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I Entanglement correlations vs. timelike measurement events?

  1. Mar 22, 2017 #1

    referframe

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    The experiments that have been done to demonstrate that entangled particles are correlated (or anti-correlated) have been designed so that Alice and Bob's measurement events are intentionally space-like separated. Thus demonstrating that the "ghostly action at a distance" is supraluminal or instantaneous.

    Have there been any such experiments that demonstrate, either intentionally or accidentally, that the correlations still hold up when the two measurement events are time-like separated? Just wondering.

    Thanks in advance.
     
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  3. Mar 22, 2017 #2

    DrChinese

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    Great question, and the answer is yes. Specifically, photons can be entangled that have never co-existed. I'd call that time-like separation. :smile:

    While it might seem impossible to entangle particles at different times, it can be and has been done using entanglement swapping. (Side note: normal Bell experiments do not actually attempt to measure spin or polarization at the exact same time anyway.)

    https://arxiv.org/abs/0911.1314
    Quantum systems that have never interacted can become nonlocally correlated through a process called entanglement swapping. To characterize nonlocality in this context, we introduce local models where quantum systems that are initially uncorrelated are described by uncorrelated local variables. While a pair of maximally entangled qubits prepared in the usual way (i.e., emitted from a common source) requires a visibility close to 70% to violate a Bell inequality, we show that an entangled pair generated through entanglement swapping will already violate a Bell inequality for visibilities as low as 50% under our assumption.

    https://arxiv.org/abs/1209.4191
    The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.
     
  4. Mar 23, 2017 #3

    vanhees71

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    It should, however be stressed, that within "no-nonsense interpretations" of quantum theory, i.e., such as local relativistic quantum field theory, upon which the most successful theory so far is based, the Standard Model of elementary particle physics, there is no "ghostly action at a distance" but only local measurements at two far-distant places on parts of an entangled system, which is entangled, because it was prepared as such in the very beginning long before any of the two "measurement events" (usually "clicks" of photon detectors at each of the experimentalists' places), which can be taylored to be space like (i.e., simultaneous in one inertial system of reference) or time-like (i.e., objectively one later than the other). No matter what, since there is no influence of the one measurement on the outcome of the other measurement, it doesn't make any difference in the outcome. The correlations due to entangledment, that are stronger than in any local deterministic model, are due to the preparation and not to the mutual influence of the measurements at far-distant places. There are very long debates on this already in these forums, leading to basically nothing from the point of view of physics!
     
  5. Mar 24, 2017 #4

    referframe

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    Can you recommend any recent papers dealing with entanglement in QFT? My two QFT books (Ryder and Zee) don't even reference it. Thanks.
     
  6. Mar 25, 2017 #5

    vanhees71

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    Read Weinberg, Quantum Theory of Fields, vol. I. There is a careful discussion about the "linked-cluster principle", "causality", and all that.
     
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