It’s a minor new quantum thought experiment which, as often happens, is being used to promote dumb sensational views about the meaning or implications of quantum mechanics. There’s a kind of two-observer entangled system (as in “Hardy’s paradox”), and then they say, let’s also quantum-erase or recohere one of the observers so that there is no trace of their measurement ever having occurred, and then they get some kind of contradictory expectations with respect to the measurements of the two observers.
Undoing a quantum measurement in the way they propose is akin to squirting perfume from a bottle, then smelling it, and then having all the molecules in the air happening to knock all the perfume molecules back into the bottle, and fluctuations in your brain erasing the memory of the smell. Classically that’s possible but utterly unlikely, and exactly the same may be said of undoing a macroscopic quantum measurement, which requires the decohered branches of the wavefunction (corresponding to different measurement outcomes) to then separately evolve so as to converge on the same state and recohere.
Without even analyzing anything in detail, it is hardly surprising that if an observer is subjected to such a highly artificial process, designed to undo a physical event in its totality, then the observer’s inferences are going to be skewed somehow. So, you do all this and the observers differ in their quantum predictions somehow. In their first interpretation (2016), Frauchiger and Renner said that this proves many worlds. Now (2018), they say it proves that quantum mechanics can’t describe itself. Maybe if they try a third time, they’ll hit on the idea that one of the observers is just wrong.
The thought experiment involves observers being in a coherent superposition. But I’m not now 100% sure that it involves actual quantum erasure, I was relying on other people’s description. I’m hoping this will be cleared up without having to plough through the paper myself.
Anyway, LW may appreciate this analysis which actually quotes HPMOR.
I think this was also recently brought up in the Open Thread.
Mitchell_Porter wrote this:
The thought experiment involves observers being in a coherent superposition. But I’m not now 100% sure that it involves actual quantum erasure, I was relying on other people’s description. I’m hoping this will be cleared up without having to plough through the paper myself.
Anyway, LW may appreciate this analysis which actually quotes HPMOR.