I Trouble understanding the SI definition of 1 second

[PeterDonis]

it is absorption spectroscopy. They shine a laser to make a transition from the state to an excited state, and measure the amount of absorbed light.

In the NIST article I linked to in post #37, it is described this way:

"As the atoms interact with the microwave signal—depending on the frequency of that signal—their atomic states might or might not be altered. The entire round trip for the ball of atoms takes about a second. At the finish point, another laser is directed at the cesium atoms. Only those whose atomic states are altered by the microwave cavity are induced to emit light (known as fluorescence). The photons (tiny packets of light) emitted in fluorescence are measured by a detector."

Emphasis mine.

How does this match up with your description in what I quoted from you above?

 

[DrClaude]

In the NIST article I linked to in post #37, it is described this way:

"As the atoms interact with the microwave signal—depending on the frequency of that signal—their atomic states might or might not be altered. The entire round trip for the ball of atoms takes about a second. At the finish point, another laser is directed at the cesium atoms. Only those whose atomic states are altered by the microwave cavity are induced to emit light (known as fluorescence). The photons (tiny packets of light) emitted in fluorescence are measured by a detector."

Emphasis mine.

How does this match up with your description in what I quoted from you above?

I'm sorry, I was mistaken about the detection method. It is not absorption spectroscopy, but fluorescence spectroscopy, meaning that rather than measure how much of the laser light is absorbed, they measure how much fluorescence the atoms emit when irradiated with that laser light. But it is still probing with a laser, not simply looking at emission of atoms on the $F=4 \rightarrow F=3$ transition.

One difficulty of measuring directly emission I forgot to mention above is that the hyperfine transition has a long lifetime, so the probability of actually measuring spontaneous emission is very small.

To summarize how this all work, without going into all the details, is that cesium atoms are somehow prepared in the $F=3$ state, then they go through a microwave cavity twice, and then the number of atoms that have made the transition to $F=4$ is measured by shining IR laser light to excite the atoms to an excited electronic state, and the amount of fluorescence from these excited atoms gives a measurement of how many atoms made the transition $F=3 \rightarrow F=4$, and thus how close the microwave generator is to the actual frequency of cesium. Maximizing the signal is what keeps the clock ticking at the correct rate.

 

[PeterDonis]

they measure how much fluorescence the atoms emit when irradiated with that laser light.

Yes.

But it is still probing with a laser, not simply looking at emission of atoms on the transition.

I'm not sure I see a big difference here. The emission is stimulated emission, true, not spontaneous emission, but it's still the same transition.

One difficulty of measuring directly emission I forgot to mention above is that the hyperfine transition has a long lifetime, so the probability of actually measuring spontaneous emission is very small.

Yes, that's why they use the probe laser for stimulated emission. But, as above, it's still the same transition causing the emission either way.

 

[DrClaude]

I'm not sure I see a big difference here. The emission is stimulated emission, true, not spontaneous emission, but it's still the same transition.

It is not stimulated emission, but spontaneous emission. The atoms absorb the laser light and the fluorescence (by spontaneous emission) is measured.

Yes, that's why they use the probe laser for stimulated emission. But, as above, it's still the same transition causing the emission either way.

Unless I misunderstood what you wrote previously, my entire point is that it is not the same transition. In the microwave cavity, atoms get excited on the transition $F=3 \rightarrow F=4$, but the number of atoms making the transition is not measured by emission on the reversed hyperfine transition $F=4 \rightarrow F=3$, which is long-lived (it is a forbidden transition) and in the microwave part of the spectrum.

Instead, the atoms in $F=4$ are optically pumped on the D2-line to $F'=5$ and the decay back (fluorescence, by spontaneous emission) to $F=4$ is measured. The lifetime is much shorter, detecting IR light is much easier, and fluorescence is localized, making it easier to measure the emitted light.

 

[PeterDonis]

the atoms in $F=4$ are optically pumped on the D2-line to $F'=5$ and the decay back (fluorescence, by spontaneous emission) to $F=4$ is measured.

Ah, ok. The NIST page doesn't make this clear.

 

[DrClaude]

Ah, ok. The NIST page doesn't make this clear.

A better description can be found in this Physics Today article:
https://physicstoday.scitation.org/doi/10.1063/1.1366066

 

[PeterDonis]

A better description can be found in this Physics Today article:
https://physicstoday.scitation.org/doi/10.1063/1.1366066

Thanks for the reference!