Entanglement and Relativity

Discussion in 'Physics & Math' started by 1100f, Oct 26, 2015.

  1. arfa brane call me arf Valued Senior Member

    We can consider the spacetime diagram as having a causal structure, in fact, the axis labeled T can be considered the path for a clock that starts ticking at E.

    So at the point the measurements are made, assuming A and B are simultaneous, the clock's past lightcone intersects the future lightcone of E, and we have a causal diamond! So what?
    Well, the only quantum events or ops (aka Seth Lloyd) inside this causal diamond are the two polarization states of two distinct photons. The clock stops when the measurements are made.

    But in fact there are two clocks, one for each detector, the two photons are essentially synchronizing these clocks, the polarization measurements and random choice of angle correlate with the entanglement at E. You can take this experiment, an attempt to measure "speed of entanglement" via classical measurement of correlations, and upgrade it to a quantum teleportation experiment, with the role of synchronization signal handed over to a classical communication.

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    Quantum information, yuh huh.
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  3. FOLZONI Registered Senior Member

    The speed of entanglement is apparently instantaneous - like Newtonian gravity! You could try to determine a finite speed for quantum entanglement (QE) but as you say, you have to have classical time measurements to assess this. Spacetime diagrams do NOT help one iota however. They don't demonstrate a causal structure, rather they represent a deterministic one cluttered with arcane symbols.

    But perhaps I shouldn't say too much about this, lest a certain 'cocky' get into a flap about it!

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  5. paddoboy Valued Senior Member

    Some pity is required bruce, as imbeciles that are afflicted with delusions of grandeur, can be caused by many abnormal brain functions.
    1. Certainty: Individuals that hold delusions are certain in their beliefs; they believe with 100% conviction that they are real, despite significant logical evidence to the contrary.
    2. Incorrigibility: Those with delusions will not update their false beliefs even if presented with overwhelming logical evidence suggesting that the opposite is true. Despite scientific evidence to suggest that their way of thinking is flawed, they aren’t able to escape their deluded perception.
    3. Impossibility: The delusion that a person holds is not only untrue, but generally impossible or highly implausible to be true. Some of the delusions may not only seem like an obvious impossibility, but they may seem highly bizarre.
    4. :::::::::::::::::::::::::::::::::::::::::::::::::

    What causes delusions of grandeur?
    Brain anatomy:
    Delusions may be a result of anatomical abnormalities in the brain
    Brain injuries:
    Those that have endured a brain injury or multiple brain injuries may be more prone to delusions.
    Those that have used or abused drugs may experience delusions of grandeur as a side effect.
    And more at the link.
    In essence bruce, perhaps we should be trying harder to get these afflicted characters to seek medical attention.
    I mean, this forum is the only outlet they have. Other forums such as Cosmoquest, has them permanently banned.
    So they are doing very limited harm, if any at all. I mean, most know tthat what they are ranting about is total crap, and unsupported.
    Its there way of getting attention, via a "shock and Awe" methodology, and of course the delusions of grandeur affliction.
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  7. arfa brane call me arf Valued Senior Member

    The diagram in post 41 is almost a spacetime diagram, the path lengths from the EPR event and from A, to B have to be equal, as do the paths for the input photon at top left and half the EPR pair. It's important that A occurs before B however, and that's significant.

    The paradigm is that the two inputs to A are erased, then A outputs a 2-bit classical code in place of the inputs, then B inputs this with the second half of the EPR pair to "regenerate" the original photon. There's a causal or mechanical thing goin' on, most definitely yes indeed.
  8. brucep Valued Senior Member

    It looks like a light cone. The path of the entangled photons would be over the light like boundary. This can't be true because the distance between A and B, inside the light cone, has to be space like for the experiment. Do you have a paper written on this experiment?
  9. arfa brane call me arf Valued Senior Member

    The diagram is from a wiki page: https://en.wikipedia.org/wiki/Quantum_teleportation

    Specifically, it's in the section subtitled Protocol, so my guess is it's a protocol diagram. Time is represented as "moving" left to right. It's in the same domain (two entangled photons), so the first diagram I posted applies, if you map E in the first diagram to EPR in the second.

    So A and B in the second diagram are events on the future lightcone of the EPR pair. A and B can also be considered as quantum gates, with inputs and outputs. One thing in the diagram, the classical outputs from B, can be discarded (or recorded), the other inputs and outputs are part of the quantum measurement.

    Now we can assume things about the protocol such as: what if there is no classical channel (no classical information is transmitted from A to B)? what if B performs a measurement before A does? What about causality and a causal chain?
    Last edited: Nov 30, 2015
  10. brucep Valued Senior Member

    That's very interesting. Sounds like no quantum information can be teleported without the classical channel. Sounds like important protocol is broken if B performs the measurement before A as pre planned between Alice and Bob. There's a high degree of possibility that I don't fully understand what's going on.
  11. zgmc Registered Senior Member

    I'm not sure I'm understanding this. Do you have any links that could shed some more light on it? Are you saying that useful information is being teleported instantaneously?
  12. arfa brane call me arf Valued Senior Member

    Actually that isn't quite true, apparently.

    Well, the transportation of quantum states depends on measurement, for sure. It also depends on what is known about the transported state. I'm trying to think about why nonlocality is a problem for SR.
    The quantum teleportation depends on half the EPR pair reaching A simultaneously with the unknown photon, and a Bell measurement at A, unless A can store one of the inputs (but I don't know how). Then the other half of the EPR pair at B is correlated with the measurement at A "instantaneously".

    But I've seen an argument that says B can measure their half of the EPR pair before A does a Bell measurement, the correlations will still be known at A even though B just outputs a photon, hence A knows which photons output from B are correlated, hence communication with B exists (but not classically!).
    The corollary being that the classical communication of which Bell state A's two inputs are in, only lets B know what to do to ensure each output is the same (state) as the unknown photon input at A. If the protocol can deal with having to discard 3/4 of the outputs from B, there we have it.
  13. arfa brane call me arf Valued Senior Member

    It sounds pretty weird, so it's probably true. Actually experiments verify all of the above situations are possible--B's measurement of A's classical output in the timelike future of A, no measurement of classical output from A along with B's measurement in the timelike future or the timelike past of A. The correlations are "caused" by the Bell measurement at A, and by the preparation of maximally entangled states at E (the EPR pair).

    The protocol diagram doesn't need the classical channel, B doesn't need to do anything to their half of the EPR pair, when A inputs the other half of this pair and an "unknown" photon, when it does the Bell measurement and destroys both inputs, it has "communicated" with B. It doesn't matter either, where A and B are on the future lightcone of the entangled pair creation event (E).

    This is kind of disturbingly nonintuitive; B can emit a photon before A does the Bell measurement, and B's photon can be in the same state as the unknown photon input at A. Teleportation is then between A's unknown (unmeasured) input photon and B's output, where ever it is. If B does nothing to its input (half the EPR pair), and doesn't input classical information from A, B is also redundant, or is then just one half of the event at E

    Locality takes a back seat, and as Einstein noted, simultaneity (locality in time), is an unusual phenomenon.
    Last edited: Dec 1, 2015
  14. arfa brane call me arf Valued Senior Member

    Ok, so taking both diagrams together, where is the information? It seems necessary to first distinguish classical from quantum information (although this might be artificial), or what is known and what can be known or projected into a future outcome.

    Both diagrams start with equivalent events--a pair of entangled photons--so this is one known. Both diagrams have pairs of 'detectors' or quantum gates (classical devices with quantum inputs and/or outputs), so both diagrams have a pair of events, A and B, in the timelike future of E. Both diagrams imply protocols for the communication of information. In the first, speed of entanglement experiment, the communication of classical information is possible anytime after the clock(s) have stopped and each detector has a classical output.

    In the second teleportation experiment, it can be seen that B is redundant, only A is needed because a Bell operator measurement is needed, this is also a classical result, but this doesn't need to be communicated anywhere because it tells A about the other half of E, A can use the classical output to decide if this other entangled half is in the same state as the input photon just destroyed by the Bell measurement.

    But there is still a protocol problem: A knows which of B's outputs is a successful quantum teleportation, but noone else does. It seems the safer option is to have a classical communication so each of B's outputs is successful, then this is another known. In that case because classical communication is necessarily forwards in time, event B must be in the timelike future of event A.

    Phew, and all because we want a causal explanation!
  15. brucep Valued Senior Member

    Non-locality isn't a problem for SR. The non locality was brought to light through quantum theory. The fact SR has application in quantum experiments doesn't mean it has something to say about quantum phenomena.
  16. arfa brane call me arf Valued Senior Member

    Yeah, with SR and its spacetime diagrams, you have lightcones and maybe you can throw in causal diamonds. With a quantum experiment and entangled states, you "transport" or teleport these states by destroying or erasing them during a measurement, the teleportation is presumably synchronous with this measurement and its ancillary destruction of quantum particles, or at least, there isn't a good reason to believe it isn't, the speed of entanglement seems to say just that.

    But, at A, the system is in a superposition of three states, two states are a Bell pair already and one half of this pair and the third photon are discarded, or absorbed. That is they do work, and this is why there is a classical output at A. The output photon, or the second half of the initial Bell pair, has information about it at A.
    Although we say a state is teleported "instantaneously", no information, nothing physical, is actually sent, the output photon at B already exists (though it might need a unitary transformation on it).

    What if you teleport a state to an observer moving at relativistic velocity, so the events E and A would occur at different times for that observer? Does that throw a spanner in the locality?

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