Abstract

This paper shows that the Clauser–Horne–Shimony–Holt test of locality of correlations which was originally designed to be used with binary observables can actually be used for any couples of quantum-like bounded continuous observables, and then for any experimental situation describable within the mathematical framework of quantum theory.

Abstract

Everett’s Relative State Interpretation has gained increasing interest due to the progress of understanding the role of decoherence. In order to fulfill its promise as a realistic description of the physical world, two postulates are formulated. In short they are (1) for a system with continuous coordinates \({\mathbf {x}}\), discrete variable j, and state \(\psi _j({\mathbf {x}})\), the density \(\rho _j({\mathbf {x}})=|\psi _j({\mathbf {x}})|^2\) gives the distribution of the location of the system with the respect to the variables \({\mathbf {x}}\) and j; (2) an equation of motion for the state \(i\hbar \partial _t \psi = H\psi\). The first postulate connects the mathematical description to the physical reality, which has been missing in previous versions. The contents of the standard (Copenhagen) postulates are derived, including the appearance of Hilbert space and the Born rule. The approach to probabilities earlier proposed by Greaves replaces the classical probability concept in the Born rule. The new quantum probability concept, earlier advocated by Deutsch and Wallace, is void of the requirement of uncertainty.

Abstract

Recently, Frauchiger and Renner proposed a Gedankenexperiment, which was claimed to be able to prove that quantum theory cannot consistently describe the use of itself. Here we show that the conclusions of Frauchiger and Renner actually came from their incorrect description of some quantum states. With the correct description there will be no inconsistent results, no matter which quantum interpretation theory is used. Especially, the Copenhagen interpretation can satisfy all the three assumptions (C), (Q), and (S) of Frauchiger and Renner simultaneously, thus it has no problem consistently describing the use of itself.

Abstract

The essence of the path integral method in quantum physics can be expressed in terms of two relations between unitary propagators, describing perturbations of the underlying system. They inherit the causal structure of the theory and its invariance properties under variations of the action. These relations determine a dynamical algebra of bounded operators which encodes all properties of the corresponding quantum theory. This novel approach is applied to non-relativistic particles, where quantum mechanics emerges from it. The method works also in interacting quantum field theories and sheds new light on the foundations of quantum physics.

Abstract

QBism is one of the main candidates for an epistemic interpretation of quantum mechanics. According to QBism, the quantum state or the wavefunction represents the subjective degrees of belief of the agent assigning the state. But, although the quantum state is not part of the furniture of the world, quantum mechanics grasps the real via the Born rule which is a consistency condition for the probability assignments of the agent. In this paper, we evaluate the plausibility of recent criticism of QBism. We focus on the consequences of the subjective character of the quantum state, the issue of realism and the problem of the evolution of the quantum state in QBism. In particular, drawing upon Born’s notion of invariance as the mark of the real, it is argued that there is no essential difference between Einstein’s program of ‘the real’ and QBists’ realism. Also, it will be argued that QBism can account for the unitary evolution of the quantum state.

We propose a superconducting wire array that realizes a family of quantum Hamiltonians that possess combinatorial gauge symmetry — a local symmetry where monomial transformations play a central role. This physical system exhibits a rich structure. In the classical limit its ground state consists of two superimposed spin liquids; one is a crystal of small loops containing disordered $U(1)$ degrees of freedom, and the other is a soup of loops of all sizes associated to $\mathbb{Z}_2$ topological order. We show that the classical results carry over to the quantum case when fluctuations are gradually tuned via the wire capacitances, yielding ${\mathbb Z}_2$ quantum topological order. In an extreme quantum limit where the capacitances are all small, we arrive at an effective quantum spin Hamiltonian that we conjecture would sustain ${\mathbb Z}_2$ quantum topological order with a gap of the order of the Josephson coupling in the array. The principles behind the construction for superconducting arrays extends to other bosonic and fermionic systems, and offers a promising path towards topological qubits and the study of other many-body systems.

Here, we show the implementation of a complete cycle of a quantum engine fuelled by information. This engine is a quantum version of the Szilard engine, where information is used to extract heat from the environment and fully convert it into work. In our experiment, this work is used to make a weight, initially in the ground state, reach its excited state. We measure the energy and the state of each component of the engine, after each step of the cycle, and compare them with the theoretical prediction to show that the cycle is implemented with high precision. We also perform experiments to show that the engine is well isolated from the environment after the heat extraction, and we measure the entropy of the weight to show the full conversion of heat into work. Thus, we successfully demonstrate that information can be used as a fuel for single-reservoir engines.

We consider quantum systems with causal dynamics in discrete spacetimes, also known as quantum cellular automata (QCA). Due to time-discreteness this type of dynamics is not characterized by a Hamiltonian but by a one-time-step unitary. This can be written as the exponential of a Hamiltonian but in a highly non-unique way. We ask if any of the Hamiltonians generating a QCA unitary is local in some sense, and we obtain two very different answers. On one hand, we present an example of QCA for which all generating Hamiltonians are fully non-local, in the sense that interactions do not decay with the distance. On the other hand, we show that all one-dimensional quasi-free fermionic QCAs have quasi-local generating Hamiltonians, with interactions decaying exponentially in the massive case and algebraically in the critical case. We also prove that some integrable systems do not have local, quasi-local nor low-weight constants of motion; a result that challenges the standard classification of integrability.

We discuss the quantum mechanical description of a gravitational wave interacting with a cavity electromagnetic field. Quantum fluctuations of the gravitational vacuum induce squeezing in the optical field. Moreover, this squeezing experiences revivals, a purely quantum effect. Measuring these gravitationally induced revivals, although out of reach from experiments, would provide evidence on the quantum nature of gravity. We also discuss the quantum mechanical treatment of the interaction between coherent and squeezed gravitational wave states and a gravity wave detector. In the case of a coherent gravitational wave, we reproduce the result from the classical theory with a quantum mechanical calculation. The case of a squeezed gravity wave is not calculable within the classical theory, and could provide evidence on the quantum nature of gravity.

Recently a certain conceptual puzzle in the AdS/CFT correspondence, concerning the growth of quantum circuit complexity and the wormhole volume, has been identified by Bouland-Fefferman-Vazirani and Susskind. In this note, we propose a resolution of the puzzle and save the quantum Extended Church-Turing thesis by arguing that there is no computational shortcut in measuring the volume due to gravitational backreaction from bulk observers. A certain strengthening of the firewall puzzle from the computational complexity perspective, as well as its potential resolution, is also presented.

Authors: Kristina Šekrst

This paper deals with the philosophical issues of the notion of nothingness and pre-inflationary stage of the universe in physical cosmology. We presuppose that, in addition to cosmological limits, there may be both anthropic and computational limits for our ability to understand and replicate the conditions before the Big Bang. That is, the very notion of nothingness and pre-Big Bang state may be conceptually, but not computationally grasped.

Authors: Aniello Lampo, Michele Mancarella, Angelo Piga

The impact of Machine Learning (ML) algorithms in the age of big data and platform capitalism has not spared scientific research in academia. In this work, we will analyse the use of ML in fundamental physics and its relationship to other cases that directly affect society. We will deal with different aspects of the issue, from a bibliometric analysis of the publications, to a detailed discussion of the literature, to an overview on the productive and working context inside and outside academia. The analysis will be conducted on the basis of three key elements: the non-neutrality of science, understood as its intrinsic relationship with history and society; the non-neutrality of the algorithms, in the sense of the presence of elements that depend on the choices of the programmer, which cannot be eliminated whatever the technological progress is; the problematic nature of a paradigm shift in favour of a data-driven science (and society). The deconstruction of the presumed universality of scientific thought from the inside becomes in this perspective a necessary first step also for any social and political discussion. This is the subject of this work in the case study of ML.

Authors: Marius Oltean, Hossein Bazrafshan Moghaddam, Richard J. Epp

Quasilocal definitions of stress-energy-momentum—that is, in the form of boundary densities (rather than local volume densities)—have proven generally very useful in formulating and applying conservation laws in general relativity. In this paper, we present a first detailed application of such definitions to cosmology, specifically using the Brown-York quasilocal stress-energy-momentum tensor for matter and gravity combined. We compute this tensor, focusing on the energy and its associated conservation laws, for FLRW spacetimes with no pertubrations and with scalar cosmological perturbations. We show how our results recover or relate to the more typical effective local treatment of energy in cosmology, with a view towards better studying the issues of the cosmological constant and of cosmological back-reactions.

Authors: Piotr T. Grochowski, Alexander R. H. Smith, Andrzej Dragan, Kacper Dębski

Quantum time dilation occurs when a clock moves in a superposition of relativistic momentum wave packets. We utilize the lifetime of an excited hydrogen-like atom as a clock to demonstrate how quantum time dilation manifests in a spontaneous emission process. The resulting emission rate differs when compared to the emission rate of an atom prepared in a mixture of momentum wave packets at order $v^2/c^2$. This effect is accompanied by a quantum correction to the Doppler shift due to the coherence between momentum wave packets. This quantum Doppler shift affects the spectral line shape at order $v/c$. However, its effect on the decay rate is suppressed when compared to the effect of quantum time dilation. We argue that spectroscopic experiments offer a technologically feasible platform to explore the effects of quantum time dilation.

Authors: Jun-Jie Wei, Xue-Feng Wu

A nonzero-mass hypothesis for the photon can produces a frequency-dependent dispersion of light, which results in arrival-time differences of photons with different frequencies originating from a given transient source. Extragalactic fast radio bursts (FRBs), with their low frequency emissions, short time durations, and long propagation distances, are excellent astrophysical probes to constrain the rest mass of the photon $m_{\gamma}$. However, the derivation of a limit on $m_{\gamma}$ is complicated by the similar frequency dependences of dispersion expected from the plasma and nonzero photon mass effects. If a handful measurements of redshift for FRBs are available, the different redshift dependences of the plasma and photon mass contributions to the dispersion measure (DM) might be able to break dispersion degeneracy in testing the photon mass. For now, nine FRBs with redshift measurements have been reported, which can turn this idea into reality. Taking into account the DM contributions from both the plasma and a possible photon mass, we use the data on the nine FRBs to derive a combined limit of $m_{\gamma}\leq7.1\times10^{-51}\;{\rm kg}$, or equivalently $m_{\gamma}\leq4.0\times10^{-15}\; {\rm eV}/c^{2}$ at 68\% confidence level, which is essentially as good as or represents a factor of 7 improvement over previous limits obtained by the single FRBs. Additionally, a reasonable estimation for the DM contribution from the host galaxy, $\rm DM_{host}$, can be simultaneously achieved in our analysis. The rapid progress in localizing FRBs will further tighten the constraints on both $m_{\gamma}$ and $\rm DM_{host}$.

Publication date: Available online 17 June 2020

Source: Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics

Author(s): Chris Smeenk

sober, elliott (2020) Contrastive Causal Explanation and the Explanatoriness of Deterministic and Probabilistic Hypotheses Theories. [Preprint]

Crowther, Karen and Linnemann, Niels and Wuthrich, Christian (2020) Spacetime functionalism in general relativity and quantum gravity. [Preprint]

Niestegge, Gerd (2020) Local tomography and the role of the complex numbers in quantum mechanics. Proc. R. Soc. A. 476:20200063.

Nature Physics, Published online: 18 June 2020; doi:10.1038/s41567-020-0966-x

Author Correction: A brief history of nuclear fusion

Baras, Dan and Shenker, Orly R. (2020) Calling for Explanation: The Case of the Thermodynamic Past State. [Preprint]

Evans, Peter W. and Thebault, Karim P Y (2019) What can bouncing oil droplets tell us about quantum mechanics? [Preprint]

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Sega, Daniel D. and Galviz, Daniel (0220) Are the notions of past, present and future compatible with the General Theory of Relativity? [Preprint]

Chen, Eddy Keming (2020) Bell’s Theorem, Quantum Probabilities, and Superdeterminism. [Preprint]

Nature Physics, Published online: 15 June 2020; doi:10.1038/s41567-020-0933-6

The quantum effective action describing non-equilibrium dynamics of a many-body system can be inferred from experiment using analogue quantum simulators. Here is an example of how it works for a quasi-one-dimensional spinor Bose gas out of equilibrium.

Nature Physics, Published online: 15 June 2020; doi:10.1038/s41567-020-0924-7

The arrangement of a sequence of stimuli affects how humans perceive information. Here, the authors show experimentally that humans perceive information in a way that depends on the network structure of stimuli.

Romano, Davide (2020) Multi-field and Bohm’s theory. [Preprint]

De Fabrizio, Livio (2020) Physics Does It Better: So Why Be Afraid of Philosophy? [Preprint]

Huggett, Nick and Matsubara, Keizo (2020) Lost Horizon? – Modeling Black Holes in String Theory. [Preprint]

Karakostas, Vassilios and Zafiris, Elias (2020) On the Structure and Function of Scientific Perspectivism in Categorical Quantum Mechanics. The British Journal for the Philosophy of Science. ISSN 1464-3537