This is a list of this week’s papers on quantum foundations published in various journals or uploaded to preprint servers such as arxiv.org and PhilSci Archive.

Dark energy from non-unitarity in quantum theory. (arXiv:1604.04183v1 [gr-qc])

on 2016-4-16 8:28am GMT

Authors: Thibaut Josset, Alejandro Perez, Daniel Sudarsky

We consider a scheme whereby it is possible to reconcile semi-classical Einstein’s equation with the violation of the conservation of the expectation value of energy-momentum that is associated with dynamical reduction theories of the quantum state for matter. The very interesting out-shot of the formulation is the appearance of a nontrivial contribution to an effective cosmological constant (which is not strictly constant). This opens the possibility of using models for dynamical collapse of the wave function to compute its value. Another interesting implication of our analysis is that tiny violations of energy-momentum conservation with negligible local effects can become very important on cosmological scales at late times.

Time Symmetric Quantum Mechanics and Causal Classical Physics. (arXiv:1604.04231v1 [quant-ph])

on 2016-4-16 8:28am GMT

Authors: Fritz W. Bopp

A two boundary quantum mechanics without time ordered causal structure is advocated as consistent theory. The apparent causal structure of usual “near future” macroscopic phenomena is attributed to a cosmological asymmetry and to rules governing the transition between microscopic to macroscopic observations. Our interest is a heuristic understanding of the resulting macroscopic physics.

Multipartite Cellular Automata and the Superposition Principle. (arXiv:1604.04201v1 [quant-ph])

on 2016-4-16 8:28am GMT

Authors: Hans-Thomas Elze

Cellular automata can show well known features of quantum mechanics, such as a linear updating rule that resembles a discretized form of the Schr\”odinger equation together with its conservation laws. Surprisingly, a whole class of “natural” Hamiltonian cellular automata, which are based entirely on integer-valued variables and couplings and derived from an Action Principle, can be mapped reversibly to continuum models with the help of Sampling Theory. This results in “deformed” quantum mechanical models with a finite discreteness scale $l$, which for $l\rightarrow 0$ reproduce the familiar continuum limit. Presently, we show, in particular, how such automata can form “multipartite” systems consistently with the tensor product structures of nonrelativistic many-body quantum mechanics, while maintaining the linearity of dynamics. Consequently, the Superposition Principle is fully operative already on the level of these primordial discrete deterministic automata, including the essential quantum effects of interference and entanglement.

Are Local Causal Models of Quantum Theory Feasible at All?. (arXiv:1604.03959v1 [quant-ph])

on 2016-4-16 8:28am GMT

Authors: Hans H. Diel

This article presents an analysis of the extent to which local causal models or local realistic models of quantum theory (QT), including quantum field theory (QFT), are theoretically possible and practically feasible in light of the present state of these theories. Quantum physicists consider Bells famous inequality and its violation in experiments to be a strong indication that local realistic or local causal models of QT are not possible and that quantum theory as a whole is therefore not a local realistic or local causal theory. Based on a proposed definition of a “formal causal model” for a theory of physics (such as QT), this paper investigates the possibility of having a local causal model for QT. Areas of QT are identified in which the construction of a causal model is impeded because of deficiencies in the state of the respective theory. It is shown that the removal of the deficiencies can be achieved by the provision of a causal model. Whereas the construction of a causal model of QT, including QFT, appears to be feasible after the removal of certain deficiencies, the construction of a local (causal) model does not appear to be possible. As a consequence of the conclusion that local (causal) models of QT/QFT are not possible, if a strong interpretation of locality is assumed, a locality model is proposed in which the non-localities are confined to “quantum objects”.

Topos Theoretic Quantum Realism

The British Journal for the Philosophy of Science – Advance Access

on 2016-4-14 5:01am GMT

Topos quantum theory (TQT) is standardly portrayed as a kind of ‘neo-realist’ reformulation of quantum mechanics.^{1} In this article, I study the extent to which TQT can really be characterized as a realist formulation of the theory, and examine the question of whether the kind of realism that is provided by TQT satisfies the philosophical motivations that are usually associated with the search for a realist reformulation of quantum theory. Specifically, I show that the notion of the quantum state is problematic for those who view TQT as a realist reformulation of quantum theory.

**1***Introduction***2***Topos Quantum Theory***2.1***Phase space***2.2***Hilbert space***2.3***Beyond Hilbert space***2.4***Defining realism***2.5***The spectral presheaf***2.6***The logic of topos quantum theory***3***Interpreting States in Topos Quantum Theory***4***Interpreting Truth Values and Clopen Subobjects in Topos Quantum Theory***4.1***Interpreting the truth values***4.2***Interpreting**Subcl()***5***Neo-realism***5.1***The covariant approach***6***Conclusion*

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences current issue

on 2016-4-13 7:05am GMT

Landauer’s principle sets fundamental thermodynamical constraints for classical and quantum information processing, thus affecting not only various branches of physics, but also of computer science and engineering. Despite its importance, this principle was only recently experimentally considered for classical systems. Here we employ a nuclear magnetic resonance set-up to experimentally address the information to energy conversion in a quantum system. Specifically, we consider a three nuclear spins S=12 (qubits) molecule—the system, the reservoir and the ancilla—to measure the heat dissipated during the implementation of a global system–reservoir unitary interaction that changes the information content of the system. By employing an interferometric technique, we were able to reconstruct the heat distribution associated with the unitary interaction. Then, through quantum state tomography, we measured the relative change in the entropy of the system. In this way, we were able to verify that an operation that changes the information content of the system must necessarily generate heat in the reservoir, exactly as predicted by Landauer’s principle. The scheme presented here allows for the detailed study of irreversible entropy production in quantum information processors.

Human mind excels at quantum physics game

Nature – Issue – nature.com science feeds

on 2016-4-13 12:00am GMT

**Human mind excels at quantum physics game**

Nature 532, 7598 (2016). http://www.nature.com/doifinder/10.1038/532160a

Author: Elizabeth Gibney

Revelation could have implications for how scientists approach quantum physics.

Exploring the quantum speed limit with computer games

Nature – Issue – nature.com science feeds

on 2016-4-13 12:00am GMT

**Exploring the quantum speed limit with computer games**

Nature 532, 7598 (2016). doi:10.1038/nature17620

Authors: Jens Jakob W. H. Sørensen, Mads Kock Pedersen, Michael Munch, Pinja Haikka, Jesper Halkjær Jensen, Tilo Planke, Morten Ginnerup Andreasen, Miroslav Gajdacz, Klaus Mølmer, Andreas Lieberoth & Jacob F. Sherson

Humans routinely solve problems of immense computational complexity by intuitively forming simple, low-dimensional heuristic strategies. Citizen science (or crowd sourcing) is a way of exploiting this ability by presenting scientific research problems to non-experts. ‘Gamification’—the application of game elements in a non-game context—is an effective tool with which to enable citizen scientists to provide solutions to research problems. The citizen science games Foldit, EteRNA and EyeWire have been used successfully to study protein and RNA folding and neuron mapping, but so far gamification has not been applied to problems in quantum physics. Here we report on Quantum Moves, an online platform gamifying optimization problems in quantum physics. We show that human players are able to find solutions to difficult problems associated with the task of quantum computing. Players succeed where purely numerical optimization fails, and analyses of their solutions provide insights into the problem of optimization of a more profound and general nature. Using player strategies, we have thus developed a few-parameter heuristic optimization method that efficiently outperforms the most prominent established numerical methods. The numerical complexity associated with time-optimal solutions increases for shorter process durations. To understand this better, we produced a low-dimensional rendering of the optimization landscape. This rendering reveals why traditional optimization methods fail near the quantum speed limit (that is, the shortest process duration with perfect fidelity). Combined analyses of optimization landscapes and heuristic solution strategies may benefit wider classes of optimization problems in quantum physics and beyond.

Physics: Unite to build a quantum Internet

on 2016-4-12 12:00am GMT

Advances in quantum communication will come from investment in hybrid technologies, explain Stefano Pirandola and Samuel L. Braunstein.

Nature 532 169 doi: 10.1038/532169a