Quantum entanglement links particles through time, according to this study that received some publicity last year:

I have read all the reviews in the popular press, most of the "time-travel-discovered" type, but I am looking for more sober comments, i.e. material written by physicists who have studied the paper. What does this result really mean? I would appreciate pointers to reviews, comments etc.

  • $\begingroup$ What does "this result" in What does this result really mean? mean? Do you mean the concept of "timelike entanglement" or sth else? $\endgroup$ Mar 11, 2012 at 17:30
  • $\begingroup$ Piotr - yes, by "this result" I mean the general concept of timeline entanglement (particles at different times can be entangled just like particles at different locations at a given time), and the specific conclusion of Olson and Ralph:"timelike entanglement may be regarded as a non-classical resource in a manner analogous to the spacelike entanglement that is often studied in the Minkowski vacuum, since any quantum information theoretic protocol may utilize conversion of timelike entanglement to spacelike entanglement as a step in the protocol." $\endgroup$ Mar 11, 2012 at 17:43

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I would venture that the paper is less significant than it seems, since in a universe with special relativity, what looks like a successful experimental proof of spatial entanglement to one observer will always look like a mix of space and time entanglement to another one.

  • $\begingroup$ Thanks Terry. If "a successful experimental proof of spatial entanglement" is defined as experimental confirmation of entanglement between two particles out of each other's light cone (in other words, if there is not enough time for the particles to have exchanged signals at the speed of light), which is an invariant, then a successful experimental proof of spatial entanglement will look the same to all observers. $\endgroup$ Mar 13, 2012 at 15:01
  • $\begingroup$ [on the road] Well stated Giulio; nice clean fallacy capture, and mea culpa for creating the fallacy. I'll try again when I get a moment. SR should still have geometric implications even for this spacelike separation, but not the way I just said. (Anyone?) $\endgroup$ Mar 14, 2012 at 15:00
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    $\begingroup$ Giulio: I need a diagram for this, but here goes: An entangled event has two classical terminators that within a single light cone that radiates from the point of entanglement. Both origin-to-terminator paths must remain causally isolated from the rest of the universe. One short path and one long path gives time-like separation; same-length paths gives space-like separation. Neither case differs when defining the consistency relationship between the terminus points. However, to classical observers the cases are different, with time-like akin to capturing half a Stern-Gerlach population. $\endgroup$ Mar 18, 2012 at 3:47
  • $\begingroup$ First, entanglement is a property of quantum states, not relativistic events (points in spacetime). Causally separated or not, but an entangled state of a composite system is entangled, and a non-entangled state is not entangled. “A mix of space and time entanglement” looks like a smooth phrase that actually means nothing. $\endgroup$ Oct 19, 2014 at 9:16
  • $\begingroup$ Bell's Inequality converted the metaphysical "spooky action" debate of Bohr and Einstein into meaningful experiments with fully classical setups and outcomes. If properly selected, these setups and outcomes prove the existence of a quantum entanglement relationship between them. However, the setups and outcomes are classical events subject to the space-vs-time interpretation ambiguity of special relativity. The entanglement relationship that links and mutually influences these events must bridge the same spacetime interval between events, and so is subject to frame-dependent order-of-events. $\endgroup$ Oct 19, 2014 at 16:10

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