# Weekly Papers on Quantum Foundations (50)

A no-go theorem for Quantum theory ontological models. (arXiv:2012.05712v1 [quant-ph])

In this paper, we show that Quantum Mechanics does not admit ontological models, in the sense that the quantum state of a system cannot correspond to a set of physical states representing the independent reality of the system. We show, via two thought experiments based on the Wigner’s friend scenario, that if the ontic state of physical systems in the lab is the same for Wigner and for his friend, one of the following will be violated: PBR theorem, Quantum-theoretic predictions, Causality and the “No-superdeterminism” assumption.

A possibilistic semantic for quantum phenomena. (arXiv:2012.05830v1 [quant-ph])

We develop a possibilistic semantic for quantum phenomena in an operational perspective. This semantic is based on a Chu duality between preparation processes and yes/no tests, the target space being a three-valued set equipped with an informational interpretation. After having defined the notion of states, we develop a precise axiomatic for the space of states. The ‘information principle’, proposed as a building block for some quantum axiomatic program, is translated into our framework to emphasize the quantum character of our description. This principle suffices to constrain the space of states to be a locally-boolean qualitative domain. The subset of pure states is then characterized within this domain structure. After having carefully precised the notions of properties and measurements, we explore the notion of compatibility between measurements. The existence of minimally-disturbing measurements is then emphasized as a key axiom to describe the space of yes/no tests. The conditions of existence of such measurements is translated into a simple property on the space of states, and an explicit formula for these measurements is given. Having reduced the space of yes/no tests to this set of minimally-disturbing operations, our Chu space becomes bi-extensional. The space of ‘descriptions’, associated to families of compatible measurements, inherits a structure of coherence domain. A subset of properties corresponding to ‘classical properties’ is identified and explored. Endly, the symmetries of the system are characterized as a general sub-class of Chu morphisms. We prove that these symmetries preserve the class of minimally-disturbing measurements and the orthogonality relation between states. Explicit expressions of these symmetries are identified for a large class of them, called ‘linear symmetries’.

An experiment to test the discreteness of time. (arXiv:2007.08431v2 [gr-qc] UPDATED)

Time at the Planck scale ($\sim 10^{-44}~\mathrm{s}$) is an unexplored physical regime. It is widely believed that probing Planck time will remain for long an impossible task. Yet, we propose an experiment to test the discreteness of time at the Planck scale and show that it is not far removed from current technological capabilities.

Quantum Technology for Economists. (arXiv:2012.04473v2 [econ.GN] UPDATED)

Research on quantum technology spans multiple disciplines: physics, computer science, engineering, and mathematics. The objective of this manuscript is to provide an accessible introduction to this emerging field for economists that is centered around quantum computing and quantum money. We proceed in three steps. First, we discuss basic concepts in quantum computing and quantum communication, assuming knowledge of linear algebra and statistics, but not of computer science or physics. This covers fundamental topics, such as qubits, superposition, entanglement, quantum circuits, oracles, and the no-cloning theorem. Second, we provide an overview of quantum money, an early invention of the quantum communication literature that has recently been partially implemented in an experimental setting. One form of quantum money offers the privacy and anonymity of physical cash, the option to transact without the involvement of a third party, and the efficiency and convenience of a debit card payment. Such features cannot be achieved in combination with any other form of money. Finally, we review all existing quantum speedups that have been identified for algorithms used to solve and estimate economic models. This includes function approximation, linear systems analysis, Monte Carlo simulation, matrix inversion, principal component analysis, linear regression, interpolation, numerical differentiation, and true random number generation. We also discuss the difficulty of achieving quantum speedups and comment on common misconceptions about what is achievable with quantum computing.

Relational structures of fundamental theories. (arXiv:2012.05584v1 [physics.hist-ph])

Authors: Pierre Martin-Dussaud

General relativity and quantum mechanics have both revealed the relativity of certain notions that were previously thought to be absolute. I clarify the precise sense in which these theories are relational, and I argue that the various aspects of relationality pertain to the same movement in the progress of physical theories.

A Natural Introduction to Fine-Tuning. (arXiv:2012.05617v1 [physics.hist-ph])

Authors: Julian De Vuyst

A well-known topic within the philosophy of physics is the problem of fine-tuning: the fact that the universal constants seem to take non-arbitrary values in order for live to thrive in our Universe. In this paper we will talk about this problem in general, giving some examples from physics. We will review some solutions like the design argument, logical probability, cosmological natural selection, etc. Moreover, we will also discuss why it’s dangerous to uphold the Principle of Naturalness as a scientific principle. After going through this paper, the reader should have a general idea what this problem exactly entails whenever it is mentioned in other sources and we recommend the reader to think critically about these concepts.

Floating free from physics: the metaphysics of quantum mechanics. (arXiv:2012.05822v1 [physics.hist-ph])

Authors: Raoni W. ArroyoJonas R. B. Arenhart

We discuss some methodological aspects of the relation between physics and metaphysics by dealing specifically with the case of non-relativistic quantum mechanics. Our main claim is that current attempts to productively integrate quantum mechanics and metaphysics are best seen as approaches of what should be called `the metaphysics of science’, which is developed by applying already existing metaphysical concepts to scientific theories. We argue that, in this perspective, metaphysics must be understood as an autonomous discipline. It results that this metaphysics cannot hope to derive any kind of justification from science. Thus, one of the main motivations of such project, which is the obtaining of a scientifically respectable justification for the attribution of a single true metaphysical profile to the posits of a scientific theory, is doomed because of the emergence of metaphysical underdetermination from the outset. If metaphysics floats free from physics, which is a premise of such project of integration between these two areas, then it is always possible to attribute more than one metaphysical profile to dress physical entities.

Challenging the cosmic censorship conjecture from a Gauss-Bonnet sector. (arXiv:2012.05642v1 [gr-qc])

The Dvali-Gabadadze-Porrati (DGP) brane-world model is employed to study the gravitational collapse of dust, with a Gauss-Bonnet (GB) term present in the five-dimensional bulk. We find that, within the normal (non self-accelerating) DGP branch and due to the curvature effects from the GB component on the brane, the black hole singularity acquires modified features. More precisely, during collapse and for a finite comoving time, before a singularity would emerge at the zero physical radius, the first time derivative of the Hubble rate diverges, whereas the brane energy density and the Hubble rate remain finite. This is a peculiar behaviour which displays similar properties to the sudden singularity occurring in particular late-time cosmological frameworks. Furthermore, the question whether this altered singularity can be visible to an external observer or it will be hidden by a black hole horizon, is addressed. We establish that, depending on the given induced-gravity parameter and the GB coupling constant, there exists a {\em threshold mass}, for the collapsing dust, below which no trapped surfaces evolve as the collapse proceeds towards the singularity. In other words, a {\em naked sudden singularity} may form.

Deformations of GR, Geometrodynamics and Reality Conditions. (arXiv:2012.05706v1 [gr-qc])

Authors: Kirill KrasnovErmis Mitsou

In four dimensions complexified General Relativity (GR) can be non-trivially deformed: There exists an (infinite-parameter) set of modifications all having the same count of degrees of freedom. It is trivial to impose reality conditions that give versions of the deformed theories corresponding to Riemannian and split metric signatures. We revisit the Lorentzian signature case. To make the problem tractable, we restrict our attention to a four-parameter set of deformations that are natural extensions of Ashtekar’s Hamiltonian formalism for GR. The Hamiltonian of the later is a linear combination of $EEE$ and $EEB$. We consider theories for which the Hamiltonian constraint is a general linear combination of $EEE, EEB, EBB$ and $BBB$. Our main result is the computation of the evolution equations for the modified theories as geometrodynamics evolution equations for the 3-metric. We show that only for GR (and the related theory of Self-Dual Gravity) these equations close in the sense that they can be written in terms of only the metric and its first time derivative. Modified theories are therefore seen to be essentially non-metric in the sense that their dynamics cannot be reduced to geometrodynamics. We then show this to be related to the problem with Lorentzian reality conditions: the conditions of reality of the 3-metric and its time derivative are not acceptable because they are not preserved by the dynamics. Put differently, their conservation implies extra reality conditions on higher-order time derivatives, which then leaves no room for degrees of freedom.

Lessons from the Information Paradox. (arXiv:2012.05770v1 [hep-th])

Authors: Suvrat Raju

Missing the point in noncommutative geometry

Huggett, Nick and Lizzi, Fedele and Menon, Tushar (2020) Missing the point in noncommutative geometry. [Preprint]

Quantum Nonlocality in Networks Can Be Demonstrated with an Arbitrarily Small Level of Independence between the Sources

Author(s): Ivan Šupić, Jean-Daniel Bancal, and Nicolas Brunner

Quantum nonlocality can be observed in networks even in the case where every party can only perform a single measurement, i.e., does not receive any input. So far, this effect has been demonstrated under the assumption that all sources in the network are fully independent from each other. Here we in…

[Phys. Rev. Lett. 125, 240403] Published Wed Dec 09, 2020

No facts without perspectives

Abstract

Perner and Roessler (in: Aguilar J, Buckareff A (eds) Causing human action: new perspectives on the causal theory of action, MIT Press, Cambridge, MA, pp 199–228, 2010) hold that children who do not yet have an understanding of subjective perspectives, i.e., mental states, explain actions by appealing to objective facts. In this paper, we criticize this view. We argue that in order to understand objective facts, subjects need to understand perspectives. By analysing basic fact-expressing assertions, we show that subjects cannot refer to facts if they do not understand two types of perspectivity, namely, spatial and doxastic perspectivity. To avoid conceptual confusion regarding different ways of referring to facts, we distinguish between reference to facts de re and de dicto.

Many-body Delocalization via Emergent Symmetry

Author(s): N. S. Srivatsa, Roderich Moessner, and Anne E. B. Nielsen

Many-body localization (MBL) provides a mechanism to avoid thermalization in many-body quantum systems. Here, we show that an emergent symmetry can protect a state from MBL. Specifically, we propose a Z2 symmetric model with nonlocal interactions, which has an analytically known, SU(2) invariant, cr…

[Phys. Rev. Lett. 125, 240401] Published Tue Dec 08, 2020

Collectivist Foundations for Bayesian Statistics

Mayo-Wilson, Conor and Saraf, Aditya (2020) Collectivist Foundations for Bayesian Statistics. [Preprint]

Ad hoc identity, Goyal complementarity, and counting quantum phenomena

Jantzen, Benjamin C. (2020) Ad hoc identity, Goyal complementarity, and counting quantum phenomena. [Preprint]

Classical Logic in Quantum Context

Oldofredi, Andrea (2020) Classical Logic in Quantum Context. [Preprint]