International Conference on Quantum Physics of Nature (QuPoN 2015) Mitschnitt einer Veranstaltung der Fakultät für Physik am Montag, dem 18. Mai 2015 im Großen Festsaal der Universität Wien Teil 4. Nicolas Gisin: "Spins, magnets, PR-boxes and weak measurements" Schnitt: Daniel Winkler Abstract: Quantum theory describes classical measurements as weak measurement, i.e. measurements that barely disturb the system at the cost of providing only little information. An example is the measurement of the direction in which a magnet points: this doesn’t “collapse” the spins that make-up the magnet while providing good enough-but-not-complete information on the magnet direction. This raises several questions: Could weak measurements characterize “classicality”? If Alice and Bob share N singlets and Alice measures (strongly) all her spins along a direction of her choice, this prepares all Bob’s N spins parallel or anti-parallel to that direction. The fluctuation of the order √𝑁 can be arbitrarily large. Why can’t Bob deduce from such a large magnetic moment the direction chosen by Alice? If Alice and Bob share a single singlet, can Bob make a measurement weak enough that the disturbed state still contains quantum nonlocality, but at the same time strong enough that his data violate the CHSH-Bell inequality with Alice’s data? How long can a chain of independent Bobs be such that each Bob violates the CHSH-Bell inequality with Alice? Can the concept of weak measurements be generalized to one PR-box? Or at least to large ensembles of PR-boxes? If not, can PR-boxes still be considered as plausible physical models? Nicolas Gisin ist Professor für Physik an der Universität Genf. INHALT ====== Kapitel Titel Position --------------------------------------------------------------------- 1. Vorspann 00:00:00 2. Spins and magnets. Weak measurements 00:00:12 3. Quantifying classicality with weak measurements 00:09:08 4. Example: Displaced single photons 00:15:42 5. Decoherence 00:23:00 6. Classical measurements on PR-boxes 00:33:24

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