# B Quantum Darwinism question

1. Oct 18, 2016

### cube137

Quantum Darwinism is about fragments and how different observers should observe the same thing, may I know where the born rule is applied. does the proper mixed state (or one outcome) occurs after the fragments leave the objects, halfway or when the observers received the fragments?

2. Oct 18, 2016

### Staff: Mentor

3. Oct 18, 2016

### cube137

4. Oct 19, 2016

### Staff: Mentor

As far as I can tell, it isn't; the Zurek paper derives all of its key conclusions without ever using the Born rule. The paper also stresses that it is "interpretation free"; there is no claim that collapse occurs or that it doesn't occur. That question is simply not addressed.

5. Oct 19, 2016

### cube137

The paper quoted below (I'd like to understand what it means exactly by "Quantum states acquire objective existence when reproduced in many copies", I hope seasoned quantumists like Stevendaryl or Simon Phoenix can comment. Peterdonis is a relativist. And also what it means "Consensus between records deposited in fragments of E looks like collapse"):

Last edited by a moderator: Oct 19, 2016
6. Oct 19, 2016

### Staff: Mentor

First you have to understand what he means by "objective existence". Basically, it looks to me like he means "repeatability": repeated measurements of a system must give consistent results. The "many copies" is what allows that to happen: each measurement basically takes its information from a different copy. And each measurement transfers the information in the copy to the measuring device, so each copy can only be used once to obtain information about the measured system--that's why you need many copies to have repeated measurements give consistent results.

I think the key here is "looks like collapse". What he is trying to explain is why, regardless of whether there actually is collapse or not, our observations make it look like there is collapse--where here "look like collapse" basically means the same thing as "objective existence" above, i.e., repeated measurements giving consistent results. And, as above, what allows that to happen is that there are many "records" (copies), each one in a different fragment of E (the environment), and all of them containing the same information about the measured system.

Again, this is all "interpretation free", as the paper says. It doesn't say that collapse "really" happens; it leaves that question open. It just explains why it looks that way to us.

Just to clarify, I do post most often in the relativity forum, but I'm reasonably conversant with QM and QFT. The main reason I don't post more often in this forum is that it seems far more prone to threads that are more about philosophy than physics (QM is funny that way). But this thread topic does not fall into that category.

7. Oct 20, 2016

### cube137

Ok. To understand what Zurek was driving at. Supposed there was not "many copies".. but only one copy, what would happen?

This is how I understood Zurek. In the second paragraph he wrote:

"Fragility of states is the second problem with quantumclassical
correspondence: Upon measurement, a general
preexisting quantum state is erased - it "collapses" into
an eigenstate of the measured observable. How is it then
possible that objects we deal with can be safely observed,
even though their basic building blocks are quantum?"

Can we take as example a macroscopic object like a piece of wood. Is this what Zurek refer to as objects where their basic building blocks are quantum? Or was Zurek referring to object as a small isolated quantum system like a buckyball or a particle? Without the "many copies".. did Zurek mean if different people interact with the same wood, they can re-prepare it and the wood can change shape? Or was he referring only to atoms or tiny isolated quantum systems where people can re-prepare it by measuring it? But I don't think he was referring only to electrons, atoms, or other microscopic systems but big objects like wood too. Is that right?

8. Oct 20, 2016

### secur

"Preferred states of S become objective" when "many observers find out the state of S independently". So the answer to your question is the third option: "when the observers received the fragments". The outcome occurs when multiple observers receive consensus information about the state. However, it can occur without humans: i.e. the environment itself can "witness" those multiple fragments.

The fragments concept is not very important in this work, neither is the "Darwinian" ansatz. The key concepts are pointer states, einselection, envariance, equiprobability (symmetry, swap), finegraining (using ancilla C) and a couple others. Some of it's pretty clever but it's not "interpretation-free". Quantum Darwinism is a flavor of MWI.

More or less nothing. Only one copy would not be enough to allow objective outcome.

Yes.

No. "Re-prepare" applies only to quantum objects.

9. Oct 20, 2016

### cube137

You agreed that a piece of wood is what Zurek refers to as objects where their basic building blocks are quantum. Hence wood can be considered as quantum objects in quantum Darwinism... hence you can re-prepare a piece of wood in the world of Zurek. But then, maybe let's refer to "Re-prepare" for electrons, photons, and small quantum system, while reserved the word "Transmute" for changing the shape of the wood. If you won't agree. But Zurek just said macroscopic object can considered as quantum object too like buckyball. Remember in the world of quantum decoherence, everything is quantum. Our classical bias only came because of Copenhagen and because of Everett where each branch is a solid classical world hence trapping one to the mindset that there is a classical world when everything is quantum at the core.

10. Oct 20, 2016

### Staff: Mentor

If there were only one copy, you would not be observing a macroscopic object and you would not be getting measurement results of the sort we usually associate with such objects. You would be looking at, say, a quantum computing experiment where a single qubit is being manipulated. And the results you get would not look like something with "objective existence" in the sense Zurek is using that term.

All objects have basic building blocks that are quantum.

No. Zurek is saying that it is impossible for a piece of wood not to have "many copies" of the information about it in the environment, because it is composed of such a huge number of quantum building blocks and it is not at all isolated--each building block is continually interacting with other building blocks and with the environment. To be able to "re-prepare" a piece of wood, you would have to first isolate it from its environment, and then control all the interactions between its quantum building blocks with sufficient accuracy to allow quantum interference effects to be observable. In principle that could be done, but in practice it is impossible.

The reason we can "re-prepare" atoms or tiny isolated quantum systems (like qubits in a quantum computer) by measuring them is that they are tiny isolated quantum systems; we can prevent them from interacting with their environment, and they contain a small enough number of quantum building blocks (just one, in the ideal case) that we can control all their interactions.

11. Oct 20, 2016

### Staff: Mentor

I think this is misstated. A correct statement would be that re-preparing is only possible in practical terms for objects containing a small number of quantum building blocks which can be isolated sufficiently from the environment that all of their interactions can be controlled. But there is no magic boundary at which objects stop being quantum. All objects are composed of quantum building blocks. It's just that not all objects meet the practical conditions for us being able to re-prepare them.

12. Oct 20, 2016

### Staff: Mentor

Not in practical terms. See my previous posts.

13. Oct 20, 2016

### secur

To back up a little, QD attempts to answer the question why large-scale quantum systems - for instance a block of wood - present one stable state. I.e., why we cannot, in practice, "transmute" or re-prepare them. The answer: decoherence via interaction with the environment causes einselection of pointer states, wherein all phase information is lost. Once that happens we obtain "objective reality". All subsequent measurements will give the same state. You can no longer re-prepare that block of wood, as you can with small quantum objects such as an electron.

Obviously this is a practical program. QD tries to explain why, in practice, a block of wood can't be re-prepared, even though in theory it can. Here's a relevant quote:

Note, he never defines the term "re-prepare" - anywhere on the net, that I could find. We're all supposing it means a second change, after the initial "collapse". QD usually uses it in that sense. Also he doesn't define "quantum object" but it seems to mean the wavefunction itself. I was using it to mean "small objects"; that might not be exactly consistent with Zurek. Fortunately you both understood what I meant.

Note, I'm only answering the question, "What does QD say?" I don't necessarily agree with it.

This is, indeed, typical "quantum philosophy", contrary to your statement above. It can easily lead to picking terminological nits, attempting to mind-read Zurek, and - in a word - the endless philosophical debate of QM interpretation. So, if you still disagree with my take on "re-prepare", I'll happily concede the point.

OTOH QD is pretty interesting, and I'm up to speed on it now. So that topic is worth pursuing.

14. Oct 20, 2016

### Staff: Mentor

Yes, small quantum objects. (Which, if unpacked, basically means what I said before, objects made of a small enough number of quantum building blocks that they can be isolated and their interactions controlled.) But your previous post that I objected to didn't include the qualifier "small", which is why I objected and why it appears to have confused the OP.

Or, in terms that the OP seems more comfortable with, it tries to explain why, in practice, only small quantum objects can be re-prepared, even though all objects are quantum objects and in theory any quantum object can be re-prepared.

15. Oct 20, 2016

### cube137

Supposed for sake of discussion there were really only one copy or not even a single copy. So what would the piece of wood look like? (please don't use example of single qubit but a big object like wood). Would the wood become invisible or became a blob in superposition? Or what should the piece of wood look like (just a rough description if you can't describe completely)?

16. Oct 20, 2016

### Staff: Mentor

In order to do this, you would have to completely isolate the piece of wood from its environment. That is, you would have to somehow prevent all interactions between every single quantum building block in the piece of wood and anything else. In practice that is impossible, by many, many orders of magnitude.

But suppose you could, in fact, do the above. You would still have to deal with the fact that the piece of wood is composed of a huge number of quantum building blocks, something like $10^{25}$ of them, and they are interacting with each other, and you can't control the interactions, which means you can't run controlled experiments on the piece of wood to test its quantum state, the way you can for small quantum systems like qubits. Nor can we possibly model the detailed interactions between all the building blocks mathematically.

So I don't think anyone knows what such a piece of wood would look like, nor does it matter since we can't make one anyway.

17. Oct 20, 2016

### cube137

If that's true. Then the "objects" he meant in the following are only quantum objects like electrons, photons, and small quantum system and not a block of wood or larger object where it can't be isolated. Correct? But the way he writes it.. it sounds like any objects like cars.

"Fragility of states is the second problem with quantumclassical
correspondence: Upon measurement, a general
preexisting quantum state is erased - it "collapses" into
an eigenstate of the measured observable. How is it then
possible that objects we deal with can be safely observed,
even though their basic building blocks are quantum?"

In the following article. http://www.nature.com/news/2004/041223/full/news041220-12.html They emphasized the "objects" meant macroscopic object like the Buckingham palace:

"If it wasn't for quantum darwinism, the researchers suggest in Physical Review Letters1, the world would be very unpredictable: different people might see very different versions of it. Life itself would then be hard to conduct, because we would not be able to obtain reliable information about our surroundings... it would typically conflict with what others were experiencing.

The difficulty arises because directly finding out something about a quantum system by making a measurement inevitably disturbs it. "After a measurement," say Wojciech Zurek at Los Alamos National Laboratory in New Mexico and his colleagues, "the state will be what the observer finds out it is, but not, in general, what it was before."

Because, as Zurek says, "the Universe is quantum to the core," this property seems to undermine the notion of an objective reality. In this type of situation, every tourist who gazed at Buckingham Palace would change the arrangement of the building's windows, say, merely by the act of looking, so that subsequent tourists would see something slightly different.

So this is very bad example? They should not use example of macroscopic object like Buckingham Palace! But then.. for other mentors.. in case there is a chance PeterDonis may have misunderstood the issue. Please let us know the case. Thanks.

18. Oct 20, 2016

### Staff: Mentor

No. All objects are quantum objects, in the sense that they are composed of quantum building blocks. But the number of building blocks in the object makes a difference. An object with only 1 building block, like an electron, is very different from an object with $10^{25}$ building blocks, like a piece of wood. Just because everything is a quantum object doesn't mean everything has to behave exactly the same.

First of all, this isn't strictly true. If the system is already in an eigenstate of the observable being measured, then the measurement doesn't change its state. But that's not really a practical issue, because if we already know the system is in an eigenstate, we don't need to measure it anyway because we already know its state.

However, once again, the size of the disturbance relative to the size of the object matters. If you are measuring an object that has only one quantum building block, like an electron, any measurement you make is going to disturb it significantly--heuristically, because the measurement itself has a minimum size which is basically one quantum building block. (For example, if we try to measure the electron by bouncing photons off of it, the minimum measurement we can make is to use one photon.) But if you are measuring an object with $10^{25}$ building blocks, like a piece of wood, there are lots of ways to measure it without significantly affecting its state, simply because of the huge number of building blocks. In fact, measuring an object of that size is really no different from what its environment is continually doing to it anyway--which is part of Zurek's point. The reason macroscopic objects like pieces of wood or Buckingham Palace look the same to everybody is that none of us have to do anything special to measure them; they're already being measured, all the time, just by being embedded in their environment. All Zurek is doing is giving more details about how that works and why it privileges particular states, the ones we think of as "classical" states and are intuitively familiar with.

No. See above.

19. Oct 20, 2016

### cube137

Yes. That's what standard decoherence does.. constantly interacting with objects and environment where they are entangled. And the improper mixed state becomes proper mixed state. We accept the proper mixed state which could be Bohmian, branches in Many Worlds, etc.

What Zurek was trying to do (please confirm my understanding) was like trying to derive how improper mixed state becomes proper mixed state. In the objects, they are in improper mixed state with environment. Then decoherence chose the pointer states and there are many fragments in the environment to make up classical world. In Copenhagen, Bohmian, Many worlds.. Classical world is a priori, observation is the primitive so you don't need quantum Darwinism.. but in Zurek quantum Darwinism, quantum state is the primitive. So far my analysis is correct (let's now use the natural language of Density Matrix)? So by this context if there is a way to block the many fragments (just theoretically), then the object would become invisible (because you can't perceive superposition and the eigenstate information flow is blocked). This is just for sake of illustration although I know there is no way to block the fragments.

20. Oct 20, 2016

### Staff: Mentor

Not quite. See below.

I don't think this is correct. Decoherence explains why we can neglect interference terms between different "classical" alternatives, but that means going from a superposition to a mixed state, not from an improper mixed state to a proper mixed state.

No. What Zurek was trying to do is to explain why, when decoherence happens, it always ends up putting systems in mixed states with "classical" alternatives--live cats or dead cats, blocks of wood over here or over there, Buckingham Palace built vs. not built, etc.--instead of mixed states that don't look anything like the classical states we observe--for example, a mixture of ("block of wood here" plus "block of wood there") and ("block of wood here" minus "block of wood there"), which is a perfectly valid mixed state, but not one that describes any block of wood we've ever observed.

In other words, according to Zurek, it's not enough to explain, as standard decoherence theory does, why macroscopic systems end up in mixed states, with interference terms negligible. You have to also explain why they end up in the particular mixed states we actually observe, and never all the other possible mixed states that are possible mathematically. As I understand it, that is what Quantum Darwinism tries to do.