Does Decoherence get rid of all Quantumness?

  • #1
The answer is no and even when decoherence occurs for Wigner's Friend in the lab, quantum coherence remains. Let's start with the paper that illustrates this.

Assisted Macroscopic Quantumness

It is commonly expected that quantum theory is universal, in that it describes the world at all scales. Yet, quantum effects at the macroscopic scale continue to elude our experimental observation. This fact is commonly attributed to decoherence processes affecting systems of sufficiently large number of constituent subsystems leading to an effective macro-scale beyond which a quantum description of the whole system becomes superfluous. Here, we show both theoretically and experimentally that the existence of such a scale is unjustifiable from an information-theoretic perspective. We introduce a variant of the Wigner's friend experiment in which a multiparticle quantum system is observed by the friend. The friend undergoes rapid decoherence through her interactions with the environment. In the usual version of this thought experiment, decoherence removes the need for Wigner to treat her as a quantum system.
CONT.
However, for our variant we prove theoretically and observe experimentally that there exist partitions of the subsystems in which the friend is entangled with one of the particles in assistance with the other particle, as observed by Wigner. Importantly, we show that the friend is indispensable for the entanglement to be observed. Hence Wigner is compelled to treat the friend as part of a larger quantum system. By analyzing our scenario in the context of a quantum key distribution protocol, we show that a semi-classical description of the experiment is suboptimal for security analysis, highlighting the significance of the quantum description of the friend.
https://arxiv.org/abs/1711.10498

Wow, I recently read this paper and the results are simply fascinating. Many people think decoherence means coherence is lost to the environment. This shows this isn't the case. Coherence remains even in the face of decoherence. So Wigner's Friend, in this scenario Alice, can be in a decohered state in the lab and Wigner outside of the lab can still measure coherence which supports my recent posts that said you can't confuse decoherence with collapse of the wavefunction. Here's more from the paper.

A commonly conjectured roadblock for macroscopic quantum effects is quantum decoherence related to the size of the considered system [12, 13]. Loosely speaking, according to some (rather intuitive) definitions of macroscopic quantumness wherein the size of the system (and thus the number of its constituent elements) is a decisive parameter [14–21], the larger the system is the harder it is to isolate it from interactions with the environment. Such interactions, in turn, destroy the coherence of the system, hence, no quantum property of a macroscopic, i.e. sufficiently large, system can be observed unless via extremely high-precision measurements [14, 17]. In other words, it is assumed that there exists a “scale” beyond which physical systems become “macroscopic” and can be analyzed without any reference to the quantum formalism.

In this contribution we challenge this view by showing in theory and experiment that, with the assistance of a second system, a macroscopic system (in the sense of being made up by a very large number of subsystems) can be proven to be entangled even after complete decoherence. We set the scene as in a Wigner’s friend experiment, with the crucial difference that the observer is not assumed to preserve quantum coherence, and analyze Wigner’s perspective, see Fig. 1. In our variant, Wigner’s friend is in possession of two particles labeled a and t. We find that, even in the presence of decoherence and regardless of its dynamics, the joint subsystem of Wigner’s friend and particle t exhibits entanglement with particle a, while there is no entanglement in any pair of subsystems alone. Consequently, as far as the information content of Wigner’s friend is concerned, she constitutes a quantum system in assistance with particle t regardless of her size. More specifically, Wigner has to use the quantum formalism to describe the entanglement in this particular partition, independent of our choice of interpretation of quantum theory. Hence, there seems to be no escape from the conclusion that Wigner’s friend, despite being a macroscopic observer by any sensible definition, is an informationally indispensable part of the quantum system.
So Wigner's Friend Alice in the lab is in a decohered state but Wigner outside of the lab can still measure coherence and he can only measure coherence if macroscopic Alice is included as an informationally indispensable part of the quantum system. Without Alice, there's no entanglement measured in a and t.

awig.png


So even when the system has decohered and coherence appears to be lost, the information about coherence can still be recovered on if macroscopic Alice is included as part of the quantum system. Here's more:

We now briefly discuss the crucial differences in the analysis and interpretation of our scenario compared to the conventional Wigner’s friend experiment. There has recently been an increasing interest in the modified Wigner’s friend scenarios, both in theory [26, 27] and experiment [28, 35]. These works are primarily devoted to the disconnect between Wigners perception of reality and that of the friend. In contrast, our aim is to investigate the extent to which the assumption of the existence of a macro-scale beyond which a quantum description becomes redundant is valid.

Here, we have extended the macroscopic system “Alice” of the conventional proposals to an even larger macroscopic system Aa (that is, Alice plus the qubit a). This effectively debilitates the decoherence mechanism’s ability to reach “classicality”, since it only leads to a loss of coherence between Alice and the microscopic subsystems.
So while decoherence has occured it doesn't remove coherence from an information-theoretic perspective. So in this case, decoherence is only relative to Wigner's Friend in the lab. Here's more:

Technically, while decoherence destroys the entanglement in the partition (A|at), this is insufficient to achieve macroscopic “classicality”, since the macroscopic system aA (which is larger than Alice) remains entangled to the qubit t. At the same time, there is no entanglement in the partition (a|t), implying that the observed entanglement is not merely due to the microscopic subsystems a and t.

At this point one may be tempted to think of Alice as simply holding a piece of (classical) information, which identifies one or the other entangled quantum state of subsystem at. We emphasize that this requires one to refer to the individual subsystem A which, in turn, means working with either the bipartition (A|at) or the tripartition (A|a|t).
So decoherence appears to destroy the entanglement in partition (A|at) but this is only relative to A(Alice) in the lab. This isn't enough to achieve macroscopic classicality because the larger system measured by Wigner outside of the lab includes aA which includes Alice and qubit a remains entangled to qubit t even though a and t aren't entangled.

This goes to what Rovelli says in the Relational interpretation. Wigner O' has information about (aA|t) which is a larger system than (A|at) which appears to Alice as a decohered state. It's not though and you need A to describe (aA|t) as an entangled system.

While this view justifies some kind of classical-quantum description of the whole system across the partitions (A|at) or (A|a|t), it fails to provide such a description for the partition (aA|t), which includes the larger macroscopic system aA of interest as an entity here, because the classical theories required to describe A do not possess the structure to accommodate entanglement. As such, the entanglement across the partition (aA|t) can only be captured when endowing the macroscopic system aA with a fully quantum description. Indeed, conditioning on Alice’s information is a way to transfer the entanglement from the partition (aA|t) to the partition (a|t). This emphasizes the crucial role Alice’s information plays for the entanglement of the whole system. Consequently, we come to the conclusion that there are micro-macro systems that show quantum features, even in the presence of decoherence.
So decoherence doesn't remove quantumness. So Wigner's Friend in the lab can be in a state of decoherence and appear to see a classical outcome in the lab, Wigner outside of the lab can still measure coherence. This result has practical implications for quantum key distribution.
 
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Allisrelative. Everyone knows decoherence doesnt get rid of all quantumness, as the following passages in wiki showed. How does your account differ to it??

"Decoherence has been used to understand the collapse of the wave function in quantum mechanics. Decoherence does not generate actual wave-function collapse. It only provides an explanation for apparent wave-function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wave function are decoupled from a coherent system and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. Specifically, decoherence does not attempt to explain the measurement problem. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble"."
 
  • #3
Allisrelative. Everyone knows decoherence doesnt get rid of all quantumness, as the following passages in wiki showed. How does your account differ to it??
No, everyone doesn't know. I was just in a debate on this forum where people confused decoherence and wavefunction collapse. They said if decoherence occurred in the lab with Wigner then it was IMPOSSIBLE to recover any quantumness.

So everyone doesn't know. I quoted the same article from Wiki in that debate.

This is a huge problem for many worlds and many decoherent histories interpretations. This is because in many worlds you have a global physical wavefunction which isn't scientific. In decoherent histories, decoherence is supposed to be an objective, physical , irreversible process.

This means when decoherence happens then coherence or quantumness is lost to the environment. This is supposed to be an objective truth so when decoherence occurs for Wigner's Friend it should also occur for Wigner outside of the lab and if Wigner shared the same universe or history as his friend in the lab then he's in the same environment where coherence is scrambled and lost.

As we see with this experiment and the recent Wigner's Friend experiment this isn't the case. Decoherence can occur for Wigner's Friend in the lab yet Wigner can still measure coherence between his friend and the quantum system being measured. Without his macroscopic friend the quantum systems are not entangled.

These results make sense if the wavefunction is real and non physical. Then the many worlds are a holographic projection of information and not actual physical universes. This way Wigner's Friend in the lab can experience local, subjective "realism" and decoherence can occur. This means the measurement is relative to the observer. Is there any scientific evidence for a non physical wavefunction? Yes, there is and there's no evidence of a global physical wave function. So if there's any speculation it's any discussion of many worlds which depends on a global physical wavefunction that isn't scientific.

The wave-function is real but nonphysical: A view from counterfactual quantum cryptography

Counterfactual quantum cryptography (CQC) is used here as a tool to assess the status of the quantum state: Is it real/ontic (an objective state of Nature) or epistemic (a state of the observer's knowledge)? In contrast to recent approaches to wave function ontology, that are based on realist models of quantum theory, here we recast the question as a problem of communication between a sender (Bob), who uses interaction-free measurements, and a receiver (Alice), who observes an interference pattern in a Mach-Zehnder set-up. An advantage of our approach is that it allows us to define the concept of "physical", apart from "real". In instances of counterfactual quantum communication, reality is ascribed to the interaction-freely measured wave function (ψ) because Alice deterministically infers Bob's measurement. On the other hand, ψ does not correspond to the physical transmission of a particle because it produced no detection on Bob's apparatus. We therefore conclude that the wave function in this case (and by extension, generally) is real, but not physical. Characteristically for classical phenomena, the reality and physicality of objects are equivalent, whereas for quantum phenomena, the former is strictly weaker. As a concrete application of this idea, the nonphysical reality of the wavefunction is shown to be the basic nonclassical phenomenon that underlies the security of CQC.
https://arxiv.org/abs/1311.7127

Here's an experiment where direct counterfactual communication occurred without the transmission of a particle.

Direct counterfactual communication via quantum Zeno effect

Intuition from our everyday lives gives rise to the belief that information exchanged between remote parties is carried by physical particles. Surprisingly, in a recent theoretical study [Salih H, Li ZH, Al-Amri M, Zubairy MS (2013) Phys Rev Lett 110:170502], quantum mechanics was found to allow for communication, even without the actual transmission of physical particles. From the viewpoint of communication, this mystery stems from a (nonintuitive) fundamental concept in quantum mechanics—wave-particle duality. All particles can be described fully by wave functions. To determine whether light appears in a channel, one refers to the amplitude of its wave function. However, in counterfactual communication, information is carried by the phase part of the wave function. Using a single-photon source, we experimentally demonstrate the counterfactual communication and successfully transfer a monochrome bitmap from one location to another by using a nested version of the quantum Zeno effect.
https://www.pnas.org/content/114/19/4920

Again, when I talk about a non physical wavefunction it has it's basis in science. A global physical wavefunction has no scientific basis. It was added by Everett in an ad hoc way to reduce the role of the observer. Tesla talked about studying the non physical.

“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”

― Nikola Tesla

Also, quantum mechanics tells us local realism is dead or close to it. here's a paper that was in Nature.

Death by experiment for local realism

A fundamental scientific assumption called local realism conflicts with certain predictions of quantum mechanics. Those predictions have now been verified, with none of the loopholes that have compromised earlier tests.
https://www.nature.com/articles/nature15631

What this says is that the local experience of decoherence of Wigner's Friend in the lab isn't an objective physical occurrence but a subjective local one. This destroys many worlds and other decoherent histories interpretations. Can many worlds still occur? Yes, but not with the unscientific global physical wavefunction.
 
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They said if decoherence occurred in the lab with Wigner then it was IMPOSSIBLE to recover any quantumness.
No, that is not what was said in the other discussion you refer to. "Any quantumness" is very vague, just like "all quantumness". What was said about the Wigner's friend experiment in the other discussion was much more specific.

Expressing your own opinions is fine. Misrepresenting others' statements is not.
 
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No, that is not what was said in the other discussion you refer to. "Any quantumness" is very vague, just like "all quantumness". What was said about the Wigner's friend experiment in the other discussion was much more specific.

Expressing your own opinions is fine. Misrepresenting others' statements is not.
All quantumness isn't vague. In the face of decoherence how can coherence occur if the quantumness has decohered and is lost to the environment? If Wigner shares the same environment as his friend in the lab and decoherence is an objective, physical, irreversible process then how can coherence or quantumness be recovered by Wigner?

I'm not misrepresenting anyone. People said, if decoherence has occurred for Wigner's Friend in the lab then Wigner can't measure any quantumness because that information is lost to the environment. Maybe a different word was used but quantumness, coherence, interference is all the same thing...quantumness.

Again, if quantumness is lost because of decoherence how can Wigner see coherence?
 
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In the face of decoherence how can coherence occur if the quantumness has decohered and is lost to the environment?
The quantumness is not lost to the environment in the experiment described in the paper. They don't actually let decoherence happen; they "simulate" decoherence by doing phase flips. But the kind of "decoherence" being done by the phase flips is not the same as the kind of decoherence that happens when, for example, stray photons in the environment reflect off an object and then fly off into space, or when an electron hits a detector screen in a double slit experiment and makes a dot.

The "decoherence" done by the phase flips is trackable; that's how the "eavesdropper" Eve discussed in Section E is able to gain the quantum information she gains: in the paper's words, Eve "has access to the information that flowed to the environment of Alice". But the only way Eve can have access to such information is in a carefully prepared experimental setup such as the one described in the paper, that deals with systems with a manageably small number of degrees of freedom.

Giving such systems names like "Alice" does not change the fact that they are nothing like humans. It is a huge misrepresentation to claim that scaling such experiments up to actual humans, which is what would be required for them to actually have the implications usually discussed under the "Wigner's Friend" topic, is a trivial exercise and can just be assumed when talking about the results. Using the word "environment" in such a context is also a serious misrepresentation, since the "environment", as noted above, has to be trackable, but non-trackability is precisely what the word "environment" is supposed to imply in decoherence theory.

This experiment is certainly interesting, but it looks to me like its implications are being overstated. Unfortunately that appears to be common in the literature on this topic.
 
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The quantumness is not lost to the environment in the experiment described in the paper. They don't actually let decoherence happen; they "simulate" decoherence by doing phase flips. But the kind of "decoherence" being done by the phase flips is not the same as the kind of decoherence that happens when, for example, stray photons in the environment reflect off an object and then fly off into space, or when an electron hits a detector screen in a double slit experiment and makes a dot.

The "decoherence" done by the phase flips is trackable; that's how the "eavesdropper" Eve discussed in Section E is able to gain the quantum information she gains: in the paper's words, Eve "has access to the information that flowed to the environment of Alice". But the only way Eve can have access to such information is in a carefully prepared experimental setup such as the one described in the paper, that deals with systems with a manageably small number of degrees of freedom.

Giving such systems names like "Alice" does not change the fact that they are nothing like humans. It is a huge misrepresentation to claim that scaling such experiments up to actual humans, which is what would be required for them to actually have the implications usually discussed under the "Wigner's Friend" topic, is a trivial exercise and can just be assumed when talking about the results. Using the word "environment" in such a context is also a serious misrepresentation, since the "environment", as noted above, has to be trackable, but non-trackability is precisely what the word "environment" is supposed to imply in decoherence theory.

This experiment is certainly interesting, but it looks to me like its implications are being overstated. Unfortunately that appears to be common in the literature on this topic.
Your problem isn't with the experiment, it's with Quantum Mechanics. If you have a better theory then let's hear it. Get it published. This is the same decoherence as with a human and yes you can scale it up to the size of a human. Formalism of QM doesn't make a difference between macroscopic or microscopic observers.

Where talking about science here not your opinion. In both papers they explain why this is the case. Here's the first paper.

”Formally, an observation is the act of extracting and storing information about an observed system. Accordingly, we define an observer as any physical system that can extract information from another system by means of some interaction and store that information in a physical memory…...Notably, the formalism of quantum mechanics does not make a distinction between large (even conscious) and small physical systems, which is sometimes referred to as universality. Hence, our definition covers human observers, as well as more commonly used nonconscious observers such as (classical or quantum) computers and other measurement devices—even the simplest possible ones, as long as they satisfy the above requirements."

https://advances.sciencemag.org/content/5/9/eaaw9832

It irks me a little when people say these things. Your opinion that "it's overstated" is pure speculation and opinion vs. actual experiments that stick within the bounds of formalism of quantum mechanics. In this paper it says this:

In this contribution we challenge this view by showing in theory and experiment that, with the assistance of a second system, a macroscopic system (in the sense of being made up by a very large number of subsystems) can be proven to be entangled even after complete decoherence. We set the scene as in a Wigner’s friend experiment, with the crucial difference that the observer is not assumed to preserve quantum coherence, and analyze Wigner’s perspective, see Fig. 1. In our variant, Wigner’s friend is in possession of two particles labeled a and t. We find that, even in the presence of decoherence and regardless of its dynamics, the joint subsystem of Wigner’s friend and particle t exhibits entanglement with particle a, while there is no entanglement in any pair of subsystems alone. Consequently, as far as the information content of Wigner’s friend is concerned, she constitutes a quantum system in assistance with particle t regardless of her size. More specifically, Wigner has to use the quantum formalism to describe the entanglement in this particular partition, independent of our choice of interpretation of quantum theory. Hence, there seems to be no escape from the conclusion that Wigner’s friend, despite being a macroscopic observer by any sensible definition, is an informationally indispensable part of the quantum system.

Again, decoherence has occurred and it's no different than with a human. Entanglement was present then Alice carried out a measurement and coherence was gone. Then coherence only occurred when Alice was included and Wigner had to use quantum formalism to describe the entanglement. The paper says this:

Wigner must therefore conclude, independent of his of interpretation of quantum theory, that: (i) Alice is part of a large entangled state within partitions (aA|t) and (a|tA); (ii) the amount of entanglement within both partitions is determined by the distinguishability of Alice’s memory states; (iii) the entanglement is not merely due to qubits, as Alice is necessary for obtaining any entanglement.
So with all due respect, I'm going to need more than opinion.

Where's the experiment that shows Wigner can't measure interference after coherence is no longer present?

Where's the experiment that shows that when an entangled system is no longer entangled and loses it's coherence because of measurement, the observer that measured the system is prohibited from being entangled with the system because of decoherence?
 
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@allisrelative you are simply restating your opinion without actually addressing any of the points I've raised. We can't have a productive discussion that way. You have expressed your opinion and it will remain visible in this thread, but I see no point in any further discussion.

Thread closed.
 
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