Quantum mechanics defies causal order

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Discussion Overview

The discussion centers on the implications of a recent experiment in quantum mechanics that suggests a defiance of causal order. Participants explore the novelty of the experiment, its consistency with standard quantum mechanics, and its potential impact on quantum computing and interpretations of quantum theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants express difficulty in understanding the experiment and its implications, questioning whether it is novel.
  • One participant notes that the experiment is an improvement over prior versions and emphasizes its consistency with standard quantum mechanics.
  • There is speculation about the existence of a deeper theory that might reveal causal order, but current experiments suggest no such theory exists.
  • Another participant challenges the interpretation of the experiment as "defying causal order," arguing that a clear causal order is present in the operations performed.
  • Participants seek further insights into the experiment's meaning for quantum computing and potential applications.
  • A video from the Brukner Group is shared, which some participants find helpful in understanding the concepts discussed.
  • One participant references a key paper by Chiribella, suggesting that the results of experiments on causal order may be inconsistent with quantum mechanics predictions, drawing an analogy to Bell's theorem.
  • There is a suggestion that accepting the results of such experiments implies a non-deterministic nature of reality, though it is noted that this may not change existing interpretations of quantum mechanics.

Areas of Agreement / Disagreement

Participants express a mix of understanding and confusion regarding the implications of the experiment. There is no consensus on the interpretation of causal order, with some arguing for its presence and others suggesting it is defied. The discussion remains unresolved regarding the broader implications for quantum theory and computing.

Contextual Notes

Participants highlight the complexity of the experiment and its implications, indicating that there may be missing assumptions or unresolved mathematical steps in the discussion.

Who May Find This Useful

Readers interested in quantum mechanics, experimental physics, and the philosophical implications of quantum theory may find this discussion relevant.

kurt101
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I came across this article at physicsworld.com which has the headline "Quantum mechanics defies causal order, experiment confirms".
https://physicsworld.com/a/quantum-mechanics-defies-causal-order-experiment-confirms/

The actual experiment is described here:
https://arxiv.org/abs/1803.04302

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?
 
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kurt101 said:
The actual experiment is described here:
https://arxiv.org/abs/1803.04302

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?

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.
 
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.
 
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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?
 
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StevieTNZ said:
There is this video, , from the "Brukner Group" at www.quantum.at (https://www.quantumfoundations.org/index.html check out links at the top of the latest publications list)


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.

https://arxiv.org/abs/1109.5154

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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