Undergrad Quantum Jumps and Schrodinger's Cat are predictable

[Tom.G]

Quantum Jumps are predicted using microwave monitoring. Weird.

https://phys.org/news/2019-06-physicists-schrodinger-cat.html

 

[PeterDonis]

I'm always highly skeptical of sensational-sounding claims on phys.org. I'm doubly skeptical when there isn't even a link to a paper (not even an arxiv preprint) in the article, which tells me that the article writer doesn't want me to look up the actual paper and find out that, while their article says "man bites dog!", the actual paper is more like "dog bites man, and now we have a more detailed model of the tooth marks".

 

[f95toli]

The paper just appeared in Nature

https://www.nature.com/articles/s41586-019-1287-z

 

[A. Neumaier]

Quantum Jumps are predicted using microwave monitoring. Weird.

Quantum jumps were observed (as predicted) in the lab long ago; see ''Are there quantum jumps?'' from my Theoretical Physics FAQ.

The paper just appeared in Nature

https://www.nature.com/articles/s41586-019-1287-z

But there are no relations to Schrödinger's cat.

From the abstract:

we experimentally demonstrate that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable ‘flight’, by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of each completed jump is continuous, coherent and deterministic. We exploit these features, using real-time monitoring and feedback, to catch and reverse quantum jumps mid-flight—thus deterministically preventing their completion. Our findings, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory

This is serious work about tracking and controlling the continuous measurement of single quantum systems. One of the coauthors is Carmichael, a well-known expert in quantum optics and author of two volumes on Statistical methods in quantum optics.

From the main text:

despite the long-term unpredictability of the jumps from |G〉 to |D〉, they are preceded by an identical no-click record from run to run. Whereas the jump starts at a random time and can be prematurely interrupted by a click, the deterministic nature of the uninterrupted flight comes as a surprise given the quantum fluctuations in the heterodyne record Irec during the jump—an island of predictability in a sea of uncertainty. [...]

From the experimental results of Fig. 2a one can infer, consistent with Bohr’s initial intuition and the original ion experiments, that quantum jumps are random and discrete. Yet the results of Fig. 3 support a contrary view, consistent with that of Schrödinger: the evolution of the jump is coherent and continuous. The difference in timescales in the two figures allows the coexistence of these seemingly opposed point of views and the reconciliation of the discreteness of countable events, such as jumps, with the continuity of the deterministic Schrödinger’s equation. [...]

although all 6.8 × 106 recorded jumps (Fig. 3) are entirely independent of one another and stochastic in their initiation and termination, the tomographic measurements as a function of Δtcatch explicitly show that all jump evolutions follow an essentially identical, predetermined path in Hilbert space—not a randomly chosen one—and, in this sense, they are deterministic. These results are further corroborated by the reversal experiments shown in Fig. 4, which exploit the continuous, coherent, and deterministic nature of the jump evolution

I'm always highly skeptical of sensational-sounding claims on phys.org.

The claim there is the usual unjustified amplification of science made sensational for the public.

 

[Demystifier]

The paper just appeared in Nature

https://www.nature.com/articles/s41586-019-1287-z

As far as I can see, this is not much more than an experimental demonstration of the known fact that decoherence lasts a finite time.

 

[Cthugha]

This is a very nice work with respect to the experiments involved, but I consider it as somewhat oversold. If you go to the methods section, their operational definition of a quantum jump is "Sections of the (continuous) measurement record are converted into state assignments, as discussed above, such as B, G or D. In the experiment, long sequences of such measurements yield the same result, that is, GGG… or DDD… When the string of results suddenly switches its value, we say that a quantum jump has occurred".

Here, they just drive the bright state transition and the dark state transition simultaneously and consider the absence of deexcitation from the bright state as some evidence that the system is in the dark state. However, I do not fully agree to this idea. When you drive the dark state transition with a low Rabi frequency and you notice that there is no emission from the bright state, this first and foremost means that the probability amplitude for dark state occupation is close to one in the Rabi oscillation cycle. Of course you will get an "identical no-click record from run to run" in this scenario as the probability of the system ending up in the dark state follows the standard Rabi cycle. Accordingly, their "catching the quantum jump" is essentially just going back the Rabi cycle downwards towards the ground state after going up part of the way to the dark state. In other words: they do not "reverse" quantum jumps, but they drive the system towards states, where there is a finite probability amplitude for this quantum jump to occur (if the system becomes perturbed randomly), introduce a threshold for this probability amplitude and as soon as the experimental parameters cross this threshold, they reduce this probability again. The jump is not reversed. It never happens.

However, doing that in the presence of the comparably strong driving field towards the bright state is a really nice experimental demonstration of what is possible with fast electronics, FPGAs and feedback.

 

[nettleton]

If this is the case, is the the triggering of the decay of a individual atom in a radioactive solid also predictable?

 

[A. Neumaier]

If this is the case, is the the triggering of the decay of a individual atom in a radioactive solid also predictable?

Probably not. The system in the experiments reported is very special, and a single system, while a radioactive solid consists of a huge number of radiactive atoms, of which a random one will decay, and then another random one.

Based on what is in the paper, it could perhaps be feasible one day to prepare a single radioactive atom in some controllable ion trap and know just a little ahead of the time that it is going to decay. But this would be surely another experimental challenge.

 

[Heikki Tuuri]

There is an anecdote that Erwin Schrödinger threatened to quit physics, if people keep talking about "quantum jumps".

The wave function always develops smoothly in time. There are no jumps. It is the measurement which makes the wave function to collapse in the standard Copenhagen interpretation.

Here we answer this question affirmatively: we experimentally demonstrate that the jump from the ground state to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable ‘flight’, by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the evolution of each completed jump is continuous, coherent and deterministic.

https://www.nature.com/articles/s41586-019-1287-z
They "monitor a population of an auxiliary energy level". That sounds like making a "weak measurement" of the system in the Aharonov style. If I am right, the system develops in separate steps toward an end state. They notice when it has made a step, and then force it back to the start state.

The authors say that the result is consistent with standard quantum mechanics.

The authors should not talk about quantum jumps, as the term does not exist in standard quantum mechanics. Also, talking about a predictable "flight" sounds like a hidden variable theory. It is a bad choice of words.

 

[A. Neumaier]

There is an anecdote that Erwin Schrödinger threatened to quit physics, if people keep talking about "quantum jumps".

Please give a reliable source. Or did you mean that he said the following?

Wenn es bei dieser verdammten Quantenspringerei bleiben soll, so bedaure ich, mich mit der Quantentheorie überhaupt befaßt zu haben.

''If I had known we were going to go on having all this damned quantum-jumping, I would never have got involved in the subject.''
(This is a - not completely faithful - translation from here, with reference to the original.)

 

[Heikki Tuuri]

https://motls.blogspot.com/2019/06/experimenters-and-especially.html
Lubos Motl criticizes harshly the language which the authors of the Nature paper use. A "quantum leap" and a "trajectory" are not terms of the standard quantum mechanics.

Nature should be more careful in their editorial policy.

 

[StevieTNZ]

https://www.quantamagazine.org/quantum-leaps-long-assumed-to-be-instantaneous-take-time-20190605/
Another article written about the experiment - well, one wonders how accurate it is (it may be, it may not be).

EDIT: I see the post above deals with this article.

 

[PeroK]

''If I had known we were going to go on having all this damned quantum-jumping, I would never have got involved in the subject.'' (not completely faithful) translation from here, with reference to the original)

It's interesting how much slicker German can be than English. The German has not a wasted word, which is quite hard to achieve in English. In any case, perhaps a better translation is:

"If there is to be no end to this damned quantum-jumping, then I'm sorry I ever had anything to do with quantum theory."

Or:

"If this damned quantum-jumping is here to stay, then I'm sorry I ever had anything to do with quantum theory."

 

[A. Neumaier]

Lubos Motl criticizes harshly the language which the authors of the Nature paper use. A "quantum leap" and a "trajectory" are not terms of the standard quantum mechanics.

Nature should be more careful in their editorial policy.

Or Lubos Motl in his critique; he is not the ultimate arbiter of science.

Terms in physics evolve and are adapted to whatever they are needed for. Otherwise we would have never progress in physics.

 

[Demystifier]

Terms in physics evolve and are adapted to whatever they are needed for. Otherwise we would have never progress in physics.

Yes, but in typical experimental quantum foundations papers in Nature, the purpose of changing the terminology is not a progress in understanding quantum foundations. The purpose is to rise hypes, which helps to publish in Nature, which helps to get funds for producing next papers of a similar kind, ...

 

[Demystifier]

Progress in understanding quantum foundations can be achieved in two ways: either by further developments of the theory, or by experiments the results of which differ from predictions of standard quantum theory. The present work achieved neither of those two.

 

[A. Neumaier]

Yes, but in typical experimental quantum foundations papers in Nature, the purpose of changing the terminology is not a progress in understanding quantum foundations. The purpose is to rise hypes, which helps to publish in Nature, which helps to get funds for producing next papers of a similar kind

They don't change terminology for the purpose of getting publicity. They use long established terminology in their field that just happens to be rejected and miscredited by Lubos Motl.

Progress in understanding quantum foundations can be achieved in two ways: either by further developments of the theory, or by experiments the results of which differ from predictions of standard quantum theory. The present work achieved neither of those two.

Many Nobel prizes were given for experimental work demonstrating existing features of Nature that were either predicted or later explained by standard quantum theory.

 

[Demystifier]

Many Nobel prizes were given for experimental work demonstrating existing features of Nature that were either predicted or later explained by standard quantum theory.

Demonstrating the existence of a feature is one thing, understanding its existence is another. Typically the former comes from experiments and the latter from theories.

 

[A. Neumaier]

Demonstrating the existence of a feature is one thing, understanding its existence is another. Typically the former comes from experiments and the latter from theories.

The paper under discussion is about such a demonstration (though not of Nobel prize quality), and an explanation of it in terms of existing theory. Thus it is progress.

 

[A. Neumaier]

https://motls.blogspot.com/2019/06/experimenters-and-especially.html
Lubos Motl criticizes harshly the language which the authors of the Nature paper use. A "quantum leap" and a "trajectory" are not terms of the standard quantum mechanics.

Read the original Nature paper rather than discussions of popular versions of it!

The authors of the Nature paper give reference to [5-9] where ''modern quantum trajectory theory'' is discussed in detail. Reference [5] is a well-known and very respectable textbook on quantum optics by Carmichael (one of the authors), and [8] is a thick and time-honored (almost 1200 citations in google scholar) survey paper about ''The quantum jump approach to dissipative processes in quantum optics'' by Plenio and Knight, both very accomplished quantum optics experts.

Note that quantum jumps are so much part of the Copenhagen interpretation (the long-time gold standard for quantum mechanics interpretations) that Erwin Schrödinger, who never liked them, wrote even as late as 1952 (but just before the interpretation questions came to the forefront again) a paper with the title ''https://www.jstor.org/stable/pdf/685552.pdf" - in vain.

Note also that nobody seriously claimed (or defended) that quantum jumps actually happen instantaneously, this was always just an idealization of the same sort of the idealization of measurements in Born's rule, which also take time.

The authors of the Nature paper do not talk about a "quantum leap", which is a typical pop-science notion without relevance in quantum physics. Indeed, this notion is used only in the pop-science account of the experiment in Quanta Magazine, to which Lubos Motl (who is an expert not in quantum optics but only in the completely unrelated subject of string theory) mainly refers.

 

[Demystifier]

The paper under discussion is about such a demonstration (though not of Nobel prize quality), and an explanation of it in terms of existing theory. Thus it is progress.

So I must have missed it, how does the paper explain it in terms of existing theory?

 

[A. Neumaier]

So I must have missed it, how does the paper explain it in terms of existing theory?

Read my post #20, the appendix of the paper, and references [5-9] on ''modern quantum trajectory theory''. Nowhere is any theory beyond that assumed.

 

[Lord Jestocost]

Indeed, the association with quantum foundations has convinced many observers that quantum trajectory theory is different from standard quantum mechanics, and therefore to be regarded with deep suspicion⁹. While it is true that stochastic Schr¨odinger and master equations of the type treated in quantum trajectories are sometimes postulated in alternative quantum theories^10–13, these same types of equations arise quite naturally in describing quantum systems interacting with environments (open systems) which are subjected to monitoring by measuring devices. In these systems, the stochastic equations arise as effective evolution equations, and are in no sense anything other than standard quantum mechanics (except, perhaps, in the trivial sense of approaching the limit of continuous measurement).

Todd A. Brun in "A simple model of quantum trajectories" https://arxiv.org/abs/quant-ph/0108132

 

[A. Neumaier]

Indeed, the association with quantum foundations has convinced many observers that quantum trajectory theory is different from standard quantum mechanics, and therefore to be regarded with deep suspicion [...] these same types of equations arise quite naturally in describing quantum systems interacting with environments (open systems) which are subjected to monitoring by measuring devices. In these systems, the stochastic equations arise as effective evolution equations, and are in no sense anything other than standard quantum mechanics

This criticism (should it be one) is very strange.

Quantum trajectory theory is nothing else than effective stochastic evolution equations for a class of open quantum optics systems. Thus it is quite natural and nothing to be suspicious about.

 

[Heikki Tuuri]

The quantum jump method is an approach which is much like the master-equation treatment except that it operates on the wave function rather than using a density matrixapproach. The main component of the method is evolving the system's wave function in time with a pseudo-Hamiltonian; where at each time step, a quantum jump (discontinuous change) may take place with some probability. The calculated system state as a function of time is known as a quantum trajectory, and the desired density matrix as a function of time may be calculated by averaging over many simulated trajectories.

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

Indeed, it is a numerical method for solving the wave equation. The authors should stress that this has nothing to do with quantum leaps of popular science.

 

[A. Neumaier]

The authors should stress that this has nothing to do with quantum leaps of popular science.

No. Scientific papers should ignore popular science, not give it credit for using poor terms.

 

[Cthugha]

They don't change terminology for the purpose of getting publicity. They use long established terminology in their field that just happens to be rejected and miscredited by Lubos Motl.

I am not too sure how I feel about that reply. In some sense you are of course right.

However, you took a similar approach some time ago in your insights article (https://www.physicsforums.com/insights/vacuum-fluctuations-experimental-practice/) which is an unjustified mixture of quoting stuff out of context, misapplying terminology that is used differently in different subfields and rejecting and miscrediting its usage in other fields.

Having reread the present Nature article several times now, I think it is indeed worthy of being published in Nature. However, I consider the experimental ability to control a quantum system as the really important point here. In some sense, this is similar to Haroche's seminal work on cavity QED (https://www.nature.com/articles/nature05589, https://www.nature.com/articles/nature10376).

 

[A. Neumaier]

you took a similar approach some time ago in your insights article (https://www.physicsforums.com/insights/vacuum-fluctuations-experimental-practice/) which is an unjustified mixture of quoting stuff out of context, misapplying terminology that is used differently in different subfields and rejecting and miscrediting its usage in other fields.

I don't think the paper I analyzed there is comparable in the quality of terminology to the one under discussion here. I'd appreciate if you would either moderate your severe accusations, or justify them in detail in the discussion thread associated with that Insight article.

Having reread the present Nature article several times now, I think it is indeed worthy of being published in Nature. However, I consider the experimental ability to control a quantum system as the really important point here. In some sense, this is similar to Haroche's seminal work on cavity QED (https://www.nature.com/articles/nature05589, https://www.nature.com/articles/nature10376).

This is an important experimental point.

The interest for foundations is that monitoring and controlling the state of an individual quantum system can now be done to a point where earlier, more idealized instantaneous quantum jumps can be resolved in time, adding insight to the nature of these ''jumps''.

 

[Cthugha]

I don't think the paper I analyzed there is comparable in the quality of terminology to the one under discussion here. I'd appreciate if you would either moderate your severe accusations, or justify them detail in the discussion thread associated with that Insight article.

I do not mean to be insulting and apologize if my statement came across as such, but I really think your comment is questionable and based on erroneous assumptions. But that is indeed a discussion to be continued elsewhere.

This is an important experimental point.

The interest for foundations is that monitoring and controlling the state of an individual quantum system can now be done to a point where earlier, more idealized instantaneous quantum jumps can be resolved in time, adding insight to the nature of these ''jumps''.

Here, I fully agree. To me it is interesting that this kind of experiment (this is good to see in the ones by Haroche) actually show examples where "weak" measurements actually bring some benefit. In most cases, they are just performed because the experimentalist could do something that sounds cool.

 

[A. Neumaier]

As you bring this "quantum jumping" up again, just have a look at this (all standard QT, no discontinous jumps) (including the theory part which is in the supplements):

https://www.nature.com/articles/s41586-019-1287-z

Since this is about the measurement of a single quantum system, I'd be interested how the minimal (statistical) interpretation makes sense of the correspondence between state and measurement results in this particular case.