I Quantum Jumps and Schrodinger's Cat are predictable

  • Thread starter Tom.G
  • Start date
  • Featured

Tom.G

Science Advisor
2,544
1,368
Quantum Jumps are predicted using microwave monitoring. Weird.

 
24,765
6,194
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

Science Advisor
Gold Member
2,911
408
The paper just appeared in Nature

 

A. Neumaier

Science Advisor
Insights Author
6,189
2,315
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

But there are no relations to Schrödinger's cat.

From the abstract:
Minev et al. said:
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:
Minev et al. said:
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

Science Advisor
Insights Author
2018 Award
9,729
2,757
The paper just appeared in Nature

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

Science Advisor
1,839
200
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.
 
If this is the case, is the the triggering of the decay of a individual atom in a radioactive solid also predictable?
 

A. Neumaier

Science Advisor
Insights Author
6,189
2,315
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.
 
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

Science Advisor
Insights Author
6,189
2,315
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?
Erwin Schrödinger said:
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.)
 
Last edited:
1,444
343

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

Science Advisor
Homework Helper
Insights Author
Gold Member
2018 Award
9,351
3,398
''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."
 
Last edited:

A. Neumaier

Science Advisor
Insights Author
6,189
2,315
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

Science Advisor
Insights Author
2018 Award
9,729
2,757
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

Science Advisor
Insights Author
2018 Award
9,729
2,757
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

Science Advisor
Insights Author
6,189
2,315
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

Science Advisor
Insights Author
2018 Award
9,729
2,757
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

Science Advisor
Insights Author
6,189
2,315
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

Science Advisor
Insights Author
6,189
2,315
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 ''Are there quantum jumps?" - 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.
 
Last edited:

Demystifier

Science Advisor
Insights Author
2018 Award
9,729
2,757
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

Science Advisor
Insights Author
6,189
2,315
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.
 
Last edited:

Lord Jestocost

Gold Member
2018 Award
331
199
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 theories10–13, 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

Science Advisor
Insights Author
6,189
2,315
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.
 
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.
 

Want to reply to this thread?

"Quantum Jumps and Schrodinger's Cat are predictable" You must log in or register to reply here.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top