# Weekly Papers on Quantum Foundations (12)

Self-Testing of a Single Quantum System: Theory and Experiment. (arXiv:2203.09003v1 [quant-ph])

Certifying individual quantum devices with minimal assumptions is crucial for the development of quantum technologies. Here, we investigate how to leverage single-system contextuality to realize self-testing. We develop a robust self-testing protocol based on the simplest contextuality witness for the simplest contextual quantum system, the Klyachko-Can-Binicio\u{g}lu-Shumovsky (KCBS) inequality for the qutrit. We establish a lower bound on the fidelity of the state and the measurements (to an ideal configuration) as a function of the value of the witness under a pragmatic assumption on the measurements we call the KCBS orthogonality condition. We apply the method in an experiment with randomly chosen measurements on a single trapped $^{40}{\rm Ca}^+$ and near-perfect detection efficiency. The observed statistics allow us to self-test the system and provide the first experimental demonstration of quantum self-testing of a single system. Further, we quantify and report that deviations from our assumptions are minimal, an aspect previously overlooked by contextuality experiments.

Kochen and Specker’s view on functional relations conflicts with the collapse postulate. (arXiv:2203.09357v1 [quant-ph])

A key ingredient of the Kochen-Specker theorem is the so-called functional composition principle, which asserts that hidden states must ascribe values to observables in a way that is consistent with all functional relations between them. This principle is motivated by the assumption that, like functions of observables in classical mechanics, a function g(A) of an observable A in quantum theory is simply a logically possible observable derived from A, and that measuring g(A) consists in measuring A and post-processing the resulting value via g. In this paper we show that, under suitable conditions, this reasonable assumption is in conflict with the collapse postulate. We then discuss possible solutions to this conflict, and we argue that the most justifiable and less radical one consists in adapting the collapse postulate by incorporating measurement contexts into it. This dependence on contexts arises for precisely the same reason why noncontextual hidden-variable models are ruled out by the Kochen-Specker theorem, namely that an observable A can typically be written as a function A=g(B)=h(C) of noncommuting observables B,C.

Superconductors and gravity. (arXiv:2203.09417v1 [gr-qc])

We review and discuss some recent developments on the unconventional interaction between superconducting systems and the local gravitational field. While it is known that gravitational perturbations (such as gravitational waves) can affect supercondensates and supercurrents dynamics, here we want to focus on the more subtle superfluid back-reaction acting on the surrounding gravitational field, analysing some specific favourable situations. To this end, we will consider suitable quantum macrosystems in a coherent state, immersed in the static weak Earth’s gravitational field, investigating possible slight local alterations of the latter not explained in terms of classical physics.

How to write a contemporary scientific article. (arXiv:2203.09405v1 [physics.ed-ph])

Authors: Vladimir M. Krasnov

Today scientists are drowned in information and have no time for reading all publications even in a specific area. The information is sifted and only a small fraction of articles is being read. Under circumstances, scientific articles have to be properly adjusted to pass through the superficial sifting. Here I present written down instructions that I used to give to my students with almost serious advises on how to write (and how not to write) a contemporary scientific article. I argue that it should tell a story and should answer on the three main questions: Why, What and So what?

What is Time? — Thoughts of a Physicist. (arXiv:2203.09415v1 [physics.hist-ph])

Authors: Gernot Münster

Discussion of some aspects of the concept of time for a general audience: nature of time, A- and B-series, time in physics, the “now”, measurement of time, definition of time units, time’s arrow, relativity of time, relativity of simultaneity.

Cosmological constraints on dark matter interactions with ordinary matter

Publication date: 26 May 2022

Source: Physics Reports, Volume 961

Author(s): Manuel A. Buen-Abad, Rouven Essig, David McKeen, Yi-Ming Zhong

Nonlocality of quantum matter corrections and cosmological constant running. (arXiv:2203.09232v1 [gr-qc])

Authors: E.V. GorbarIlya L. Shapiro

Semiclassical contributions to the gravitational action include various terms with zero, two, and four derivatives of the metric as well as nonlocal form factors for these terms. Contributions to some of these terms could be confused with others on a specific metric background or for a particular gauge fixing. We present a critical analysis of the recent works where the tensor structure and the number of derivatives in the action of gravity were not properly taken into account. Taking these relevant aspects into account, we show that although some contributions owing to the quantum fluctuations of massive or massless scalar as well as fermion and vector fields may be attributed to the “running” of the cosmological constant, in reality they correspond to the fourth derivative terms of the action.

Maximal acceleration and black hole evaporation. (arXiv:2203.09483v1 [gr-qc])

Authors: Ricardo Gallego Torromé

We discuss how in certain theories of spacetime admitting a maximal proper acceleration Hawking radiation does not completely evaporate the black hole. The black hole remnant’s mass depends on the inverse of the maximal acceleration. Furthermore, as a consequence of a duality between the minimum mass that a Schwarzschild black hole can have in such spacetimes and the maximal acceleration, we show that in certain theories with a maximal acceleration there must be an upper uniform bounds on the value of the force that can be exerted on test particles.

Quantum Hair and Black Hole Information. (arXiv:2112.05171v2 [hep-th] UPDATED)

Authors: Xavier CalmetStephen D.H. Hsu

It has been shown that the quantum state of the graviton field outside a black hole horizon carries information about the internal state of the hole. We explain how this allows unitary evaporation: the final radiation state is a complex superposition which depends linearly on the initial black hole state. Under time reversal, the radiation state evolves back to the original black hole quantum state. Formulations of the information paradox on a fixed semiclassical geometry describe only a small subset of the evaporation Hilbert space, and do not exclude overall unitarity.

Hidden Variables of Gravity and Geometry and the Cosmological Constant Problem. (arXiv:2202.06977v2 [hep-th] UPDATED)

Authors: Nemanja Kaloper

We extend General Relativity by adding $4$-form field strengths to the gravitational sector of the theory. This promotes Planck scale and the cosmological constant into integration constants of these $4$-forms. When we include the charges of the $4$-forms, these `constants’ can jump discretely from region to region. We explain how the cosmological constant problem can be solved in this framework. When the cosmological constant picks up contributions from two different $4$-forms, with an irrational ratio of charges, the spectrum of its values is a very fine discretuum. If the membrane charges satisfy $\frac{2\kappa_{\tt eff}^2 \kappa^2 |{\cal Q}_i|}{3{\cal T}^2_i} < 1$, the discharge processes populating this discretuum dynamically relax $\Lambda$, ceasing as $\Lambda$ approaches zero. Thus the theory exponentially favors a huge hierarchy $\Lambda/\mpl^4 \ll 1$ instead of $\Lambda/\mpl^4 \simeq 1$.

Enhancing Gravitational Interaction between Quantum Systems by a Massive Mediator

Author(s): Julen S. Pedernales, Kirill Streltsov, and Martin B. Plenio

In 1957 Feynman suggested that the quantum or classical character of gravity may be assessed by testing the gravitational interaction due to source masses in superposition. However, in all proposed experimental realizations using matter-wave interferometry, the extreme weakness of this interaction r…

[Phys. Rev. Lett. 128, 110401] Published Tue Mar 15, 2022

The Chemical Bond is a Real Pattern

Seifert, Vanessa A. (2022) The Chemical Bond is a Real Pattern. [Preprint]

On systems of coordinate transformations and wave mechanics

Lyons, Tony (2022) On systems of coordinate transformations and wave mechanics. [Preprint]

Cailin O’Connor and James Owen Weatherall’s The Misinformation Age

Nash, Erin (2019) Cailin O’Connor and James Owen Weatherall’s The Misinformation Age. BJPS Review of Books.

Walter Ott and Linda Patton’s Laws of Nature

Hildebrand, Tyler (2019) Walter Ott and Linda Patton’s Laws of Nature. BJPS Review of Books.