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I Entanglement Swapping and measurement vs. observation

  1. Aug 19, 2015 #1
    I think entanglement swapping shows the difference between measurement and observation. A measurement isn't as important as observation at least to subatomic particles.

    Here's some key points

    http://arstechnica.com/science/2012...cts-results-of-measurements-taken-beforehand/

    Two things immediately jump out.

    1. How do the particles know that Alice and Bob haven't made a choice to observe or share information?

    2. Why do the particles correlate themselves with Victor's choice even after the fact?

    The key point is Alice and Bob haven't made a choice to observe which measurement has occurred so the particles correlate themselves with Victor's observation.

    Why doesn't measurement occur independent of Victor's choice?

    I think conscious knowledge of the system is important on a fundamental level and we also see this in the delayed choice quantum eraser experiment.

    A particle in that experiment at D0 behaves like a particle or an interference pattern based on whether information about it's entangled pair can be known or not after the particle has already hit D0.

    Why does observation matter to subatomic particles?

    When you look at entanglement swapping the particles are like Schrodinger's cat until Victor makes a choice even after Alice and Bob's particles have been measured. If Alice and Bob don't observe and share information about what measurement has occurred the particles going to Alice and Bob correlate themselves with Victor's observation.

    Without a conscious observer knowing which state the particle will be in how could our universe exist? In other words, our universe formed because on a subatomic level they knew we would be here to observe there states like Victor.
     
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  3. Aug 19, 2015 #2

    DrChinese

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    This is a complicated setup, and is certainly subject to the usual quantum interpretations.

    1. They don't. In the delayed choice version, they are in fact already measured before their partners arrive at Victor.

    2. The choices of Alice, Bob AND Victor are all part of the quantum "context". The ordering is irrelevant to the observed statistics, which depend on the overall context.

    There is no known sense in which consciousness is relevant. That is pure speculation on your part at this point.
     
  4. Aug 19, 2015 #3
    1. Of course they do and this is one of the points of the whole set up. This is the reason you don't have Alice and Bob sharing information or knowing the outcome of their measurements until after Victor has made his choice to entangle/not entangle. How do the particles know that Alice and Bob haven't made the choice to share information about which measurement has occurred? The measurement with Alice and Bob should be independent of Victor's choice because Victor's choice occurs after the fact that Bob and Alice's particles have been measured.

    2. Of course the ordering is irrelevant and that's the point!! Observation is more important than measurement at least to particles who seem to correlate themselves based on observed/not observed. This places more importance on a conscious observer knowing which state the particle is in vs. the state being measured. When Alice and Bob make the choice to observe the state their particles are in that state will depend on Victor's future choice but when you break it down, past, present and future lose there meaning but observation doesn't.
     
  5. Aug 19, 2015 #4

    DrChinese

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    1. We're talking about different things, it appears. Observers Alice and Bob can be well separated (outside each others' light cones) and the results do not vary. They can agree to what settings to use or not, that doesn't matter either.

    2. The particles measured by Alice and Bob are correlated if Victor swaps entanglement via their partners. The key is that entanglement swapping can occur after Alice and Bob make their observations, and there is still correlation. Also:

    a. It is impossible to comprehend what you mean as a difference between the words "observe" and "measure". They are normally used interchangeably on this forum.

    b. There is no known sense in which the conscious mind plays a part in quantum mechanics.
     
  6. Aug 19, 2015 #5
    This is wrong and this goes to my point. Alice and Bob don't make observations until after Victor has made his choice. Like I said, this is key. If Alice and Bob made an observation before Victor's choice then Victor's choice wouldn't matter. This is because Alice and Bob's observation would already be part of their environment and there's nothing Victor can do to change what Bob and Alice observed. Here's more from the actual paper of the experiment.

    In our experiment, the primary events are the polarization measurements of photons 1 and 4 by Alice and Bob. They keep their data sets for future evaluation. Each of these data sets by itself and their correlations are completely random and show no structure whatsoever. The other two photons (photons 2 and 3) are delayed until after Alice and Bob’s measurements, and sent to Victor for measurement. His measurement then decides the context and determines the interpretation of Alice and Bob’s data.

    Bingo

    Alice and Bob's data should be independent of Victor's choice. Why should it matter to the particles that went to Bob and Alice if Victor will choose to entangle/not entangle? The reason this matters is because Bob and Alice Don't observe their results. It goes onto say:

    In the entanglement swapping 1-3 procedure, two pairs of entangled photons are produced, and one photon from each pair is sent to Victor. The two other photons from each pair are sent to Alice and Bob, respectively. If Victor projects his two photons onto an entangled state, Alice’s and Bob’s photons are entangled although they have never interacted or shared any common past. What might be considered as even more puzzling is Peres’ idea of “delayed-choice for entanglement swapping”4,5 . In this gedanken experiment, Victor is free to choose either to project his two photons onto an entangled state and thus project Alice’s and Bob’s photons onto an entangled state, or to measure them individually and then project Alice’s and Bob’s photons onto a separable state. If Alice and Bob measure their photons’ polarization states before Victor makes his choice and projects his two photons either onto an entangled state or onto a separable state, it implies that whether their two photons are entangled (showing quantum correlations) or separable (showing classical correlations) can be defined after they have been measured.

    Alice and Bob's have been measured but not observed. There's no interaction between Alice and Bob's particles after they have been measured. This is why the future evaluation of Alice and Bob's particles of correlation/separable are determined by Victor's future choice. If measurement was the determing factor then Victor's choice would be meaningless.
     
  7. Aug 19, 2015 #6
    There is an interesting article by Reinhold Bertlmann entitled "Time-ordering dependence of measurements in teleportation" (http://link.springer.com/article/10.1140/epjd/e2013-30647-y), and I quote from the first page of the article:
    Reference [1] refers to the delayed-choice entanglement swapping experiment performed by Zeilinger et al., published in 2012.
     
  8. Aug 19, 2015 #7
    Very good point.

    This is exactly what I'm saying. If Alice and Bob's measurement is observed by Alice and Bob then Victor's choice is rendered meaningless to Alice and Bob.

    I have always said if you had a hypothetical time machine you can change the past only if it hasn't been observed yet. So if hypothetical time travelers saw that Romney won the election while looking at our timeline, they can go back further in time to change the future and elect Obama as long as the event hasn't occurred and been observed yet by us.

    This is similar because Alice and Bob's particles haven't been observed yet therefore Victor can collapse the quantum state and swap entanglement with his choice. He can't do this if Alice and Bob's particles communicate.

    So it's like they're in one universe where they all exist together unless Alice and Bob's particles interact then Victor's particles will be classically separated from Alice and Bob's particles. It's like Alice and Bob's particles are in another universe.
     
  9. Aug 19, 2015 #8

    DrChinese

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    We are discussing several different ideas. We should probably get agreement on the setup, from your reference:
    1. Two independent sources (labeled I and II) produce pairs photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. (I'm not sure why the third party is named "Victor".)
    2. Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing.
    3. At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons.
    As I said, Alice and Bob measure before Victor. And they can be sufficiently separated so that their photons have never existed within a common light cone. Ergo they have no causal influence on one another, unless such is FTL. Of course, Alice and Bob DO observe their results. They are correlated if Victor later decides to entangle them. Otherwise, they are not correlated. This is the point of the experiment, that a correlation is made to occur after particles are already detected and the outcomes recorded/

    I repeat that there is no meaning on this forum to any appreciable difference between the words observe and measure. Further, the existence of conscious observers is irrelevant as far as anyone knows. And there is no "other universe" (whatever you mean by that). They only relevant regions of spacetime in this setup are those defined by light cones from the 2 photons sources, labelled I and II.
     
  10. Aug 19, 2015 #9
    If this is the case let's see the quote from the original paper.

    The quantum state hasn't been observed if it was then Victor's decision wouldn't change anything.

    Measurement and the observers knowledge of which measurement occurred are two different things. If there's no knowledge of what measurement occurred then Victor's choice can determine correlation/separable for Alice and Bob's particle.

    Alice and Bob's local measurements are only independent of Victor's choice if Alice and Bob's particles interact.

    Here's a quote from the original paper.

    On the basis of Victor’s measurement settings and results, Alice and Bob can group their earlier and locally totally random results into subsets which each have a different meaning and interpretation. This formation of subsets is independent of the temporal order of the measurements. According to Wheeler, Bohr said: “No elementary phenomenon is a phenomenon until it is a registered phenomenon.”7,8 We would like to extend this by saying: “Some registered phenomena do not have a meaning unless they are put in relationship with other registered phenomena.

    Exactly,

    In other words local measurements are meaningless in isolation of the whole state.

    Why isn't measurement of Alice and Bob's particle independent of Victor's choice? It's because the quantum state which includes particles 1,2,3 and 4 hasn't been observed until Victor makes a choice. If Alice and Bob's particles are observed and knowledge about each state is known, then Victor is blocked off from determining correlation/separable for Alice and Bob's particles.

    This goes back to the question, how do Alice and Bob's particles know that they're part of a system that includes particles 1,2,3 and 4 or they're not? How do they know when Victor has entangled or not entangled?

    When Victor doesn't entangle, particles 1&2 and particles 3&4 remain entangled. When Victor makes the choice to entangle, they lose their correlation and it's swapped.

    WHY DOES VICTOR'S CHOICE HAVE ANY MEANING AFTER ALICE AND BOB'S PARTICLES HAVE BEEN MEASURED?

    The 4 particles have to remain as a single state and the only thing that allows them to remain a single state is there's no communication or interaction between Alice and Bob.
     
  11. Aug 19, 2015 #10

    bhobba

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    Nor is a distinction made in any textbook I am aware of - and I have read quite a few. Observation and measurement are basically synonymous. That said observation is a bad term we are stuck with for historical reasons. Observation in QM does not imply a concious observer despite its usual day to day meaning.

    Thanks
    Bill
     
  12. Aug 19, 2015 #11

    bhobba

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    Choice has nothing to do with anything. It could be done by a machine - it makes no difference. This whole conciousness thing comes from a misinterpretation of what observe means in QM and some ideas from the early days of QM that are now very fringe. They are still about because they cant be disproved, but very few physicists ascribe to it.

    Thanks
    Bill
     
  13. Aug 20, 2015 #12

    zonde

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    You might be missing some details of experiment. Victor's choice does not matter. What matters is Victor's measurement. And based on result of Victor's measurement Alice and Bob's results are postselected in different subsets that show appropriate correlations.
     
  14. Aug 20, 2015 #13
    Of course Victor's choice matters. That's the delayed CHOICE portion of the experiment. Here's a quote directly from the experiment.

    In the entanglement swapping1-3 procedure, two pairs of entangled photons are produced, and one photon from each pair is sent to Victor. The two other photons from each pair are sent to Alice and Bob, respectively. If Victor projects his two photons onto an entangled state, Alice’s and Bob’s photons are entangled although they have never interacted or shared any common past. What might be considered as even more puzzling is Peres’ idea of “delayed-choice for entanglement swapping”4,5 . In this gedanken experiment, Victor is free to choose either to project his two photons onto an entangled state and thus project Alice’s and Bob’s photons onto an entangled state, or to measure them individually and then project Alice’s and Bob’s photons onto a separable state. If Alice and Bob measure their photons’ polarization states before Victor makes his choice and projects his two photons either onto an entangled state or onto a separable state, it implies that whether their two photons are entangled (showing quantum correlations) or separable (showing classical correlations) can be defined after they have been measured.

    Victor is free to choose.

    Whether this choice is controlled by a quantum random number generator or a human brain, it's still a free choice. Victor's CHOICE matters and the experimenters explain why.

    In order to experimentally realize Peres’ gedanken experiment, we place Victor’s choice and measurement in the time-like future of Alice’s and Bob’s measurements, providing a “delayed-choice” configuration in any and all reference frames. This is accomplished by (1) proper optical delays for Victor’s photons and (2) a high-speed tunable bipartite state analyzer, which (3) is controlled in real time by a quantum random number generator (QRNG)6 . Both delay and randomness are needed to avoid the possibility that the photon pairs can “know” in advance which setting will be implemented after they are registered and can behave accordingly by producing results of a definite entangled or a definite separable state. Whether Alice’s and Bob’s photons can be assigned an entangled state or a separable state depends on Victor’s later choice. In Peres’ words: “if we attempt to attribute an objective meaning to the quantum state of a single system, curious paradoxes appear: quantum 2 effects mimic not only instantaneous action-at-a-distance but also, as seen here, influence of future actions on past events, even after these events have been irrevocably recorded.”4

    DEPENDS ON VICTOR'S LATER CHOICE.

    Choice is very important and I would say CHOICE creates reality. Victor's CHOICE to entangle/not entangle created a reality in that environment.

    If I go to the store to buy a bag of Funyuns or a bag of Doritos and I make the choice to buy Funyuns, I then go to the cooler to buy a drink I like with my funyuns, I bump into a High School friend, he invites me to a cookout, I go and meet his sister that's in town, we start dating, get married and have 2 kids.

    My choice to buy funyuns created a reality and history in this environment. I could have made a choice to buy Doritos and went straight to the checkout and never bumped into my HS friend.

    The point is, choice causes a measurement to occur and a history unfolds from that choice. We see the same thing with Victor. He makes a choice to entangle and a history unfolds where Alice and Bob's particles are entangled. He makes a choice not to entangle and a history unfolds where Alice and Bob's particles aren't correlated.

    Choice is very important because Victor makes a choice after Alice and Bob's particles have been measured. He used a quantum random number generator so Victor's choice could be random and therefore Bob and Alice's particles couldn't "know" Victor's choice. So it comes down to the knowledge the observer has about Alice and Bob's particles. If Alice and Bob have no communication then their particles correlate themselves with Victor's choice.
     
  15. Aug 20, 2015 #14
    This means a human's choice is the same as a machine's choice , and points to us humans having no free choice. I mean how much of a free choice can a machine have...
     
  16. Aug 20, 2015 #15
    It's two different things at least in this experiment. The choice was random and controlled by a quantum random number generator.

    Humans can make a random choice by flipping a shuffled deck of cards. Humans can also make choices based on information so in that sense it's different from a machine. A human can make the choice to flip over the cards and pause at every third card. So the choice is non random based on the knowledge the human has about pausing every three cards.

    This can be a human making a choice to invest in stocks using the Kelly Criterion. The choice is non random based on the persons knowledge of the Kelly Criterion.
     
  17. Aug 20, 2015 #16

    bhobba

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    Machines don't choose by the definition of choice. Not everything in QM is like observation that has a meaning different to the usual use.

    Thanks
    Bill
     
    Last edited: Aug 20, 2015
  18. Aug 20, 2015 #17

    bhobba

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    Physics is not a semantic word game. Machines do not choose. I suggest you think about why you want to use that word.

    If you say - Why does the independent entangling or not entangling by Victor have any meaning after Alice's and Bob's particles have been measured - what's going on is much clearer and less loaded with the semantic baggage of 'choice' - which in usual use means a decision by a rational agent.

    This whole business has to do with the mathematics of entanglement.

    Consider the entangled state 1/√2 |a>|b> + 1/√2 |b>|a>. If system 1 is observed in state |a> then system two cant be in state |a> because only |a>|b> or |b>|a> is allowed by entanglement. This means the other system must be in state |b>. It analogous to Bertlmann's socks:
    http://cds.cern.ch/record/142461/files/198009299.pdf

    The difference is, as Bell showed, is its different to classical correlations, but it still just a correlation.

    In general all delayed choice experiments do is show in simple cases decoherence can be unscrambled - there is nothing mysterious about if the observation made later unscrambles it or not. Its only on the surface a mysterious phenomena. But when you understand all that's going on is if the decoherence is unscrambled - its actually trivial.

    If people want to understand in a particular set-up how it gets unscambled then go ahead - do a post about that. But that's what's going on.

    As seen from the above here its very simple. It doesn't even require invoking undoing decoherence - its entirely due to if Victor entangles them or not.

    Normally I wouldn't care about such semantic quibbling - if you use choice or whatever is not really germane - the issue comes when you want to use it to draw conclusions like the above. Replacing Victor etc with machines and avoiding loaded words like choice clearly shows such can not be concluded.

    Thanks
    Bill
     
    Last edited: Aug 20, 2015
  19. Aug 20, 2015 #18

    zonde

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    There is another quote from the same paper:

    However, there is never a paradox if the quantum state is viewed as no more than a ‘catalogue of our knowledge’2. Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observers’ events is irrelevant and no physical interactions whatsoever between these events, especially into the past, are necessary to explain the delayed-choice entanglement swapping. What, however, is important is to relate the lists of Alice, Bob and Victor’s measurement results. On the basis of Victor’s measurement settings and results, Alice and Bob can group their earlier and locally totally random results into subsets that each have a different meaning and interpretation. This formation of subsets is independent of the temporal order of the measurements.
     
  20. Aug 20, 2015 #19

    DrChinese

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    This example does not serve as an analogy to the experiment at hand. Please note that NONE of the following make any difference to the experimental statistical results:

    1. Whether Alice, Bob, or Victor are conscious or are robots.
    2. Whether Alice or Bob are close enough to communicate or not.
    3. Whether the Victor entangles before or after Alice and Bob make their observations.
    4. Whether Alice makes her observation after Victor entangles, and Victor makes his decision after Bob observer.
    5. Whether the photons Alice and Bob observe ever even co-existed.

    The only thing that matters is the overall context, which is not limited to a classical light cone and is not constrained by classical causality (as occurs in your example). Yes, Victor's choice matters. But it can be made at any time. In your example, to be a proper analogy: you could get married to your friend's sister and later re-connect with your friend and get introduced to her.

    Note to self: stay away from the funyuns. :)
     
  21. Aug 20, 2015 #20
    The analogy was a response to people who act like Victor's choice didn't matter and it's obvious that it does. The Author's of the paper make that clear.I was then showing how choice creates reality on a quantum (Victor) or classical(funyuns) level. On a side note, I don't eat funyuns, too salty.

    I'm glad you see that:

    "We found that whether Alice's and Bob's photons are and show quantum correlations or are separable and show classical correlations can be decided after they have been measured", explains Xiao-song Ma, lead author of the study.

    DECIDED BY VICTOR'S CHOICE

    Victor's choice created reality and that was my point about the funyun example.

    If Victor decides to entangle, a history unfolds in the environment where Alice and Bob's particles are entangled. If Victor chooses not to entangle, then you have a history n that environment where Alice and Bob's particles are separable.

    This all depends on CHOICE.

    Victor's choice can occur after Alice and Bob's particles have been measured because there's no communication between Bob and Alice.

    This has nothing to do with unscrambling decoherence but Victor's CHOICE. Show me in the paper where it talks about unscrambling decoherence. In fact later test were done that showed choice can suppress decoherence and delay it.

    Experimental demonstration of delayed-choice decoherence suppression

    http://www.nature.com/ncomms/2014/140729/ncomms5522/full/ncomms5522.html

    The important thing here also is Victor's choice causes correlation in all 3 bases. It's important to look at this because you can still have superposition say in the computational basis while there's decoherence in the energy basis. There's a lot of myths out there about decoherence and it has become a catch all phrase when it's not. You can still have superposition in other bases after decoherence time and even at equilibrium.

    Decoherence: myths and realities

    https://dwave.wordpress.com/2011/06/10/decoherence-myths-and-realities/

    This was also seen in an experiment on quantum entanglement and photosynthesis.

    Quantum entanglement in photosynthetic light-harvesting complexes

    http://www.nature.com/nphys/journal/v6/n6/abs/nphys1652.html#supplementary-information

    So, it has nothing to do with the unscrambling of decoherence. It has everything to do with Victor's CHOICE which creates a reality and history in the the environment.
     
    Last edited: Aug 20, 2015
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