I Physical Meaning of Atomic Oscillations

[Sciencemaster]

TL;DR Summary

When atomic clocks measure the 'frequency' of an atom, does it just mean how often it transitions between states, or does it have anything to do with spatial vibrations?

A physics question for those more atomically inclined than myself. Atomic clocks are said to measure the frequency of oscillations. By this definition of atomic oscillation, is anything physically vibrating, or does it just mean switching between the two energy levels without excess explicit spatial vibration? I know that atoms in general do vibrate (for example, with temperature), but I'm specifically referring to the oscillation that an atomic clock's laser matches the frequency of, and that is observed to measure time in such equipment. Also, while I'm sure it's not that simple due to the wave function and all that, there are processes that are affected by physical, spatial vibrations and their frequencies, so I imagine there's an explicit answer to this question.
So, are atomic oscillations physical vibrations, or not explicitly a spatial movement?
Also, to make sure I'm thinking of these systems correctly, these oscillations are the same as atomic transitions, correct?
Thank you for the help!

 

[PeterDonis]

Atomic clocks are said to measure the frequency of oscillations.

Not really, no.

What atomic clocks measure is the frequency of light emitted by a particular transition between two atomic energy levels.

"Oscillations" is a very poor term to refer to this, although unfortunately it is often used. Nothing is physically vibrating; that's not at all what's happening.

 

[A. Neumaier]

TL;DR Summary:

are atomic oscillations physical vibrations, or not explicitly a spatial movement?

The frequencies measured in atomic spectra are precisely the frequencies of the quantum expectation values <A(t)> of the observables A(t) of the atom. See, e.g., Sections 6.1-6.2 of my online book Classical and quantum mechanics via Lie algebras. Thus they are the frequencies which with the atom oscillates.

 

[Sciencemaster]

Not really, no.

What atomic clocks measure is the frequency of light emitted by a particular transition between two atomic energy levels.

"Oscillations" is a very poor term to refer to this, although unfortunately it is often used. Nothing is physically vibrating; that's not at all what's happening.

Oh, that's...not very intuitive from how it's described haha. But it does make sense, as I've seen a paper that showed the probability spread of frequencies in the clock, and it seems more like the probability that an emitted photon has a given frequency in a transition than the probability that an atom vibrates at that frequency, given the language surrounding it. That being the case, what do atomic clocks actually count? It seems like the frequency of "oscillations" determines the accuracy of the clock, and now I'm unsure by what mechanism the time elapsed is measured.

 

[PeterDonis]

what do atomic clocks actually count?

They don't count anything. What they do is tune a controllable microwave source to maximize the number of photons emitted in the chosen atomic transition. The frequency of the microwave source, which is known because it's controllable, is then used as a clock.

 

[Sciencemaster]

They don't count anything. What they do is tune a controllable microwave source to maximize the number of photons emitted in the chosen atomic transition. The frequency of the microwave source, which is known because it's controllable, is then used as a clock.

Okay, so they point a light source at a sample of some element, and tune it in such a way that maximizes absorption, giving a precise value for the source's wavelength because of allowed atomic transitions, right? That makes sense. Looking back, even Wikipedia defined the second as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom." But how do they use this tuned source to, say, determine what fraction of a second occurs between two measurements or how long a second is from a Caesium source? Of course, there's going to be a relationship between the frequency at which the source emits and the amount of time it takes to emit a singular wavelength, but I'm unsure what exactly is being measured/kept track of in the device to be used as a clock, or by what physical mechanism.

 

[PeterDonis]

so they point a light source at a sample of some element, and tune it in such a way that maximizes absorption, giving a precise value for the source's wavelength because of allowed atomic transitions, right?

Basically, yes. Note that the "light" is microwaves, not visible light.

how do they use this tuned source to, say, determine what fraction of a second occurs between two measurements

Once you have a tuned source of microwaves that is known to have a certain frequency, you just count the crests or troughs of the waves. 9,192,631,770 of them is one second. The microwaves from the tuned source are well within the regime where the phase of the waves can be monitored.

 

[Sciencemaster]

Basically, yes. Note that the "light" is microwaves, not visible light.


Once you have a tuned source of microwaves that is known to have a certain frequency, you just count the crests or troughs of the waves. 9,192,631,770 of them is one second. The microwaves from the tuned source are well within the regime where the phase of the waves can be monitored.

Alright, that makes sense. And by 'light', I just meant EM radiation. Out of curiosity, do atomic clocks continually 'check' their wavelength against the 'sample' atoms (such as the Caesium inside) and apply corrections if needed?

 

[PeterDonis]

do atomic clocks continually 'check' their wavelength against the 'sample' atoms (such as the Caesium inside) and apply corrections if needed?

I believe they do check and apply corrections; I don't know if they do so "continuously".

 

[Sciencemaster]

I believe they do check and apply corrections; I don't know if they do so "continuously".

Alright, please correct me if I'm wrong, but this is my current understanding of an atomic clock system.
A microwave emitter of some sort emits a constant output of microwaves. This output is sent through a sample of an element (in our previous examples, Caesium), and the initial laser is tuned such that the absorption is maximized/emission is minimized. With the source tuned, the phase of the resulting microwaves is measured, maybe through a voltage detector or some sort of interferometer, and a mechanism then uses these results to 'count' the peaks/troughs/periods of the microwaves. Ultimately, that 'count' is the clock, it's the measurement of the amount of 'ticks' between two events or the time elapsed since the clock is activated. Then, the clock probably checks its frequency against the element sample at certain intervals and corrects accordingly if needed, but it mostly just continues to monitor the phase and 'count' accordingly from the source and its measurements of the microwaves emitted.

 

[javisot20]

Alright, please correct me if I'm wrong, but this is my current understanding of an atomic clock system.
A microwave emitter of some sort emits a constant output of microwaves. This output is sent through a sample of an element (in our previous examples, Caesium), and the initial laser is tuned such that the absorption is maximized/emission is minimized. With the source tuned, the phase of the resulting microwaves is measured, maybe through a voltage detector or some sort of interferometer, and a mechanism then uses these results to 'count' the peaks/troughs/periods of the microwaves. Ultimately, that 'count' is the clock, it's the measurement of the amount of 'ticks' between two events or the time elapsed since the clock is activated. Then, the clock probably checks its frequency against the element sample at certain intervals and corrects accordingly if needed, but it mostly just continues to monitor the phase and 'count' accordingly from the source and its measurements of the microwaves emitted.

https://en.m.wikipedia.org/wiki/Atomic_clock, "The timekeeping accuracy of the involved atomic clocks is important because the smaller the error in time measurement, the smaller the error in distance obtained by multiplying the time by the speed of light is (a timing error of a nanosecond or 1 billionth of a second (10−9 or 1⁄1,000,000,000 second) translates into an almost 30-centimetre (11.8 in) distance and hence positional error).

The main variety of atomic clock uses caesium atoms cooled to temperatures that approach absolute zero. The primary standard for the United States, the National Institute of Standards and Technology (NIST)'s caesium fountain clock named NIST-F2, measures time with an uncertainty of 1 second in 300 million years (relative uncertainty 10−16). NIST-F2 was brought online on 3 April 2014.[3][4]"

 

[PeterDonis]

this is my current understanding of an atomic clock system

That's basically my understanding as well.