.
For any given definition, there are a handful of possible "defining experiments" being carried out at a handful of national labs. Their deviations with respect to each other are constantly being monitored and metrologists are constantly chasing down error terms. Yes, they choose one standard to "bless" as the official definition, but it's their job to live below that abstraction and to maintain it. If their "blessed" definition started to drift with respect to the other candidate definitions they would notice very quickly and react appropriately. The fact that most of us entertain a single definition isn't a consequence of philosophical confusion as to whether or not one exists, it's a consequence of delegating the ongoing experiments backing our simplifying assumptions to a group of people who are very good at them.
For instance, the disagreement between solar time and atomic time is monitored so closely that the slowing of Earth's rotation due to tidal forces is a gigantic signal compared to measurement noise:
If, say, Earth flew through a cloud of dark matter that somehow messed with Cs absorption lines, we would see Cs clocks drift with respect to Rb clocks, unlocked quartz clocks, Earth rotation, Optical Lattice Clocks, etc, etc. The event would not go unnoticed. Cs clocks would be demoted from their position as primary time standards and the next best candidate would be promoted.
In a simple case yes, but if all other time keeping mechanisms would also be messed with by dark matter, there might be no way of saying which one is the right own.
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.
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.
It may matter how fast you're moving non-inertially and how massive nearby objects are. (General and Special Relativity)
And then, there might be advanced quantum effects we don't know about yet.
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.
If we could reasonably harvest the energy of gamma rays, it could also significantly improve the efficiency of nuclear power plants, and maybe even make fusion plants somehow feasible.
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.
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.
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.
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.
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...
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?
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.
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.
*acousto-optic, of course. (I swear, auto-incorrect is getting worse.)
To expound slightly on your (correct) explanation: The phonons impart (or remove) energy to (from) the laser light, the same way a moving mirror does: via doppler shift.
I made a differential heterodyne interferometer in undergrad using the method you describe.
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...
[1] https://en.wikipedia.org/wiki/Caesium_standard
Look, I think philosophy is important and fundamental, and yes philosophy is a language game. What I don't like is how most philosophers don't actually contribute, in other words don't actually do philosophy.
The most philosophical thing I have seen is formal verification software. Like metamath.
Philosophers could easily gain a lot of my respect if they switched from natural language philosophy, to slowly formalizing physics, law, norms and values, natural language dictionaries into actual formal concepts, in a collaborative way, so that all philosophy can actually be integrated into a theory.
Back to the clocks, what you claim is patently false. It is perfectly possible given 2 types of clock A and B to assess which type of clock is more precise:
Let's model an imprecise clock as one that reports as time passed: the actual time passed plus an error term. The error term undergoes a random walk, or diffusion.
This means all you have to do is make an ensemble of 2 (or more) clocks of type A: A1 and A2, and similarily 2 (or more) clocks of type B: B1 and B2, reset all clocks and then observe for which type of clock X we have a smaller difference between X1 and X2, as time passes.
if the difference in reported time between A1 and A2 wanders away from 0 slower than the difference in reported time betwween B1 and B2 then you know clock type A is more precise.
The clock drift of A1 and A2 could be correlated for some reason that we don't currently understand.
That is true, but you did say precision, not accuracy.
I assumed you accurately chose your wording, when you used the word "precise".
It's mostly a philosophical conundrum, not a practical one.
For any given definition, there are a handful of possible "defining experiments" being carried out at a handful of national labs. Their deviations with respect to each other are constantly being monitored and metrologists are constantly chasing down error terms. Yes, they choose one standard to "bless" as the official definition, but it's their job to live below that abstraction and to maintain it. If their "blessed" definition started to drift with respect to the other candidate definitions they would notice very quickly and react appropriately. The fact that most of us entertain a single definition isn't a consequence of philosophical confusion as to whether or not one exists, it's a consequence of delegating the ongoing experiments backing our simplifying assumptions to a group of people who are very good at them.
For instance, the disagreement between solar time and atomic time is monitored so closely that the slowing of Earth's rotation due to tidal forces is a gigantic signal compared to measurement noise:
https://en.wikipedia.org/wiki/Leap_second#/media/File:Deviat...
If, say, Earth flew through a cloud of dark matter that somehow messed with Cs absorption lines, we would see Cs clocks drift with respect to Rb clocks, unlocked quartz clocks, Earth rotation, Optical Lattice Clocks, etc, etc. The event would not go unnoticed. Cs clocks would be demoted from their position as primary time standards and the next best candidate would be promoted.
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.
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...
It may matter how fast you're moving non-inertially and how massive nearby objects are. (General and Special Relativity) And then, there might be advanced quantum effects we don't know about yet.
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.
If we could reasonably harvest the energy of gamma rays, it could also significantly improve the efficiency of nuclear power plants, and maybe even make fusion plants somehow feasible.
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.
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.
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.
Full preprint/paper:
https://arxiv.org/abs/1710.11398
That's a 2017 paper. This one looks more like it: https://arxiv.org/abs/1905.06308
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.
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
There are some lasers near many range (Ar2* Excimer), but not many (as you mention) that are tunable to that exact wavelength.
Thorium-229 Nuclear Clock - Benedict Seiferle @ ThEC2018 https://www.youtube.com/watch?v=nrbGhAy-fXI