Authors: Wei-Wei Pan, Xiao-Ye Xu, Yaron Kedem, Qin-Qin Wang, Zhe Chen, Munsif Jan, Kai Sun, Jin-Shi Xu, Yong-Jian Han, Chuan-Feng Li, Guang-Can Guo

Entanglement and wave function description are two of the core concepts that make quantum mechanics such a unique theory. A method to directly measure the wave function, using Weak Values, was demonstrated by Lundeen et al., Nature \textbf{474}, 188(2011). However, it is not applicable to a scenario of two disjoint systems, where nonlocal entanglement can be a crucial element since that requires obtaining the Weak Values of nonlocal observables. Here, for the first time, we propose a method to directly measure a nonlocal wave function of a bipartite system, using Modular Values. The method is experimentally implemented for a photon pair in a hyper-entangled state, i.e. entangled both in polarization and momentum degrees of freedom.

Authors: Roumen Tsekov

A theoretical parallel between the classical Brownian motion and quantum mechanics is explored. It is shown that, in contrast to the classical Langevin force, quantum mechanics is driven by turbulent velocity fluctuations with diffusive behavior. In the case of simultaneous action of the two stochastic sources, the quantum Brownian motion takes place, which is theoretically described as well.

Authors: Yusuf Turek

In this study, we give a new proposal to measure the temperature of the spin in a sample in magnetic resonance force microscopy system by using postselected weak measurement, and investigate the fisher information to estimate the precision of our scheme. We showed that in high temperature regime the temperature of the spin can be measured via weak measurement technique with proper postselection and our scheme able to increase the precision of temperature estimation.

Publication date: Available online 18 October 2019

Source: Physics Reports

Author(s): Maximilian Schlosshauer

Abstract

Authors: Edward Anderson

This Series of Articles provides a local resolution of this major longstanding foundational problem between QM and GR, or, more generally, between Background Dependent and Background Independent Physics. We focus on the classical version; the concepts we use are moreover universal enough to admit quantum counterparts. This requires a series of articles to lay out because the Problem of Time is multi-faceted and is largely about interferences between facets, with traditional solutions to individual facets breaking down in attempted joint resolutions of facets. [98] already covered this at both the classical and semiclassical quantum levels. This Series serves, firstly, to isolate this resolution from [98]’s introductory and field-wide comparative material, passing from an 84-part work down to just a 14-part one. Secondly, to clarify that, at the usual differential-geometric level of structure used in physical theories, our classical local resolution is entirely catered for by Lie’s mathematics. This renders our local classical part of the Problem of Time understandable by a very large proportion of Theoretical Physics or Mathematics majors.

In this first article, we cover a first aspect: Temporal Relationalism. This generalizes the facet traditionally known as the Frozen Formalism Problem, that follows from the quantum form of GR’s Hamiltonian constraint.

Authors: Edward Anderson

In this article, we consider a second Problem of Time Facet. This started life as Wheeler’s Thin Sandwich Problem, within the narrow context of 1) GR-as-Geometrodynamics, in particular its momentum constraint. 2) A Lagrangian variables level treatment. Conceiving in terms of Barbour’s Best Matching is a freeing from 1), now in the context of first-class linear constraints. Conceiving in terms of the underlying Background Independence aspect – the titular Configurational Relationalism – serves moreover to remove 2) as well. This is implemented by the $G$-act, $G$-all method. I.e. given an object $O$ that is not $G$-invariant, we act on it with $G$ and then apply an operation involving the whole group. This has the effect of double-cancelling the introduction of our group action, thus yielding a $G$-independent version of $O$. A first example of whole-group operation is group averaging. This is a valuable prototype by its familiarity to a large proportion of Mathematics and Physics majors through featuring in elementary Group Theory and Representation Theory courses (and which can be traced back to Cauchy). In particular, this is much more familiar than Thin Sandwiches or Best Matching! Secondly, extremization over the group, of which Best Matching, and taking infs or sups over the group, are examples. Some further significant examples of this method in modern geometry and topology include those of Kendall, Younes, Hausdorff and Gromov. In this way, we populate this approach with examples well beyond the usual GR literature’s by DeWitt, Barbour and Fischer (which we also outline).

Dethier, Corey (2019) Forces in A True and Physical Sense: From Mathematical Models to Metaphysical Conclusions. [Preprint]

Salom, Igor (2019) The hard problem and the measurement problem: a no-go theorem and potential consequences. [Preprint]

Abstract

The Wigner’s friend type of thought experiments manifest the conceptual challenge on how different observers can have consistent descriptions of a quantum measurement event. In this paper, we analyze the extended version of Wigner’s friend thought experiment (Frauchiger and Renner in Nat Commun 3711:9, 2018) in detail and show that the reasoning process from each agent that leads to the no-go theorem is inconsistent. The inconsistency is with respect to the requirement that an agent should make use of updated information instead of outdated information. We then apply the relational formulation of quantum measurement to resolve the inconsistent descriptions from different agents. In relational formulation of quantum mechanics, a measurement is described relative to an observer. Synchronization of measurement result is a necessary requirement to achieve consistent descriptions of a quantum system from different observers. Thought experiments, including EPR, Wigner’s Friend and it extended version, confirm the necessity of relational formulation of quantum measurement when applying quantum mechanics to composite system with entangled but space-like separated subsystems.

Abstract

A recent paper (Martin-Dussaud et al. in Found Phys 49:96, 2019) has given a lucid treatment of Bell’s notion of local causality within the framework of the relational interpretation of quantum mechanics. However, the authors went on to conclude that the quantum violation of Bell’s notion of local causality is no more surprising than a common cause. Here, I argue that this conclusion is unwarranted by the authors’ own analysis. On the contrary, within the framework outlined by the authors, I argue that far from saving the notion of ‘locality’ from the grip of Bell’s theorem, the authors have deprived it of a meaningful definition.

Janas, Michael and Cuffaro, Michael E. and Janssen, Michel (2019) Putting probabilities first. How Hilbert space generates and constrains them. [Preprint]

Nature Physics, Published online: 21 October 2019; doi:10.1038/s41567-019-0678-2

Strongly interacting bosons in an optical lattice exhibit anomalous subdiffusive evolution when subjected to a dissipative process. The experimental observations are attributed to a mechanism termed ‘interaction-impeding of decoherence’.

Le Bihan, Baptiste (2019) Spacetime Emergence in Quantum Gravity: Functionalism and the Hard Problem. [Preprint]

Abstract

According to quantum mechanics, the informational content of isolated systems does not change in time. Considering composite systems, it would be very useful to identify suitable indicators able to quantify the informational content of the single parts and to describe their evolution through balance equations, as it happens in the case of energy. Reasoning on the basic concepts of quantum mechanics, we show that there is an intrinsic quantum information encoded in the coherence of quantum states. Such information is measured by a function, called here coherent entropy, which turns out to be complementary to the von-Neumann entropy. We show that the total quantum information of multipartite systems is determined by the coherent entropy of the single subsystems plus their mutual information. Interestingly, the coherent entropy is found to be equal to the information conveyed in the future by quantum states, providing a further inspiring interpretation to this quantity. The vision proposed in this paper also suggests a natural and simple definition of an indicator for nonlocal correlations.