# Weekly Papers on Quantum Foundations (45)

Gravitational Decoherence: A Thematic Overview. (arXiv:2111.02462v1 [gr-qc])

Gravitational decoherence (GD) refers to the effects of gravity in actuating the classical appearance of a quantum system. Because the underlying processes involve issues in general relativity (GR), quantum field theory (QFT) and quantum information, GD has fundamental theoretical significance. There is a great variety of GD models, many of them involving physics that diverge from GR and/or QFT. This overview has two specific goals along one central theme: (i) present theories of GD based on GR and QFT and explore their experimental predictions; (ii) place other theories of GD under the scrutiny of GR and QFT, and point out their theoretical differences. We also describe how GD experiments in space in the coming decades can provide evidences at two levels: a) discriminate alternative quantum theories and non-GR theories; b) discern whether gravity is a fundamental or an effective theory.

Spectra of Neutron Wave Functions in Earth’s Gravitational Field. (arXiv:2111.02769v1 [quant-ph])

The time evolution of a quantum wave packet in the linear gravity potential is known as Quantum Bouncing Ball. The qBounce collaboration recently observed such a system by dropping wave packets of ultracold neutrons by a height of roughly 30 microns. In this article, space and momentum spectra as well as Wigner functions of the neutron wave functions in the gravitational field of the Earth are analyzed. We investigate the quantum states in the “preparation region”, into which they transition after exiting a narrow double-mirror system and where we would expect to observe free fall and bounces in classical physics. For this, we start from the stationary solutions and eigenvalues of the Schr\”odinger equation in terms of Airy functions and their zeros. Subsequently, we examine space and momentum distributions as well as Wigner functions in phase space for pure and mixed quantum states. The eventual influence of Yukawa-like forces for small distances of several micrometers from the mirror is included through first order perturbation calculations. Those allow us to study the resulting modifications of space and momentum distributions, and phase space functions.

Is there a classical model of Wigner’s friend?. (arXiv:2111.02807v1 [quant-ph])

“Wigner’s friend” refers to a quantum process of which different observers, following the rules of quantum mechanics, give contradictory descriptions. Lostaglio and Bowles have recently claimed to describe a classical system showing the same effect. It is argued that this claim is not justified.

Genuine hidden nonlocality without entanglement: from the perspective of local discrimination. (arXiv:2111.02891v1 [quant-ph])

Quantum nonlocality without entanglement is a fantastic phenomenon in quantum theory. This kind of quantum nonlocality is based on the task of local discrimination of quantum states. Recently, Bandyopadhyay and Halder [arXiv:2104.11933] studied the problem: is there any set of orthogonal states which can be locally distinguishable, but under some orthogonality preserving local measurement, each outcome will lead to a locally indistinguishable set. We say that the set with such property has hidden nonlocality. Moreover, if such phenomenon can not arise from discarding subsystems which is termed as local irredundancy, we call it genuine hidden nonlocality. There, they presented several sets of entangled states with genuine hidden nonlocality. However, they doubted the existence of a set without entanglement but with genuine hidden nonlocality. In this paper, we eliminate this doubt by constructing a series of sets without entanglement but whose nonlocality can be genuinely activated. We derive a method to tackle with the local irredundancy problem which is a key tricky for the systems whose local dimensions are composite numbers. As Bandyopadhyay and Halder have been pointed out, sets with genuine hidden nonloclity would lead to some applications on the data hiding.

The de Broglie-Bohm Quantum Theory and its Application to Quantum Cosmology. (arXiv:2111.03057v1 [gr-qc])

We review the de Broglie-Bohm quantum theory. It is an alternative description of quantum phenomena in accordance with all the quantum experiments already performed. Essentially, it is a dynamical theory about objectively real trajectories in the configuration space of the physical system under investigation. Hence, it is not necessarily probabilistic, and it dispenses with the collapse postulate, making it suitable to be applied to cosmology. The emerging cosmological models are usually free of singularities, with a bounce connecting a contracting era with an expanding phase, which we are now observing. A theory of cosmological perturbations can also be constructed under this framework, which can be successfully confronted with current observations, and can complement inflation or even be an alternative to it.

Non-Local Boxes for Networks. (arXiv:2102.03597v2 [quant-ph] UPDATED)

Nonlocal boxes are conceptual tools that capture the essence of the phenomenon of quantum non-locality, central to modern quantum theory and quantum technologies. We introduce network nonlocal boxes tailored for quantum networks under the natural assumption that these networks connect independent sources and do not allow signaling. Hence, these boxes satisfy the No-Signaling and Independence (NSI) principle. For the case of boxes without inputs, connecting pairs of bipartite sources and producing binary outputs, we prove that the sources and boxes producing local random outputs and maximal 2-box correlations, i.e. $E_2=\sqrt{2}-1$, $E_2^o=1$, are essentially unique.

Violation of equivalence in an accelerating atom-mirror system in the generalized uncertainty principle framework. (arXiv:2104.10531v2 [quant-ph] UPDATED)

We study the spontaneous excitation of a two-level atom in the presence of a perfectly reflecting mirror, when the atom, or the mirror, is uniformly accelerating in the framework of the generalised uncertainty principle (GUP). The quantized scalar field obeys a modified dispersion relation leading to a GUP deformed Klein-Gordon equation. The solutions of this equation with suitable boundary conditions are obtained to calculate the spontaneous excitation probability of the atom for the two separate cases. We show that in the case when the mirror is accelerating, the GUP modulates the spatial oscillation of the excitation probability of the atom, thus breaking the symmetry between the excitation of an atom accelerating relative to a stationary mirror, and a stationary atom excited by an accelerating mirror. An explicit violation of the equivalence principle seems to be thus manifested. We further obtain an upper bound on the GUP parameter using standard values of the system parameters.

On a Quantum Weyl Curvature Hypothesis. (arXiv:2111.02137v1 [gr-qc] CROSS LISTED)

Roger Penrose’s Weyl Curvature Hypothesis states that the Weyl curvature is small at past singularities, but not at future singularities. We review the motivations for this conjecture and present estimates for the entropy of our Universe. We then extend this hypothesis to the quantum regime by demanding that the initial state of primordial quantum fluctuations be the adiabatic vacuum in a (quasi-) de~Sitter space. We finally attempt a justification of this quantum version from a fundamental theory of quantum gravity and speculate on its consequences in the case of a classically recollapsing universe.

Complexity Equals Anything?. (arXiv:2111.02429v1 [hep-th])

We present a new infinite class of gravitational observables in asymptotically Anti-de Sitter space living on codimension-one slices of the geometry, the most famous of which is the volume of the maximal slice. We show that these observables display universal features for the thermofield-double state: they grow linearly in time at late times and reproduce the switch-back effect in shock wave geometries. We argue that any member of this class of observables is an equally viable candidate as the extremal volume for a gravitational dual of complexity.

Misinterpreting Modified Gravity as Dark Energy: a Quantitative Study. (arXiv:2111.02866v1 [astro-ph.CO])

Standard cosmological data analyses typically constrain simple phenomenological dark-energy parameters, for example the present-day value of the equation of state parameter, $w_0$, and its variation with scale factor, $w_a$. However, results from such an analysis cannot easily indicate the presence of modified gravity. Even if general relativity does not hold, experimental data could still be fit sufficiently well by a phenomenological $w_0w_a$CDM, unmodified-gravity model. Hence, it would be very useful to know if there are generic signatures of modified gravity in standard analyses. Here we present, for the first time to our knowledge, a quantitative mapping showing how modified gravity models look when (mis)interpreted within the standard unmodified-gravity analysis. Scanning through a broad space of modified-gravity (Horndeski) models, and assuming a near-future survey consisting of CMB, BAO, and SNIa observations, we report values of the best-fit set of cosmological parameters including $(w_0, w_a)$ that would be inferred if modified gravity were at work. We find that modified gravity models that can masquerade as standard gravity lead to very specific biases in standard-parameter spaces. We also comment on implications for measurements of the amplitude of mass fluctuations described by the parameter $S_8$.

Unitarily inequivalent quantum cosmological bouncing models. (arXiv:2111.02963v1 [gr-qc])

By quantising the background as well as the perturbations in a simple one fluid model, we show that there exists an ambiguity in the choice of relevant variables, potentially leading to incompatible observational physical predictions. In a classical or quantum inflationary background, the exact same canonical transformations lead to unique predictions, so the ambiguity we put forward demands a semiclassical background with a sufficiently strong departure from classical evolution. The latter condition happens to be satisfied in bouncing scenarios, which may thus be having predictability issues. Inflationary models could evade such a problem because of the monotonic behavior of their scale factor; they do, however, initiate from a singular state which bouncing scenarios aim at solving.

Wavefunction of the universe: Reparametrization invariance and field redefinitions of the minisuperspace path integral. (arXiv:2103.15168v2 [hep-th] UPDATED)

We consider the Hartle-Hawking wavefunction of the universe defined as a Euclidean path integral that satisfies the “no-boundary proposal.” We focus on the simplest minisuperspace model that comprises a single scale factor degree of freedom and a positive cosmological constant. The model can be seen as a non-linear $\sigma$-model with a line-segment base. We reduce the path integral over the lapse function to an integral over the proper length of the base and use diffeomorphism-invariant measures for the ghosts and the scale factor. As a result, the gauge-fixed path integral is independent of the gauge. However, we point out that all field redefinitions of the scale factor degree of freedom yield different choices of gauge-invariant path-integral measures. For each prescription, we compute the wavefunction at the semi-classical level and find a different result. We resolve in each case the ambiguity in the form of the Wheeler-DeWitt equation at this level of approximation. By imposing that the Hamiltonians associated with these possibly distinct quantum theories are Hermitian, we determine the inner products of the corresponding Hilbert spaces and find that they lead to a universal norm, at least semi-classically. Quantum predictions are thus independent of the prescription at this level of approximation. Finally, all wavefunctions of the Hilbert spaces of the minisuperspace model we consider turn out to be non-normalizable, including the no-boundary states.

Complete Incompatibility, Support Uncertainty, and Kirkwood-Dirac Nonclassicality

Author(s): Stephan De Bièvre

For quantum systems with a finite dimensional Hilbert space of states, we show that the complete incompatibility of two observables—a notion we introduce—is equivalent to the large support uncertainty of all states. The Kirkwood-Dirac (KD) quasiprobability distribution of a state—which depends on th…

[Phys. Rev. Lett. 127, 190404] Published Fri Nov 05, 2021

Drawing Scales Apart: The Origins of Wilson’s Conception of Effective Field Theories

Rivat, Sébastien (2021) Drawing Scales Apart: The Origins of Wilson’s Conception of Effective Field Theories.

Troubles with mathematical contents

Facchin, Marco (2021) Troubles with mathematical contents. [Preprint]

“Fundamental” “constants” and precision tests of the standard model

Koberinski, Adam (2021) “Fundamental” “constants” and precision tests of the standard model. In: UNSPECIFIED.

A generic approach to the quantum mechanical transition probability

Niestegge, Gerd (2021) A generic approach to the quantum mechanical transition probability. [Preprint]

Closing the hole argument

Halvorson, Hans and Manchak, JB (2021) Closing the hole argument. [Preprint]

Structuralist Approaches to Bohmian Mechanics

Lorenzetti, Lorenzo (2021) Structuralist Approaches to Bohmian Mechanics. [Preprint]

Bogoliubov’s correlations confirmed

Nature Physics, Published online: 04 November 2021; doi:10.1038/s41567-021-01406-w

Interacting quantum systems are difficult to formulate theoretically, but Nikolai Bogoliubov offered a workaround more than 70 years ago that has stood the test of time. Now, correlations that are a crucial feature of his theory have been observed.

Observation of pairs of atoms at opposite momenta in an equilibrium interacting Bose gas

Nature Physics, Published online: 04 November 2021; doi:10.1038/s41567-021-01381-2

Interactions between atoms in a Bose–Einstein condensate cause quantum fluctuations and the creation of additional correlations between pairs of atoms. These effects have now been directly observed, confirming long-standing theoretical predictions.

The Darwinian Tension: Romantic Science and the Causal Laws of Nature

Greif, Hajo (2015) The Darwinian Tension: Romantic Science and the Causal Laws of Nature. Studies in History and Philosophy of Biological and Biomedical Sciences, 53. pp. 53-61. ISSN 1369-8486