Weekly Papers on Quantum Foundations (7)

Quantum correlations and ergotropy. (arXiv:2202.05050v1 [quant-ph])

Understanding the role of classical and quantum correlations in work extraction is a problem of fundamental importance in thermodynamics. We approach this problem by considering that, in closed quantum systems, the maximum cyclic work extractable is equal to the ergotropy. Thus, we aim to identify and investigate the contributions to the ergotropy coming from different kinds of initial correlations (total, classical, discord and entanglement correlations). In particular, our results suggest that only discord correlations always give a positive contribution to work extraction.

Entanglement of Local Hidden States. (arXiv:2107.12944v2 [quant-ph] UPDATED)

Steering criteria are conditions whose violation excludes the possibility of describing the observed measurement statistics with local hidden state (LHS) models. When the available data do not allow to exclude arbitrary LHS models, it may still be possible to exclude LHS models with a specific separability structure. Here, we derive experimentally feasible criteria that put quantitative bounds on the multipartite entanglement of LHS. Our results reveal that separable states may contain hidden entanglement that can be unlocked by measurements on another system, even if no steering between the two systems is possible.

Generalising Aumann’s Agreement Theorem. (arXiv:2202.02156v2 [quant-ph] UPDATED)

Aumann’s celebrated theorem says that a group of agents who once shared a common prior probability distribution cannot assign different posteriors to a given proposition, should these agents have common knowledge about their posteriors. In other words, rational agents cannot agree to disagree. Aumann’s agreement theorem was one of the first attempts to formalise and explore the role played by common knowledge in decision theory. Recently, we have seen a resurfacing of the debate around possible (quantum) extensions of Aumann’s results. This paper contributes to this discussion. First, as expected, we argue that agreeing to disagree is impossible in quantum theory. Secondly, and based on the quantum argument, we show that agreeing to disagree is also forbidden in any generalised probability theory.

Recollections of the Feynman Lectures in Physics. (arXiv:2202.05210v1 [physics.hist-ph])

Authors: James Hartle

Brief recollections by the author about how he contributed to the production of the Feynman Lectures in Physics

Mitigating the quantum hype. (arXiv:2202.01925v3 [physics.soc-ph] UPDATED)

Authors: Olivier Ezratty

We are in the midst of quantum hype with some excessive claims of quantum computing potential, many vendors’ and even some research organizations’ exaggerations, and a funding frenzy for very low technology readiness level startups. Governments are contributing to this hype with their large quantum initiatives and their technology sovereignty aspirations. Technology hypes are not bad per se since they create emulation, drive innovations and also contribute to attracting new talents. It works as scientists and vendors deliver progress and innovation on a continuous basis after a so-called peak of expectations. It fails with exaggerated overpromises and underdeliveries that last too long. It could cut short research and innovation funding, creating some sort of quantum winter. After looking at the shape and form of technology and science hypes and driving some lessons from past hypes, we investigate the current quantum hype and its specifics. We find that, although there is some significant uncertainty on the potential to create real scalable quantum computers, the scientific and vendor fields are relatively sane and solid compared to other technology hypes. The vendors hype has some profound and disruptive impact on the organization of fundamental research. Also, quantum technologies comprise other fields like quantum telecommunications and quantum sensing with a higher technology readiness level, which are less prone to hype. We then make some proposals to mitigate the potential negative effects of the current quantum hype including recommendations on scientific communication to strengthen the trust in quantum science, vendor behavior improvements, benchmarking methodologies, public education and putting in place a responsible research and innovation approach.

A covariant formulation of Relativistic Mechanics. (arXiv:2202.04658v1 [gr-qc])

Authors: Miguel Correia

Accretion disks surrounding compact objects, and other environmental factors, deviate satellites from geodesic motion. Unfortunately, setting up the equations of motion for such relativistic trajectories is not as simple as in Newtonian mechanics. Here, we propose a simple method aimed at generating physically accurate covariant 4-forces. We apply this method to several conservative and dissipative forces. In particular, we compute the drag due to gravitational and hard-sphere collisions in dust, gas and radiation media. We recover and covariantly extend known forces such as Epstein drag, Chandrasekhar’s dynamical friction and Poynting-Robertson drag. Variable-mass effects are also considered, namely Hoyle-Lyttleton accretion and the variable-mass rocket. We conclude with two applications: 1. The free-falling spring. We find that Hooke’s law amounts to an effective Anti-de Sitter tidal force; 2. Black hole infall with drag. We numerically compute some trajectories on a Schwarzschild background supporting a dust-like accretion disk.

A tale of tails through generalized unitarity. (arXiv:2202.04674v1 [hep-th])

Authors: Alex EdisonMichèle Levi

We introduce a novel framework to study high-order gravitational effects on a binary from the scattering of its emitted gravitational radiation. Here we focus on the radiation-reaction due to the background of the binary’s gravitational potential, namely on the so-called tail effects. We start from the effective field theory of a binary composite particle, and through multi-loop and generalized unitarity methods, we derive the effective action of the dynamical multipoles, the energy spectrum, and the observable flux due to these effects. We proceed through the third subleading such radiation-reaction effect — at the four-loop level and the seventh order in post-Newtonian gravity — shedding new light on the higher-order effects, and completing the state of the art.

Initial data on big bang singularities. (arXiv:2202.04919v1 [gr-qc])

Authors: Hans Ringström

The goal of this article is to parametrise solutions to Einstein’s equations with big bang singularities and quiescent asymptotics. To this end, we introduce a notion of initial data on big bang singularities and conjecture that it can be used to parametrise quiescent solutions. A mathematical statement of the conjecture presupposes a precise definition of the class of quiescent solutions as well as a proof of existence and uniqueness of developments corresponding to initial data on a big bang singularity. We provide one definition of quiescence here. We also appeal to existing results in order to illustrate that, in certain cases, there are unique developments corresponding to initial data on the singularity. However, our perspective leads to a large class of open problems corresponding to the general conjecture. An additional benefit of the notion of initial data developed here is that it can be used to give a unified perspective on the existing results concerning quiescent singularities. In fact, we provide several examples of how existing results can be considered to be special cases of the framework developed here. A second, potential, application is to oscillatory and spatially inhomogeneous big bang singularities. Considering the existing arguments in the spatially homogeneous setting, a crucial first step in the study of oscillatory behaviour is to understand how solutions approach the Kasner circle along a stable manifold, and then depart via an unstable manifold. In order to carry out a similar analysis in the spatially inhomogeneous setting, it is of central importance to first identify the stable manifold. Building on the work of Fournodavlos and Luk, we here propose such an identification.

Quantum Gravity and Phenomenology: Dark Matter, Dark Energy, Vacuum Selection, Emergent Spacetime, and Wormholes. (arXiv:2202.05104v1 [hep-th])

We discuss the relevance of quantum gravity to the frontier questions in high energy phenomenology: the problems of dark matter, dark energy, and vacuum selection as well as the problems of emergent spacetime and wormholes. Dark matter and dark energy phenomenology, and the problem of vacuum selection are discussed within the context of string theory as a model of quantum gravity. Emergent spacetime and wormholes are discussed in a more general context of effective theories of quantum gravity.

Cosmology and Fundamental Physics in the Era of Gravitational-Wave Astronomy. (arXiv:2202.05105v1 [gr-qc])

Authors: Alexander C. Jenkins

The advent of gravitational-wave (GW) astronomy has presented us with a completely new means for observing the Universe, allowing us to probe its structure and evolution like never before. In this thesis, we explore three distinct but complementary avenues for using GW observations to gain new insights into cosmology and fundamental physics.

In chapter 1, we study the astrophysical GW background (AGWB): the cumulative GW signal arising from a large number of compact binary coalescences (CBCs) throughout the Universe. Since these compact binaries reside in galaxies, the AGWB contains anisotropies that trace out the large-scale structure of the cosmic matter distribution. We investigate the angular power spectrum of the AGWB, with the goal of developing predictions that can be confronted with directional AGWB searches.

In chapter 2, we calculate the nonlinear GW memory emitted by cusps and kinks on cosmic string loops, which are among the most promising cosmological sources of GWs. We show that, surprisingly, the cusp memory signal diverges for sufficiently large loops, indicating a breakdown in the validity of the weak-field description of the cusp. We then present one tentative possible solution to this divergence, in which the portion of the string surrounding the cusp collapses to form a primordial black hole (PBH).

Finally, in chapter 3 we develop a powerful new method for GW detection based on precision measurements of the orbits of binary systems. In the presence of a stochastic GW background (GWB) the trajectories of the binary’s components are perturbed, giving rise to a random walk in the system’s orbital parameters over time. We calculate the sensitivity of binary pulsars and lunar laser ranging to the GWB through this effect, and show that present data are already sensitive enough to place the strongest constraints to date in the $\mu$Hz frequency band.

Nonlocality for Generic Networks

Author(s): Marc-Olivier Renou and Salman Beigi

Bell’s theorem shows that correlations created by a single entangled quantum state cannot be reproduced classically. Such correlations are called nonlocal. They are the elementary manifestation of a broader phenomenon called network nonlocality, where several entangled states shared in a network cre…

[Phys. Rev. Lett. 128, 060401] Published Wed Feb 09, 2022

Noise phased out

Nature Physics, Published online: 07 February 2022; doi:10.1038/s41567-021-01486-8

Environmental noise can severely hinder the storage and transmission of quantum information. Experiments now reveal that trapped ions are promising candidates for reliable quantum memories.