# Weekly Papers on Quantum Foundations (11)

Observability, redundancy and modality for dynamical symmetry transformations

Wallace, David (2019) Observability, redundancy and modality for dynamical symmetry transformations. [Preprint]

When scale is surplus

Gryb, Sean and Sloan, David (2021) When scale is surplus. [Preprint]

Sonne und Mond, or, the good, the bad, and the ugly: comments on the debate between MOND and ΛCDM

Helbig, Phillip (2021) Sonne und Mond, or, the good, the bad, and the ugly: comments on the debate between MOND and ΛCDM. The Observatory, 140 (1279). pp. 225-247. ISSN 0029-7704

From Unobservable to Observable: Entity Change and Scientific Realism

Allzén, Simon (2021) From Unobservable to Observable: Entity Change and Scientific Realism. [Preprint]

Wigner’s friend and the quasi-ideal clock. (arXiv:2009.10218v2 [quant-ph] UPDATED)

In 1962, Eugene P. Wigner introduced a thought experiment that highlighted the incompatibility in quantum theory between unitary evolution and wave function reduction in a measurement. This work resulted in a class of thought experiments often called Wigner’s Friend Scenarios, which have been providing insights over many frameworks and interpretations of quantum theory. Recently, a no-go theorem obtained by Daniela Frauchiger and Renato Renner brought attention back to the Wigner’s Friend and its potential of putting theories to test. Many answers to this result pointed out how timing in the thought experiment could be yielding a paradox. In this work, we ask what would happen if the isolated friend in a Wigner’s Friend Scenario did not share a time reference frame with the outer observer, and time should be tracked by a quantum clock. For this purpose, we recollect concepts provided by the theory of quantum reference frames and the quantum resource theory of asymmetry, to learn how to internalize time in this scenario, and introduce a model for a feasible quantum clock proposed by Mischa P. Woods, Ralph Silva and Jonathan Oppenheim, called the quasi-ideal clock. Our results have shown that no decoherent behavior comes from this approach, and the disagreement between the superobserver and its friend persists even for an imprecise clock on Wigner’s side. However, the gaussian spread of this clock model can control what observables do not raise a paradox, indicating the relevance of deepening this analysis.

Phase Space Logic. (arXiv:2102.06572v2 [quant-ph] UPDATED)

We propose a phase space logic that can capture the behavior of quantum and quantum-like systems. The proposal is similar to the more generic concept of epistemic logic: it encodes knowledge or perhaps more correctly, predictions about outcomes of future observations on some systems. For a quantum system, these predictions are statements about future outcomes of measurements performed on specific degrees of freedom of the system. The proposed logic will include propositions and their relations including connectives, but importantly also transformations between propositions on different degrees of freedom of the systems. A key point is the addition of a transformation that allows to convert propositions about single systems into propositions about correlations between systems. We will see that subtle choices of the properties of the transformations lead to drastically different underlying mathematical models; one choice gives stabilizer quantum mechanics, while another choice gives Spekkens’ toy theory. This points to a crucial basic property of quantum and quantum-like systems that can be handled within the present phase space logic by adjusting the mentioned choice. It also enables a discussion on what behaviors are properly quantum or only quantum-like, relating to that choice and how it manifests in the system under scrutiny.

Positivity Bounds on Dark Energy: When Matter Matters. (arXiv:2103.06855v1 [astro-ph.CO])

Positivity bounds – constraints on any low-energy effective field theory imposed by the fundamental axioms of unitarity, causality and locality in the UV – have recently been used to constrain scalar-tensor theories of dark energy. However, the coupling to matter fields has so far played a limited role. We show that demanding positivity when including interactions with standard matter fields leads to further constraints on the dark energy parameter space. We demonstrate how implementing these bounds as theoretical priors affects cosmological parameter constraints and explicitly illustrate the impact on a specific Effective Field Theory for dark energy. We also show in this model that the existence of a standard UV completion requires that gravitational waves must travel superluminally on cosmological backgrounds.

Wave optics of quantum gravity for light and particles. (arXiv:2012.02288v2 [gr-qc] UPDATED)

Authors: S.L. CherkasV.L. Kalashnikov

Effects of the quantum gravity under Minkowsky space-time background are considered. It is shown that despite the absence of the full theory of quantum gravity, very concrete and definite predictions could be made for the influence of the quantum gravitational fluctuations on the propagation of the particles and light. It is shown that the operator of the gravitational potential in a vacuum has the correlator $<0|\hat \Phi({\bf r})\hat \Phi({\bf r}^\prime)|0>\sim \delta({\bf r}-{\bf r}^\prime)/M_p^3$. As a result, nonrelativistic massive particle waves lose their coherence at a distance proportional to the fourth degree of the particle velocity. Coherent electromagnetic wave attenuates due to metric fluctuations inversely proportionally to the fifth degree of a frequency.

The Spacetime Picture in Quantum Gravity. (arXiv:2012.03994v3 [gr-qc] UPDATED)

Authors: Alejandro Ascárate

We propose an approach which, by combining insights from Loop Quantum Gravity (LQG), Topos theory, Non-commutative Geometry \`a la Connes, and spacetime relationalism, provides fertile ground for the search of an adequate spacetime picture in Quantum Gravity. With this approach, we obtain a novel way of deducing the quantization of the possible values for the area of a surface. One gets the same area values than those from the area operator in standard LQG, but our approach makes a further prediction: some smaller values and sub-divisions are also allowed. In addition, the area arises as a noncommutative distance between two noncommutative points, and thus they should be interpreted as irreducible string-like objects at the physical level (where the area interpretation for the noncommutative distance holds).

A path integral formulation for particle detectors: the Unruh-DeWitt model as a line defect. (arXiv:2012.14912v2 [hep-th] UPDATED)

Particle detectors are an ubiquitous tool for probing quantum fields in the context of relativistic quantum information (RQI). We formulate the Unruh-DeWitt (UDW) particle detector model in terms of the path integral formalism. The formulation is able to recover the results of the model in general globally hyperbolic spacetimes and for arbitrary detector trajectories. Integrating out the detector’s degrees of freedom yields a line defect that allows one to express the transition probability in terms of Feynman diagrams. Inspired by the light-matter interaction, we propose a gauge invariant detector model whose associated line defect is related to the derivative of a Wilson line. This is another instance where nonlocal operators in gauge theories can be interpreted as physical probes for quantum fields.

A puzzle about laws and explanation

Abstract

In this paper, we argue that the popular claim that laws of nature explain their instances (explanatory laws) creates a philosophical puzzle when it is combined with the widely held requirement that explanations need to be underpinned by ‘wordly’ relations (explanatory realism). We argue that a “direct solution” to the puzzle that accounts for both explanatory laws and explanatory realism requires endorsing at least a radical metaphysics. Then, we examine the ramifications of a “skeptical solution”, i.e., dissolving it by giving up at least one of these two claims, and argue that adopting it is more favorable to Humean reductionists than to anti-reductionists about laws of nature.

Weak Measurement of a Superconducting Qubit Reconciles Incompatible Operators

Author(s): Jonathan T. Monroe, Nicole Yunger Halpern, Taeho Lee, and Kater W. Murch

Traditional uncertainty relations dictate a minimal amount of noise in incompatible projective quantum measurements. However, not all measurements are projective. Weak measurements are minimally invasive methods for obtaining partial state information without projection. Recently, weak measurements …

[Phys. Rev. Lett. 126, 100403] Published Thu Mar 11, 2021

Typicality in the Foundations of Statistical Physics and Born’s Rule

Duerr, Detlef and Struyve, Ward (2020) Typicality in the Foundations of Statistical Physics and Born’s Rule. pp. 35-44.

The visibility of philosophy of science in the sciences, 1980–2018

Khelfaoui, Mahdi and Gingras, Yves and Lemoine, Maël and Pradeu, Thomas (2021) The visibility of philosophy of science in the sciences, 1980–2018. Synthese. ISSN 0039-7857

What Are the Minimal Conditions Required to Define a Symmetric Informationally Complete Generalized Measurement?

Author(s): Isabelle Jianing Geng, Kimberly Golubeva, and Gilad Gour

Symmetric informationally complete (SIC) positive operator valued measures (POVMs) are a class of quantum measurements which, in addition to being informationally complete, satisfy three conditions: (1) every POVM element is rank one, (2) the Hilbert-Schmidt inner product between any two distinct el…

[Phys. Rev. Lett. 126, 100401] Published Mon Mar 08, 2021

A Brief Historical Perspective on the Consistent Histories Interpretation of Quantum Mechanics

Rocha, Gustavo and Rickels, Dean and Boge, Florian J. (2021) A Brief Historical Perspective on the Consistent Histories Interpretation of Quantum Mechanics. [Preprint]

When randomness is intrinsic, formalize it

De Peuter, Steven (2021) When randomness is intrinsic, formalize it. [Preprint]