Is decoherence necessary to observe a quantum event?

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  • #1
Heidi
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Hi Pfs,
I wonder if in quantum mechanics the question "Did this event occur?" has a yes or no answer like in classical phyics or in our daily life.
We are accustomed to consider linear superpositions of states like "dead" and "alive"
but id "dead" is a state the death of the cat is an event. i wonder if we can have superposition of events.
the title ask if decoherence is necessary to observe something.
it seems that getting an measurement result needs an inernoment in which information is lost.
this needs that the measured state becomes no more pure density matrix.
there is a theorem telling us that we can extend the system to a bigger one in which the purity is conserved. in this bigger system we could say that no such measurement occured.
the problem is that decoherence is not question of point of view. it is a physical process.
can we say that what is physical in the smaller system becomes unphysical in the bigger one?
 
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  • #2
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@Heidi, the questions you are asking are really about the interpretation of QM, and their answers will depend on which interpretation of QM you adopt.
 
  • #3
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Moderator's note: Thread moved to QM interpretations subforum.
 
  • #4
Demystifier
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there is a theorem telling us that we can extend the system to a bigger one in which the purity is conserved. in this bigger system we could say that no such measurement occured.
the problem is that decoherence is not question of point of view. it is a physical process.
can we say that what is physical in the smaller system becomes unphysical in the bigger one?
Such contradictory conclusions resulting from simultaneous considerations of a bigger and a smaller system is known in literature as the Wigner friend paradox. Recently this paradox has been sharpened by Frauchiger and Renner in https://arxiv.org/abs/1604.07422 , where they argued that all interpretations of QM avoid the paradox one way or another, but different interpretations do it differently. It has been discussed in this forum as well, in several threads, most recently in
https://www.physicsforums.com/threads/current-status-of-fr-type-thought-experiments.1046499/
 
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  • #5
Heidi
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I do not know if it is exactly the Wigner's friend problem.
was decoherence process known then?
Decoherence is a physical process that occurs when information is lost in the environment.
we have in a measurement "one result" => "there was decoherence"
if environment + apparatus + particle has no information loss what about a global desity matrix?
does it decohere or remain pure during the apparatus + particle interaction?
I read that severall things are contradictory. it seems that it is due to that they are "level" dependent.
eg Bob and Alice see no interference (diagonal density matrices) but a the global level when we look a the pairs of result we see that interferences reappear. see Birgit Dopfer experiment.
 
  • #6
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we have in a measurement "one result" => "there was decoherence"
No, we don't. Decoherence can explain why different "branches" of the wave function corresponding to different measurement results do not interfere with each other. But it cannot explain why only one "branch" actually survives the measurement, as some QM interpretations (the ones in which collapse of the wave function is an actual process, not just a calculational convenience) assert.

if environment + apparatus + particle has no information loss
This will always be true under unitary evolution, and the theory of decoherence assumes unitary evolution. The "information loss" in decoherence arises because the information contained in the environment is not retrievable.
 
  • #7
Heidi
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Do you say that in the measurement process (giving later one result) the decoherence may be absent?
 
  • #9
Heidi
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So we are not far from answering yes to the question in the title?
 
  • #11
StevieTNZ
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I must admit I'm awfully confused as to what exactly your asking.
What is your definition of 'quantum event'? A superposition of states? If so, quantum mechanics already caters to that.
What is decoherence suppose to help us observe? All you write is 'something'. "title ask if decoherence is necessary to observe something."
Decoherence is nothing more than tracing over information and discarding some. It doesn't really achieve much - in principle superposition still exists, despite appearing as if we have classical probabilities of things that exist in definite states.
 
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  • #12
Heidi
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Sorry if i was not clear.
the absorption of a photon by an atom is an event. a spontaneous decay is also an event.
a superposition of states is not an event it is a state.
i wrote that the dead cat is in a given state and i added that it was different from the death of the cat which is an event. i did not think that there was a problem with that
My question in the title was not about measuring properties of states like eigen values but if events
can be observed without decohence (with a global pure density matrix)
 
  • #13
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the absorption of a photon by an atom is an event. a spontaneous decay is also an event.
You need to be more specific. These processes can happen without being measured. Are they "events" if they are not measured?

if events
can be observed without decohence
What do you mean by "observed"?

One fairly common definition defines "observation" as "decoherence occurs". With that definition, the answer to your question is obviously "no" by definition. But does that tell you anything useful about the actual physics?
 
  • #15
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We can look at this wikipedia article
https://en.wikipedia.org/wiki/Event_(particle_physics)
it seems correct.
This is a very specific definition of "event" used by particle physicists. It is not, and is not intended to be, a general definition of "quantum event", which is what would be relevant to this discussion.

What does quantum mechanics says about not observed properties? can we talk about them?
What is a "not observed property"?

It might help if, instead of using vague ordinary language, you would give a reference to a textbook or peer-reviewed paper that gives precise definitions of the concepts you are interested in.
 
  • #16
Demystifier
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So we are not far from answering yes to the question in the title?
Yes. Decoherence is a necessary (but not sufficient) ingredient of a measurement.
 
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  • #17
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I do not know if it is exactly the Wigner's friend problem.
was decoherence process known then?
Not in the way we understand it today, and not under this word, but the essential ideas of decoherence were already implicit in the von Neumann theory of quantum measurements from 1932.

See also my https://arxiv.org/abs/1406.3221 to see how the same ideas of quantum measurement are described without and with the explicit notion of decoherence. The paper is about Schrodinger cat, but the conclusion briefly discusses the relevance to the Wigner friend paradox. In fact, the cat and the Wigner friend are essentially the same thing.
 
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  • #18
Lord Jestocost
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At the end, decoherence boils down mathematically to the idea of adding one more link to the quantum-mechanical von Neumann measurement chain.
 
  • #19
Heidi
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thank you for this link. There a many interpretaions and the Bohmian one is successful. i learned it through the 2 splits experiment and it was stunning.
i am fond of Heisenberg who said to speak only of what the appartus measure eg not the jumps in an atom but the absorption or emission lines.
i like the relational quantum interpretation in which properties live in particular physical interactions. and it seems that decoherence is necessary to these interactions
 
  • #20
Morbert
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We are accustomed to consider linear superpositions of states like "dead" and "alive"
but id "dead" is a state the death of the cat is an event. i wonder if we can have superposition of events.
Before worrying about decoherence, it's better to understand what quantum theories say about events* more fundamentally.

Quantum theories won't tell you whether or not an event "really" occurred, regardless of decoherence, without some further interpretational commitment. Instead, quantum theories will assign probabilities to events, and offer logical inferences/implications** concerning events.

E.g. Quantum mechanics won't tell you whether the particle in the box really decayed or not, or whether the cat is really alive or not, but quantum mechanics will let you construct implications between the decay of the particle, the health of the cat, and the appearance of the cat when you open the box. What ontological commitments you make is up to you.

It is common for physicists to assume macroscopic events like "the detector produced datum X" are really occurring, while microscopic events like "this particle has property Q" are at least useful propositions for understanding how their detectors will respond to sources. Quantum theories are fine with this so long as the reasoning about events is consistent.

* I am taking "event" to mean an element of an event algebra.
** The Boolean algebra formed from the event algebra
 
  • #21
Heidi
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Yes. Decoherence is a necessary (but not sufficient) ingredient of a measurement.
I resume:
for any decoherence of a density matrix (non unitary process) we may add to the system new degrees of freedom so that the new bigger system evolve unitarily and tha partial trace on its density matrix give back the "little" decohered density matrix
"one measurement result" => "there was decoherence" is equivalent to
"there was not decoherence" => "no measurement result"
so in the bigger system there is no "collapse" no unique result
Here Wigner and his friend do not see the same thing.
Of course if a measurement is done on the bigger system about the same property it will give the same unique result.
And it is HERE that the different interpretations give different explanations (why we have the same result).
 
  • #22
Demystifier
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I resume:
for any decoherence of a density matrix (non unitary process) we may add to the system new degrees of freedom so that the new bigger system evolve unitarily and tha partial trace on its density matrix give back the "little" decohered density matrix
"one measurement result" => "there was decoherence" is equivalent to
"there was not decoherence" => "no measurement result"
so in the bigger system there is no "collapse" no unique result
Here Wigner and his friend do not see the same thing.
Of course if a measurement is done on the bigger system about the same property it will give the same unique result.
And it is HERE that the different interpretations give different explanations (why we have the same result).
Almost, but not quite. In some interpretations, those that violate the consistency principle C in the Frauchiger-Renner paper, Wigner and his friend may disagree on the result of measurement. Examples of such interpretations are QBism (measurement outcomes are subjective experiences of an agent) and relational interpretation (reality is not absolute, but relative to something else). But the disagreement is a result of interpretation (Wigner thinks within a paradigm of a given interpretation and concludes that his friend didn't get the result that the friend claims that he obtained), not of a direct comparison of measurement outcomes (Wigner cannot directly see what was the outcome seen by his friend), so there is no any direct contradiction in the experimental data.
 
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  • #23
Heidi
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In Rovelli relational interpretation, Wigner and his friend measure independently the same thing on a particle. there are two interactions giving two measurement results that may be different.
he writes then that to compare these results (that will be identical) they have to interact together and that the equality comes from this interaction.
Each one keeps his result in his memory before interacting with the other.
Do you know how do Rovelli solve this problem?
 
  • #24
Heidi
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As Wigner and his friend do measurement on the "same" particle at close but different times, a continuous evolution of the particle between t and t + delta t depends of the theories.
this can be avoiding in considering that at t and t + delta t we have two particles maximally entangles
Wigner and his friend share these two maximally entangled particles just like Bob ans Alice in the famous entanglement experiment. in their experiment the measured particles have a space like distance . here they are time like close but it has the same result:
if they compare the result of their measurement they are the same;
it is a kind of entanglement swapping between their apparatus and the entangled particles.
Of course it is only my way to see how relational quantum interpretation could avoid a paradox
 
  • #25
Morbert
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The probability that wigner observes result a, conditioned on his friend observing a different result a', is the conditional probability p(a|a') = p(a∩a')/p(a). If you compute this with quantum theory, p(a|a') = 0.
 
  • #26
Heidi
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it is true but it remained to explain why. and what was implicitly admmitted.
i recall that what was concerned was the relational quantum mechanics in value a is not attached to
the particle after the measurement but only valid for the Wigner+particle relational interaction
 

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