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B Quantum mechanics defies causal order

  1. Sep 16, 2018 #1
    I came across this article at physicsworld.com which has the headline "Quantum mechanics defies causal order, experiment confirms".

    The actual experiment is described here:

    I had a difficult time understanding the experiment and the implications of this experiment. Is it novel in any way? Can anyone shed light on what the headline is implying?
  2. jcsd
  3. Sep 16, 2018 #2


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    Not sure I can add anything to your understanding of this paper from a top team. Sure, it's an improvement over some similar prior versions of the experiment. That we are even reading about experiments like this is nothing short of amazing.

    The key thing is that this result is fully consistent with standard QM. There is no known causal order in many quantum operations/setups. There has been speculation that a deeper theory (than current QM) might point out such causal order. Experiments such as this tend to show that no such theory exists.
  4. Sep 16, 2018 #3


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    They implemented the operation "if C then A*B else B*A", where C is a qubit and A,B are unitary operations on a second qubit. For some reason I can't fathom they interpreted this as "defying causal order". To my eyes there's a very clear causal order. First the setup performs "if C then A else B", then it performs "if C then B else A". That's the order.
  5. Sep 16, 2018 #4
    I am also interested in this.. I read about it before but didn't fully grasp it.
    Any more insights about this experiment?
    What does it mean for quantum computing?
    And what are possible applications?
  6. Sep 16, 2018 #5
  7. Sep 16, 2018 #6


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    Wow, they managed to "dumb it down" so I can almost understand it! Thanks for the link, I definitely recommend it. Although the 3:49 video took me a bit longer as I had to stop and replay a few sections a couple of times. LOL.

    The video mentions this key paper by Chiribella. Its result demonstrates that the result of an experiment to discriminate causal order produces results inconsistent with the predictions of QM. (Somewhat analogous to Bell in that respect.) So the OP's citation is the latest/greatest implementation of this idea.


    Accepting this result is another way of saying that nature is not deterministic. I doubt this will change anyone's favorite interpretation, but this is all very impressive stuff.
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