Scott Aaronson | On the Hardness of Detecting Macroscopic Superpositions

Abstract: When is decoherence "effectively irreversible"? We examine this question of quantum foundations using the tools of quantum computational complexity. We prove that, if one had a quantum circuit to determine if a system was in an equal superposition of two orthogonal states (for example, the |Alive⟩ and |Dead⟩ states of Schrödinger's cat), then with only a slightly larger circuit, one could also swap the two states (e.g., bring a dead cat back to life). In other words, observing interference between the |Alive⟩ and |Dead⟩
states is a "necromancy-hard" problem, technologically infeasible in any world where death is permanent. We also prove several related results, and we show that our results are quantitatively tight. Given the subject of the workshop, I'll also discuss some potential implications for how one should think about the Everett interpretation.

About the speaker: Scott Aaronson is David J. Bruton Jr. Centennial Professor of Computer Science at the University of Texas at Austin. He is well known for his research into quantum computation and computational complexity theory, as well as for his blog, Shtetl-Optimised.

[Self-Duality]

Extremely intriguing post! Thank you! Subscribed.

[bobfrankel]

Thanks, was very informative and interesting. Surprisingly I was able to follow, even without a computer background. Only wish that the discussion phase was longer and shared with the public with more follow up. Thanks again

[Achrononmaster]

@46:00 that "Babak" guy missed the whole point of the "thought rotation" gedankenexperiment. If "someone" could easily rotate your consciousness they'd be a god. So it would not only rule out the von-Neumann version of Copenhagen, it would confirm the existence of gods. Now, this is not to say such gods do not exist, but if they did where are they? I'm being facetious, but semi-serious there too. Point is, it is not easy to test MWI, period, at least not this way by comparing macroscopic superposition, and if all we can test is small scale superpositions we are not doing much, because they'd be compatible with almost all versions of QM interpretations, in fact they have to be, up to whatever limits the model says collapse or subjective awareness in no-collapse theories, occurs.
For me, there is no collapse, there are only indelible records that tell us limits on what states were once in superposition, so it is a version of limited sum-over-histories: not all Feynman--Hibbs histories have to be actualized, only some very large number. But in the SOH formalism you have to pretend all histories occur because you have no information to say otherwise, like rolling a dice: you only need a few hundred to confirm the die is fair.