I "Filming" a quantum measurement

[EPR]

What is a measurement? Is every interaction a measurement?

No one can tell.

 

[PeterDonis]

In the paragraph right at the page break between pp 2-3.

And in that paragraph, it says "if at least one photon is scattered into the environment, then...the coupling to the photon corresponds to the measurement of the qutrit projection..."

The photon being "scattered into the environment" means a fluorescence photon is detected. So what they are saying, though they are saying it in rather roundabout language, is that the detection of the fluorescence photon is the actual measurement. Or, more precisely, if such a photon is detected, then we can deduce that the atom is in the state $|0\rangle$. If such a photon was not detected, we cannot deduce anything about the state of the atom.

None of this is saying that the previous application of the 422 nm laser pulse is the actual measurement. It's not even a preparation procedure because applying the pulse does not guarantee that the atom is in a particular state. So I still think that the paper's description of the 422 nm laser pulse as a "measurement process" is misleading.

 

[PeterDonis]

A bit later in the paper Eq. (7) shows that for $t \rightarrow \infty$ you realize the projection

There is no $t \rightarrow \infty$. $t$ in that equation is a constant, the length of the 422 nm laser pulse, fixed by the experimental procedure; its value, as noted in the text right after that equation, is 1 microsecond.

 

[vanhees71]

OK, it's a fight about words and slang in different scientific communities. I'd also say the actual measurement is indeed the detection of the fluorescence photon.

Yes, it's not $t \rightarrow \infty$, and indeed with fixed $t$ you don't get an exact projection, but that's the point: It takes time to "measure", and increasing $\Omega$ (via the intensity of the laser) you get closer and closer to the ideal projection. That's what they want to demonstrate, and in my opinion they have done so. They use the fact that what has to get large is $\mathrm{Re} [\Omega^2/(\Gamma+4 \mathrm{i} \Delta)] t$ to get close to the ideal projective preparation.

It's not surprising after 95 years of modern quantum mechanics being successul but nice to have been measured and demonstrated.

 

[PeterDonis]

It takes time to "measure"

But varying the power of the 422 nm laser pulse does not vary the "time to measure". It varies the probability that a fluorescence photon will be detected, i.e., that a "measurement" will be made at all. The actual "time to measure" is determined by the length and power of the fluorescence detection step, which, as far as I can tell from the paper, is not varied at all from run to run.

increasing $\Omega$ (via the intensity of the laser) you get closer and closer to the ideal projection. That's what they want to demonstrate, and in my opinion they have done so.

I have no issue with saying that they have shown that varying the intensity of the laser varies the probability that a fluorescence photon will be detected ("closer and closer to the ideal projection" means that probability gets closer and closer to 1), which in turn varies the probability that the quantum coherence between the $|0\rangle$ state and the other atom states $|1\rangle$ and $|2\rangle$ is destroyed.

I just don't think that corresponds to "filming a quantum measurement in progress". If the actual measurement is the detection of the fluorescence photon (and you say you agree with that), that process is not being "filmed" at all.

 

[vanhees71]

No. They say clearly what was filmed in the paper.

It's not about how fluorescence works or is mathematically described. You are also not interested in the specific mathematical description of the inner workings of your digital voltmeter when measuring a voltage.

 

[Demystifier]

There are no quantum jumps, at least not in standard quantum mechanics. It is an old ad-hoc description of the emission and absorption of photons in the Bohr atom model by electrons jumping between different allowed orbits. Today we describe it dynamically, often sufficiently in first-order perturbation theory with an external classical em. field (semi-classical approximation), which describes of course only induced emission and absorption or the quanum em. field, which includes spontaneous emission too.

As @samalkhaiat recently said in another thread, it's a mess so let me clarify this once and for all.

In QM it's essential to distinguish the ensemble level from the individual level.

In a nutshell, the situation in standard QM can be concisely summarized as follows:

Time evolution at the ensemble level:
- Is it deterministic or random? Deterministic.
- Is it continuous or jumping? Continuous.

Time evolution at the individual level:
- Is it deterministic or random? Random.
- Is it continuous or jumping? Agnostic (minimal standard QM does not specify).

What about other (non-standard) interpretations of QM?
- On the ensemble level, they all agree with the above.
- On the individual level, many of them disagree with the above.

 

[Demystifier]

Is every interaction a measurement?

No one can tell.

I can, no it isn't.

 

[PeterDonis]

They say clearly what was filmed in the paper.

I know what they say in the paper. I am simply saying that I think what they say in the paper is misleading, because when you look at what is actually being done, what is being "filmed", which, as you say, they describe in the paper, quite apart from just using the word "measurement", is not the actual measurement (the detection of the fluorescence photons); it is something else, that they call a "measurement process" in the paper, but I've already explained why I think that's misleading as well.

 

[PeterDonis]

You are also not interested in the specific mathematical description of the inner workings of your digital voltmeter when measuring a voltage.

Measuring a voltage is a classical process so it is a bad example.

If I am doing a double slit experiment with a photon, I don't call the effect of the slits on the photon a "measurement". The measurement is when the photon hits the detector screen and makes a dot. Nobody has a specific mathematical description of this either, the mathematical description is all about the effect of the slits, but that doesn't mean the effect of the slits on the photon is the measurement.

 

[EPR]

I can, no it isn't.

No, you can't. It is your personal opinion.

In the MWI every interaction is a measurement.

 

[PeterDonis]

In the MWI every interaction is a measurement.

First, any claims about particular interpretations belong in a separate thread in the interpretations forum, not here. (And before you try to post your claim about the MWI in any thread in the interpretations forum, I strongly suggest that you check it first.)

Second, the 7 Basic Rules of QM (already linked once in this thread, I believe, and they are one of the sticky links at the top of this forum) make clear what a "measurement" is as far as basic QM is concerned, and "every interaction" is not what they say a measurement is.

 

[Demystifier]

No, you can't.

I just did.

 

[vanhees71]

Measuring a voltage is a classical process so it is a bad example.

If I am doing a double slit experiment with a photon, I don't call the effect of the slits on the photon a "measurement". The measurement is when the photon hits the detector screen and makes a dot. Nobody has a specific mathematical description of this either, the mathematical description is all about the effect of the slits, but that doesn't mean the effect of the slits on the photon is the measurement.

Ok, at least I now know, what you mean by measurement: You call only a process a measurement, where the quantum system interacts with a macroscopic measurement device which stores the result irreversably. That I can agree with. Then in the here discussed paper, what they call a "measurement" is indeed a "preparation procedure" and only the detection of the fluorescence photons is the measurement. So what's demonstrated here is the time-dependence of a preparation procedure leading in the limit of high intensity of the 422nm laser field to a preparation procedure as in what's commonly called a "von Neumann filter measurement". All this nitpicking on words is not that important, because it's clear from the description of the experiment what's meant by the words.

NB: It's of course not true that nobody has a mathematical description of photon detection. Usually it's just the photoelectric effect which FAPP can be described in 1st-order (time-dependent) perturbation theory in the dipole approximation. This also theoretically shows that the photon-detection probability is proportional to the energy density of the photons, i.e., the quantum version of the classical "intensity" $\propto \vec{E}^2(t,\vec{x})$ for a plane-wave mode. That's important, because that's indeed the only physical, i.e. gauge invariant, quantity you can define for free photons with the physical meaning of an intensity. Of course this is discussed in textbooks on quantum optics at length and nobody cares anymore in papers, because this is settled standard knowledge. You find a very nice description of all that in, e.g.,

J. Garrison and R. Chiao, Quantum optics, Oxford University
Press, New York (2008),
https://doi.org/10.1093/acprof:oso/9780198508861.001.0001

 

[PeterDonis]

You call only a process a measurement, where the quantum system interacts with a macroscopic measurement device which stores the result irreversably.

Yes.

All this nitpicking on words is not that important, because it's clear from the description of the experiment what's meant by the words.

It's clear to you and me (after some discussion and clarification), yes, but note that what is actually happening in the experiment, given the definition of "measurement" we have just agreed on, is not accurately described by the phrase "filming a quantum measurement". What is being "filmed" is a preparation process, not a measurement. So I think calling out the misleading nature of those words, which is what I have been doing, is important. Particularly since the misleading phrase is the very title of this thread.

 

[vanhees71]

Yes, in the quantum-foundation community you have the tendency to make some "headlines" and "advertisement" of research, which sometimes is not as exciting as this advertisement promises. At least that's an impression I have as an interested non-expert in this field.