Question around 'quantum object hits screen'

[timmdeeg]

Let's assume, a C60 bucky ball passes the double slit. Would you agree that thereafter in principle the bucky ball should be detectable on the screen e.g. using an atomic force microscope?
And is the transition from quantum to classical state due to instantaneous decoherence?

In contrast Zeilinger investigated decoherence as a slow process by heating bucky balls gradually, whereby the interference pattern was smeared out increasingly.

Now supposed we shoot cold and hot bucky balls against a screen. How would you describe their state before and after their arrival at the screen. What does it mean that the hot bucky ball has lost its coherent state before arriving at the screen? How can this make sense at all?
Or in short what makes the main difference of hot vs. cold in this case?

 

[hutchphd]

References??

 

[vanhees71]

https://www.nature.com/articles/44348
https://www.nature.com/articles/nature02276

 

[timmdeeg]

Hot Buckyballs Lose Quantum Coherence​

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[Demystifier]

Now supposed we shoot cold and hot bucky balls against a screen. How would you describe their state before and after their arrival at the screen. What does it mean that the hot bucky ball has lost its coherent state before arriving at the screen? How can this make sense at all?
Or in short what makes the main difference of hot vs. cold in this case?

It's very simple if you know what's decoherence in general. The hot buckyball emits radiation, so it's entangled with its own radiation, so buckyball alone is not in a pure state. The cold buckyball does not emit radiation, so it has nothing to be entangled with, so it's in a pure state. It has nothing to do with the screen, the coherence is lost by the emission of radiation.

 

[timmdeeg]

The hot buckyball emits radiation, so it's entangled with its own radiation, so buckyball alone is not in a pure state.

So can one say the cold buckyball being a quantum object has no definit path before measurement whereas the hot buckyball after decoherence is a classical object which arrives at the screen on a definite path? And after "arrival" at the screen both cases are indistinguishable.

Would the conclusion be that the first case causes the so-called "measurement problem" but the second case doesn't in this sense because the measurement happens due to interaction with the environment. Does the difference concern the time-scale only, instantaneous v. gradually?

 

[Demystifier]

So can one say the cold buckyball being a quantum object has no definit path before measurement whereas the hot buckyball after decoherence is a classical object which arrives at the screen on a definite path? And after "arrival" at the screen both cases are indistinguishable.

Would the conclusion be that the first case causes the so-called "measurement problem" but the second case doesn't in this sense because the measurement happens due to interaction with the environment. Does the difference concern the time-scale only, instantaneous v. gradually?

Answering your questions would require a more detailed discussion of decoherence in general. Since such questions are often asked, I have prepared a lecture: http://thphys.irb.hr/wiki/main/images/5/50/QFound3.pdf

 

[timmdeeg]

Answering your questions would require a more detailed discussion of decoherence in general. Since such questions are often asked, I have prepared a lecture: http://thphys.irb.hr/wiki/main/images/5/50/QFound3.pdf

Thanks!

Page 14: Quantum measurement: Measurement is associated with (almost) full decoherence.

Page 21: Decoherence eliminates certain coherent superpositions, but it still contains incoherent superpositions.

From that it seems - but I'm not sure - that the state of the hot buckyball after decoherence "still contains incoherent superpositions" and as such differs from the state after quantum measurement. If so, the hot buckyball can not be viewed as a classical particle.

If correct decoherence occurs in two stages in this case, first "partial" decoherence due to interaction with environment, second (almost) full decoherence by quantum measurement.

Please correct where necessary.

 

[Demystifier]

If correct decoherence occurs in two stages in this case, first "partial" decoherence due to interaction with environment, second (almost) full decoherence by quantum measurement.

Please correct where necessary.

No, no, there are no two stages. Decoherence is always partial, i.e. never full. But after a sufficient interaction with the environment, it can be almost full. Measuring apparatus is just an example of environment, with which the decoherence is almost full. Decoherence by itself is not sufficient to explain how the quantum object (e.g. buckyball) becomes classical. To explain classicality one must also specify an interpretation of quantum mechanics, which is always a bit controversial.

 

[timmdeeg]

No, no, there are no two stages. Decoherence is always partial, i.e. never full. But after a sufficient interaction with the environment, it can be almost full.

Ok, thanks for clarifying this point. So, how would you describe the state of the C60 ball after interaction with the environment (i)? Is it a classical state comparable to the state of a cold C60 ball after measurement? Then I would expect the C60 ball (i) has definite path before arriving at the screen.

 

[Demystifier]

Ok, thanks for clarifying this point. So, how would you describe the state of the C60 ball after interaction with the environment (i)? Is it a classical state comparable to the state of a cold C60 ball after measurement? Then I would expect the C60 ball (i) has definite path before arriving at the screen.

Yes, you can describe it that way. I say "can" because it depends on the quantum interpretation (Copenhagen, Bohm, many-worlds, ...), but certainly there are interpretations in which such a description is correct. The interpretation independent fact is that hot C60's don't show interference fringes on the screen, that's what we measure. Whether the C60 has a definite classical path or not before measurement is not what we determine by measurement, and the minimal quantum theory does not say, that's why we have various interpretations.

 

[timmdeeg]

Whether the C60 has a definite classical path or not before measurement is not what we determine by measurement, and the minimal quantum theory does not say, that's why we have various interpretations.

Ah, I see. So the cold C60 has no classical path before measurement which is not interpretation dependent, but one can't conclude from this conversely that the hot C60 (after almost full decoherence) has a classical path, because that is interpretation dependent.
Thanks for your explanations!

 

[Demystifier]

Ah, I see. So the cold C60 has no classical path before measurement which is not interpretation dependent, but one can't conclude from this conversely that the hot C60 (after almost full decoherence) has a classical path, because that is interpretation dependent.
Thanks for your explanations!

Sort of, but there are interpretations (the Bohmian one with particle ontology) in which even a cold C60 has a path, though not exactly a classical one.

 

[timmdeeg]

Sort of, but there are interpretations (the Bohmian one with particle ontology) in which even a cold C60 has a path, though not exactly a classical one.

Shouldn't cold C60 - having a path - produce a diffraction pattern in the double slit experiment? As we know this isn't the case, which in this respect seems to mean "not exactly a classical one".

I have been reading about the Bohmian interpretation, but are not in the details. It is probably too simple to say well if measurement can disprove a path then it hasn't a path but otherwise it has.

It would be great if you could clarify this subtlety a bit in "normal" language.

 

[Demystifier]

Shouldn't cold C60 - having a path - produce a diffraction pattern in the double slit experiment? As we know this isn't the case, which in this respect seems to mean "not exactly a classical one".

I have been reading about the Bohmian interpretation, but are not in the details. It is probably too simple to say well if measurement can disprove a path then it hasn't a path but otherwise it has.

It would be great if you could clarify this subtlety a bit in "normal" language.

Explaining Bohmian mechanics properly would be a separate thread. See e.g. https://arxiv.org/abs/quant-ph/0611032

A very short summary would be that the particle is a pointlike object guided by the wave, so the particle has a well defined position at any time, even if we don't measure it.

 

[physika]

Decoherence is always partial, i.e. never full.

because it cannot explain which of the available pointer states will be observed as the single outcome ?

.

 

[Demystifier]

because it cannot explain which of the available pointer states will be observed as the single outcome ?

No, it's because the realistic wave functions always have small but nonzero (typically exponentially decreasing) tails.