Aharonov meets Spekkens: What do quantum paradoxes tell us about the nature of reality?

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• Matthew Leifer (Perimeter Institute)
• Thursday 15 October 2015, 14:15-15:15
• MR4, Centre for Mathematical Sciences, Wilberforce Road, Cambridge.

If you have a question about this talk, please contact William Matthews.

Over the last few decades, Aharonov and collaborators have discovered a large number of counterintuitive phenomena in quantum theory, such as the “three box paradox”, the “quantum pigeonhole principle”, “quantum cheshire cats”, and “protective measurement”. The interpretation of these effects has always been controversial, with ongoing arguments about whether these effects have “classical” analogues as opposed to being “genuinely quantum”.

In this talk, I will reanalyse these effects in light of the work of Spekkens. Spekkens emphasizes that many seemingly strange quantum phenomena actually have straightforward explanations if we view quantum states as states of incomplete knowledge about some underlying reality. Those that do not have such an explanation are the “genuinely quantum” phenomena, and these all seem to fall under the remit of Spekkens’ generalized definition of contextuality. So now we can make the question more precise: can Aharonov’s phenomena be explained in theories in which the quantum state represents incomplete knowledge and, if not, do they constitute proofs of contextuality?

I will show that a class of effects known as “logical pre- and post-selection paradoxes”, which includes the three box paradox and the quantum pigeonhole principle, do constitute direct proofs of contextuality, so they are “genuinely quantum” in Spekkens’ sense. On the other hand, protective measurement, which was initially proposed as an argument for the reality of the quantum state, is most easily accounted for in theories in which the quantum state represents incomplete knowledge.

This talk is based on joint work with Josh Combes, Chris Ferrie, and Matt Pusey, available in the papers arXiv:1506.07850 and arXiv:1509.08893

This talk is part of the CQIF Seminar series.

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