Why ψ is incomplete?

Philippe Grangier explains a simple illustration:
“With the Nobel Prize attributed to Aspect, Clauser, and Zeilinger, the international scientific community acknowledged the fundamental importance of the experimental violation of Bell’s inequalities. It is however still debated what fails in Bell’s hypotheses, leading to these inequalities, and usually summarized as “local realism”, or maybe more appropriately “classical local realism”. The most common explanation is “quantum non-locality”, that remains however fully compatible with relativistic causality; this makes wondering whether any non-local phenomenon is really involved in these experiments. Here we want to recapitulate another option, sometimes called “predictive incompleteness”, closely related to the idea that the usual state vector ψ is incomplete indeed, as it was claimed by Einstein, Podolsky and Rosen. However, the right way to complete ψ has nothing to do with hidden variables, but requires to specify the measurement context, as it was claimed by Bohr. Here we will consider the simple case of two spin 1/2, or two qubits, in order to keep the argument simple, but it does apply generally in quantum mechanics.” See full paper on the arXiv.

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Who will win: cryptographers or cryptanalysts?

R. Renner & R. Wolf discuss this in their paper.
“Ever since its inception, cryptography has been caught in a vicious circle: Cryptographers keep inventing methods to hide information, and cryptanalysts break them, prompting cryptographers to invent even more sophisticated encryption schemes, and so on. But could it be that quantum information technology breaks this circle? At first sight, it looks as if it just lifts the competition between cryptographers and cryptanalysts to the next level. Indeed, quantum computers will render most of today’s public key cryptosystems insecure. Nonetheless, there are good reasons to believe that cryptographers will ultimately prevail over cryptanalysts. Quantum cryptography allows us to build communication schemes whose secrecy relies only on the laws of physics as well as some minimum assumptions about the cryptographic hardware – leaving basically no room for an attack. While we are not yet there, this article provides an overview of the principles and state of the art of quantum cryptography.”

Do Atoms Age?

“Time evolution generically entangles a quantum state with environmental degrees of freedom. The resulting increase in entropy changes the properties of that quantum system leading to “aging”. It is interesting to ask if this familiar property also applies to simple, single particle quantum systems such as the decay of a radioactive particle. We propose a test of such aging in an ion clock setup where we probe for temporal changes to the energies of the electronic state of an ion containing a radioactive nucleus. Such effects are absent in standard quantum mechanics and this test is thus a potent null test for violations of quantum mechanics. As a proof of principle, we show that these effects exist in causal non-linear modifications of quantum mechanics.” See full paper on the arXiv.

Fools will be rewarded!

“Ralph, like us, was willing to be a fool. And the way to get to the top of the heap in terms of developing original research is to be a fool, because only fools keep trying. You have idea number 1, you get excited, and it flop. Then you have idea number 2, you get excited, and it flops. Then you have idea number 99, you get excited, and it flops. Only a fool would be excited by the 100th idea, but it might take 100 ideas before one really pays off. Unless you’re foolish enough to be continually excited, you won’t have the motivation, you won’t have the energy to carry it through. God rewards fools”
-Martin Hellman

Image from: sciencefocus.com

How deep the theory of quantum communications goes?

“In the theory of quantum communications, a deeper structure has been recently unveiled – mainly within the physics community – showing that the capacity does not completely characterize the channel ability to transmit information due to phenomena with no counterpart in the classical world. Although how deep goes this structure is yet to be fully uncovered, it is crucial for the communication engineering community to own the implications of these phenomena for understanding and deriving the fundamental limits of communications. Hence, the aim of this paper is to shed light on these phenomena by providing the reader with an easy access and guide towards the relevant literature and the prominent results, by adopting a communication engineering perspective.”
See full paper on arXiv.