Authors: William Donnelly

The holographic principle posits that all quantum information in a region of spacetime is encoded on its boundary. While there is strong evidence for this principle in certain models of quantum gravity in asymptotically anti-de Sitter spacetime, it is yet to be established whether holography is a generic feature of quantum gravity, or a peculiar property of these models. The goal of the present work is to present a model of holographic reconstruction in the framework of perturbative quantum gravity in flat spacetime. Specifically, we consider a state in the single-particle sector of a quantum field theory and give a method to completely reconstruct the quantum state from measurement of the metric at spatial infinity. Our argument uses a relativistic generalization of the quantum-mechanical Wigner function, and gives an explicit mechanism by which the gravitational constraints encode quantum information holographically on the boundary. Moreover, it suggests how information about more general states might be recovered from soft charges at null infinity, with applications to the black hole information loss paradox.

Authors: Grzegorz Plewa

Inspired by ER=EPR conjecture, we search for a connection between quantum entanglement and geometry. Starting with several simple postulates, we present a formalism providing decomposition of the vacuum of quantum harmonic oscillator into two maximally entangled states in such a way that both can be interpreted geometrically. The interpretation is based solely on the form of the states, and it reveals certain structures at the boundary and in the bulk of $AdS_3$. The former correspond to world lines of massless particles at the boundary. The latter resemble worldsheets of interacting closed strings.

Authors: Tanmay Vachaspati, George Zahariade

We work in the Heisenberg picture to demonstrate the classical-quantum correspondence (CQC) in which the dynamics of a quantum variable is equivalent to that of a complexified classical variable. The correspondence provides a tool for analyzing quantum backreaction problems which we illustrate by a toy model in which a rolling particle slows down due to quantum radiation. The dynamics found using the CQC is in excellent agreement with that found using the much more laborious full quantum analysis.

Authors: Laura Felline

In his recent book Bananaworld. Quantum mechanics for primates, Jeff Bub revives and provides a mature version of his influential information theoretic interpretation of Quantum Theory (QT). In this paper, I test Bub s conjecture that QT should be interpreted as a theory about information, by examining whether his information theoretic interpretation has the resources to explain (or explain away) quantum conundrums. The discussion of Bub s theses will also serve to investigate, more in general, whether other approaches succeed in defending the claim that QT is about quantum information. First of all, I argue that Bub s interpretation of QT as a principle theory fails to fully explain quantum nonlocality. Secondly, I argue that a constructive interpretation, where the quantum state is interpreted ontically as information, also fails at providing a full explanation of quantum correlations. Finally, while epistemic interpretations might succeed in this respect, I argue that such a success comes at the price of rejecting some in between the most basic scientific standards of physical theories.

Authors: Johannes Mesa Pascasio, Siegfried Fussy, Herbert Schwabl, Gerhard Groessing

In the quest for an understanding of nonlocality with respect to an appropriate ontology, we propose a "cosmological solution". We assume that from the beginning of the universe each point in space has been the location of a scalar field representing a zero-point vacuum energy that nonlocally vibrates at a vast range of different frequencies across the whole universe. A quantum, then, is a nonequilibrium steady state in the form of a "bouncer" coupled resonantly to one of those (particle type dependent) frequencies, in remote analogy to the bouncing oil drops on an oscillating oil bath as in Couder's experiments. A major difference to the latter analogy is given by the nonlocal nature of the vacuum oscillations.

We show with the examples of double- and $n$-slit interference that the assumed nonlocality of the distribution functions alone suffices to derive the de Broglie-Bohm guiding equation for $N$ particles with otherwise purely classical means. In our model, no influences from configuration space are required, as everything can be described in 3-space. Importantly, the setting up of an experimental arrangement limits and shapes the forward and osmotic contributions and is described as vacuum landscaping.

Authors: Anastasios Y. Papaioannou

Using the language of the Geometric Algebra, we recast the massive Dirac bispinor as a set of Lorentz scalar, vector, bivector, pseudovector, and pseudoscalar fields that obey a generalized form of Maxwell's equations of electromagnetism. This field-based formulation requires careful distinction between geometric and non-geometric implementations of the imaginary unit scalar in the Dirac algebra. This distinction, which is obscured in conventional treatments, allows us to find alternative constructions of the field bilinears and a more natural interpretation of the discrete C, P, and T transformations.

Authors: Laura Felline

The aim of this paper is to explore the ways in which Axiomatic Reconstructions of Quantum Theory in terms of Information-Theoretic principles (ARQITs) can contribute to explaining and understanding quantum phenomena, as well as to study their explanatory limitations. This is achieved in part by offering an account of the kind of explanation that axiomatic reconstructions of quantum theory provide, and re-evaluating the epistemic status of the program in light of this explanation. As illustrative cases studies, I take Clifton's, Bub's and Halvorson's characterization theorem and Popescu's and Rohrlich's toy models, and their explanatory contribution with respect to quantum non-locality. On the one hand, I argue that ARQITs can aspire to provide genuine explanations of (some aspects of) quantum non-locality. On the other hand, I argue that such explanations cannot rule out a mechanical quantum theory.

Authors: Tomas Zimmermann

A formulation of quantum mechanics is introduced based on a $2D$-dimensional phase-space wave function $\text{\reflectbox{\text{p}}}\mkern-3mu\text{p}\left(q,p\right)$ which might be computed from the position-space wave function $\psi\left(q\right)$ with a transformation related to the Gabor transformation. The equation of motion for conservative systems can be written in the form of the Schr\"{o}dinger equation with a $4D$-dimensional Hamiltonian with classical terms on the diagonal and complex off-diagonal couplings. The Hamiltonian does not contain any differential operators and the quantization is achieved by replacing $q$ and $p$ with $2D$-dimensional counterparts $\left(q+q'\right)/2$ and $\left(p+p'\right)/2$ and by using a complex-valued factor $e^{i\left(q\cdot p'-q'\cdot p\right)/2}$ in phase-space integrals. Despite the fact that the formulation increases the dimensionality, it might provide a way towards exact multi-dimensional computations as it may be evaluated directly with Monte-Carlo algorithms.

Authors: Luciana Henaut, Lorenzo Catani, Dan E. Browne, Shane Mansfield, Anna Pappa

We introduce a simple single-system game inspired by the Clauser-Horne-Shimony-Holt (CHSH) game. For qubit systems subjected to unitary gates and projective measurements, we prove that any strategy in our game can be mapped to a strategy in the CHSH game, which implies that Tsirelson's bound also holds in our setting. More generally, we show that the optimal success probability depends on the reversible or irreversible character of the gates, the quantum or classical nature of the system and the system dimension. We analyse the bounds obtained in light of Landauer's principle, showing the entropic costs of the erasure associated with the game. This shows a connection between the reversibility in fundamental operations embodied by Landauer's principle and Tsirelson's bound, that arises from the restricted physics of a unitarily-evolving single-qubit system.

Authors: Orchidea MariaLecian

The features of correlated Michelson interferometers are for describing the analysis of Einsteinian spacetime models, and the quantum geometries pertinent with descriptions of GR compatible with particle Physics. Such apparati allow for the spectral decomposition of fractional Planck-scale displacements correlations and fractional Plancktime- interval correlations for kinematical investigations in particle Physics on emerging Minkowski background, and for models which admit GR as a limit after cosmological implementations for Poincar\'e particles content.

Publication date: 25 August 2018

**Source:**Physics Letters A, Volume 382, Issue 33

Author(s): Anna M. Nobili, Alberto Anselmi

Tests of the Weak Equivalence Principle (WEP) probe the foundations of physics. Ever since Galileo in the early 1600s, WEP tests have attracted some of the best experimentalists of any time. Progress has come in bursts, each stimulated by the introduction of a new technique: the torsion balance, signal modulation by Earth rotation, the rotating torsion balance. Tests for various materials in the field of the Earth and the Sun have found no violation to the level of about 1 part in 10^{13}. A different technique, Lunar Laser Ranging (LLR), has reached comparable precision. Today, both laboratory tests and LLR have reached a point when improving by a factor of 10 is extremely hard. The promise of another quantum leap in precision rests on experiments performed in low Earth orbit. The Microscope satellite, launched in April 2016 and currently taking data, aims to test WEP in the field of Earth to ${10}^{-15}$, a 100-fold improvement possible thanks to a driving signal in orbit almost 500 times stronger than for torsion balances on ground. The ‘Galileo Galilei’ (GG) experiment, by combining the advantages of space with those of the rotating torsion balance, aims at a WEP test 100 times more precise than Microscope, to ${10}^{-17}$. A quantitative comparison of the key issues in the two experiments is presented, along with recent experimental measurements relevant for GG. Early results from Microscope, reported at a conference in March 2017, show measurement performance close to the expectations and confirm the key role of rotation with the advantage (unique to space) of rotating the whole spacecraft. Any non-null result from Microscope would be a major discovery and call for urgent confirmation; with 100 times better precision GG could settle the matter and provide a deeper probe of the foundations of physics.

Author(s): Anna M. Nobili, Alberto Anselmi

Author(s): Yuan Yuan, Zhibo Hou, Kang-Da Wu, Guo-Yong Xiang, Chuan-Feng Li, and Guang-Can Guo

When a photon passes through an interferometer, quantum mechanics does not provide a clear answer as to its past. Quantum retrodiction is a quantitative theory, which endeavors to make statements about the past of a system based on present knowledge. Quantum retrodiction may be used to analyze the p...

[Phys. Rev. A 97, 062115] Published Fri Jun 15, 2018

Author(s): Dong-Ling Deng

Machine learning, the core of artificial intelligence and big data science, is one of today’s most rapidly growing interdisciplinary fields. Recently, machine learning tools and techniques have been adopted to tackle intricate quantum many-body problems. In this Letter, we introduce machine learning...

[Phys. Rev. Lett. 120, 240402] Published Thu Jun 14, 2018

Wuthrich, Christian (2018) When the actual world is not even possible. [Preprint]

A quantum state’s entanglement across a bipartite cut can be quantified with entanglement entropy or, more generally, Schmidt norms. Using only Schmidt decompositions, we present a simple iterative algorithm to maximize Schmidt norms. Depending on the choice of norm, the optimizing states maximize or minimize entanglement, possibly across several bipartite cuts at the same time and possibly only among states in a specified subspace. Recognizing that convergence but not success is certain, we use the algorithm to explore topics ranging from fermionic reduced density matrices and varieties of pure quantum states to absolutely maximally entangled states and minimal output entropy of channels.

Author(s): Raam Uzdin and Saar Rahav

The second law of thermodynamics can be described using the Clausius inequality, the main link between classical and quantum thermodynamics. A new thermodynamic framework addresses long-standing limitations of this inequality and reveals new bounds relevant to quantum technology experiments.

[Phys. Rev. X 8, 021064] Published Tue Jun 12, 2018

Recently it was shown that certain fluid-mechanical ‘pilot-wave’ systems can strikingly mimic a range of quantum properties, including single particle diffraction and interference, quantization of angular momentum etc. How far does this analogy go? The ultimate test of (apparent) quantumness of such systems is a Bell-test. Here the premises of the Bell inequality are re-investigated for particles accompanied by a pilot-wave, or more generally by a resonant ‘background’ field. We find that two of these premises, namely outcome independence and measurement independence, may not be generally valid when such a background is present. Under this assumption the Bell inequality is possibly (but not necessarily) violated. A class of hydrodynamic Bell experiments is proposed that could test this claim. Such a Bell test on fluid systems could provide a wealth of new insights on the different loopholes for Bell’s theorem. Finally, it is shown that certain properties of background-based theories can be illustrated in Ising spin-lattices.

Walsh, Kirsten (2017) Newton: from certainty to probability? [Preprint]

Szabo, Laszlo E. (2017) Meaning, Truth, and Physics. In G. Hofer-Szabó, L. Wronski (eds.) Making it Formally Explicit.

Conveying the quantum world is the ultimate challenge for artist duo Semiconductor, who turn the most abstruse scientific observations into captivating sensory experiences

Author(s): Jun Gao, Lu-Feng Qiao, Zhi-Qiang Jiao, Yue-Chi Ma, Cheng-Qiu Hu, Ruo-Jing Ren, Ai-Lin Yang, Hao Tang, Man-Hong Yung, and Xian-Min Jin

Quantum information technologies provide promising applications in communication and computation, while machine learning has become a powerful technique for extracting meaningful structures in “big data.” A crossover between quantum information and machine learning represents a new interdisciplinary...

[Phys. Rev. Lett. 120, 240501] Published Mon Jun 11, 2018

Colombo, Matteo and Elkin, Lee and Hartmann, Stephan (2018) Being Realist about Bayes, and the Predictive Processing Theory of Mind. [Preprint]

Chen, Eddy Keming (2018) Quantum Mechanics in a Time-Asymmetric Universe: On the Nature of the Initial Quantum State. [Preprint]

Authors: Chris Nagele, Ebubechukwu O. Ilo-Okeke, Peter P. Rohde, Jonathan P. Dowling, Tim Byrnes

The entanglement swapping protocol is analyzed in a relativistic setting, where shortly after the entanglement swapping is performed, a Bell violation measurement is performed. From an observer in the laboratory frame, a Bell violation is observed due to entanglement swapping taking place, but in a moving frame the order of the measurements is reversed, and a Bell violation is observed even though no entanglement is present. Although the measurement results are identical, the wavefunctions for the two frames are different--- one is entangled and the other is not. Furthermore, for boosts in a perpendicular direction, in the presence of decoherence, we show that a maximum Bell violation can occur across non-simultaneous points in time. This is a signature of entanglement that is spread across both space and time, showing both the non-local and non-simultaneous feature of entanglement.

Authors: David Schmid, Katja Reid, Robert W. Spekkens

The common wisdom in the field of quantum information theory is that when a system is initially correlated with its environment, the map describing its evolution may fail to be completely positive. If true, this would have practical and foundational significance. We here demonstrate, however, that the common wisdom is mistaken. We trace the error to the standard argument for how the evolution map ought to be defined. We show that it sometimes fails to define a linear map or any map at all and that these pathologies persist even in completely classical examples. Drawing inspiration from the framework of classical causal models, we argue that the correct definition of the evolution map is obtained by considering a counterfactual scenario wherein the system is reprepared independently of any systems in its causal past while the rest of the circuit remains the same, yielding a map that is always completely positive. In a post-mortem on the standard argument, we highlight two distinct mistakes that retrospectively become evident (in its application to completely classical examples): (i) the types of constraints to which it appealed are constraints on what one can infer about the final state of a system based on its initial state, where such inferences are based not just on the cause-effect relation between them-which defines the correct evolution map-but also on the common cause of the two; (ii) in a (retrospectively unnecessary) attempt to introduce variability in the input state, it inadvertently introduced variability in the inference map itself, then tried to fit the input-output pairs associated to these different maps with a single map.

Wuthrich, Christian (2012) When the actual world is not even possible. [Preprint]

Moulavi Ardakani, Reza (2017) Time Reversal Invariance in Quantum Mechanics.

Christian, Joy (2018) Quantum Correlations are Weaved by the Spinors of the Euclidean Primitives. Royal Society Open Science, 5 (180526). pp. 1-40. ISSN 2054-5703

McCoy, C.D. (2018) The implementation, interpretation, and justification of likelihoods in cosmology. Studies in History and Philosophy of Modern Physics, 62. pp. 19-35. ISSN 1355-2198

McCoy, C.D. and Callender, Craig (2017) Time in Cosmology. [Preprint]

Author(s): Yu Guo, Xiao-Min Hu, Bi-Heng Liu, Yun-Feng Huang, Chuan-Feng Li, and Guang-Can Guo

Growing interest has been invested in exploring high-dimensional quantum systems, for their promising perspectives in certain quantum tasks. How to characterize a high-dimensional entanglement structure is one of the basic questions to take full advantage of it. However, it is not easy for us to cat...

[Phys. Rev. A 97, 062309] Published Thu Jun 07, 2018

Author(s): Luca Mancino, Marco Sbroscia, Emanuele Roccia, Ilaria Gianani, Valeria Cimini, Mauro Paternostro, and Marco Barbieri

The emergence of realistic properties is a key problem in understanding the quantum-to-classical transition. In this respect, measurements represent a way to interface quantum systems with the macroscopic world: these can be driven in the weak regime, where a reduced back-action can be imparted by c...

[Phys. Rev. A 97, 062108] Published Thu Jun 07, 2018

In the following paper, the author will try to test the meaning of the transcendental approach in respect of the inner changes implied by the idea of quantum gravity. He will firstly describe the basic methodological Kant’s aim, viz. the grounding of a meta-science of physics as the a priori corpus of physical knowledge. After that, he will take into account the problematic physical and philosophical relationship between the theory of relativity and the quantum mechanics; in showing how the elementary ontological and epistemological assumptions of experience result to be changed within them, he will also show the further modifications occurred in the development of the loop quantum gravity. He will particularly focus on the tough problem of the relationship space-matter, in order to settle the decisive question about the possibility of keeping a transcendental approach in the light of quantum gravity. He will positively answer by recalling Cassirer’s theory of the invariants of experience, although he will also add some problematic issues arising from the new physical context.

Wayne, Andrew (2017) Point-particle explanations: the case of gravitational waves. Synthese. ISSN 1573-0964

A theoretical breakthrough has shown that quantum computers are not just faster versions of ordinary computers, but something much stranger

Author(s): Kavan Modi, Arun Kumar Pati, Aditi Sen(De), and Ujjwal Sen

Theorists predict that quantum information can’t be securely “locked” in a vault.

[Phys. Rev. Lett. 120, 230501] Published Tue Jun 05, 2018

Author(s): Libor Caha, Zeph Landau, and Daniel Nagaj

We present a collection of results about the clock in Feynman's computer construction and Kitaev's local Hamiltonian problem. First, by analyzing the spectra of quantum walks on a line with varying end-point terms, we find a better lower bound on the gap of the Feynman Hamiltonian, which translates ...

[Phys. Rev. A 97, 062306] Published Tue Jun 05, 2018

To solve the probability problem of the Many Worlds Interpretation of Quantum Mechanics, D. Wallace has presented a formal proof of the Born rule via decision theory, as proposed by D. Deutsch. The idea is to get subjective probabilities from rational decisions related to quantum measurements, showing the non-probabilistic parts of the quantum formalism, plus some rational constraints, ensure the squared modulus of quantum amplitudes play the role of such probabilities. We provide a new presentation of Wallace’s proof, reorganized to simplify some arguments, and analyze it from a formal perspective. Similarities with classical decision theory are made explicit, to clarify its structure and main ideas. A simpler notation is used, and details are filled in, making it easier to follow and verify. Some problems have been identified, and we suggest possible corrections.

Authors: Dimitrios Krommydas

We study violations of the Null Energy Condition (NEC) in Quantum Field Theory (QFT) and their implications. For the first part of the project, we examine these violations for classes of already known and novel (first discussed here) QFT states. Next, we discuss the implications of these violations focusing on the example of Wormhole Traversability. After reviewing the current literature on the existing restrictions on these violations, we conjecture that NEC violating states are incompatible with the Semi-Classical Gravity approximation. We argue that this conjecture provides the only way out of the problems introduced by the violations of NEC in this regime. Building on this, we propose a bound that should hold for all QFT states. Finally, we show that both our conjecture and bound hold for some relevant classes of QFT states.

Authors: Roderich Tumulka

The biggest and most lasting among David Bohm's (1917-1992) many achievements is to have proposed a picture of reality that explains the empirical rules of quantum mechanics. This picture, known as pilot wave theory or Bohmian mechanics among other names, is still the simplest and most convincing explanation available. According to this theory, electrons are point particles in the literal sense and move along trajectories governed by Bohm's equation of motion. In this paper, I describe some more recent developments and extensions of Bohmian mechanics, concerning in particular relativistic space-time and particle creation and annihilation.

Authors: H.-L. Huang, Y.-H. Luo, B. Bai, Y.-H. Deng, H. Wang, H.-S. Zhong, Y.-Q. Nie, W.-H. Jiang, X.-L. Wang, J. Zhang, Li Li, Nai-Le Liu, Tim Byrnes, J. P. Dowling, Chao-Yang Lu, Jian-Wei Pan

Wheeler's delayed-choice experiment investigates the indeterminacy of wave-particle duality and the role played by the measurement apparatus in quantum theory. Due to the inconsistency with classical physics, it has been generally believed that it is not possible to reproduce the delayed-choice experiment using a hidden variable theory. Recently, it was shown that this assumption was incorrect, and in fact Wheeler's delayed-choice experiment can be explained by a causal two dimensional hidden-variable theory [R. Chaves, G. B. Lemos, and J. Pienaar, Phys. Rev. Lett. 120, 190401 (2018)]. Here, we carry out an experiment of a device-independent delayed-choice experiment using photon states that are space-like separated, and demonstrate a loophole-free version of the delayed-choice protocol that is consistent with quantum theory but inconsistent with any causal two-dimensional hidden variable theory. This salvages Wheeler's thought experiment and shows that causality can be used to test quantum theory in a complementary way to the Bell and Leggett-Garg tests.

**Quantum electrodynamics and the proton size**

Quantum electrodynamics and the proton size, Published online: 04 June 2018; doi:10.1038/s41567-018-0166-0

Tests of one of the most fundamental theories in physics reveal an issue with the size of the proton — or the Rydberg constant. Thomas Udem explains.Butterfield, Jeremy (2018) On Dualities and Equivalences Between Physical Theories. [Preprint]

Authors: J. Sperling

It is demonstrated that the collapse of the wave function is equivalent to the continuity of measurement outcomes. The latter states that a second measurement has to result in the same outcome as the first measurement of the same observable for a vanishing time between both observations. In contrast to the exclusively quantum-physical collapse description, the equivalent continuity requirement also applies in classical physics, allowing for a comparison of both domains. In particular, it is found that quantum coherences are the single cause for measurable deviations in statistical properties due to the collapse. Therefore, the introduced approach renders it possible to characterize and quantify the unique features of the quantum-physical measurement problem within the framework of modern quantum resource theories and compare them to classical physics.

Authors: Carlo Maria Scandolo, Roberto Salazar, Jarosław K. Korbicz, Paweł Horodecki

We investigate the emergence of classicality and objectivity in arbitrary physical theories. First we provide an explicit example of a theory where there are no objective states. Then we characterize classical states of generic theories, and show how classical physics emerges through a decoherence process, which always exists in causal theories as long as there are classical states. We apply these results to the study of the emergence of objectivity, here recast as a multiplayer game. In particular, we prove that the so-called Spectrum Broadcast Structure characterizes all objective states in every causal theory, in the very same way as it does in quantum mechanics. This shows that the structure of objective states is valid across an extremely broad range of physical theories. Finally we show that, unlike objectivity, the emergence of local observers is not generic among physical theories, but it is only possible if a theory satisfies two axioms that rule out holistic behavior in composite systems.

Authors: Philippe Allard Guérin, Časlav Brukner

In general relativity, the causal structure between events is dynamical, but it is definite and observer-independent; events are point-like and the membership of an event A in the future or past light-cone of an event B is an observer-independent statement. When events are defined with respect to quantum systems however, nothing guarantees that event A is definitely in the future or in the past of B. We propose to associate a causal reference frame corresponding to each event, which can be interpreted as an observer-dependent time according to which an observer describes the evolution of quantum systems. In the causal reference frame of one event, this particular event is always localised, but other events can be "smeared out" in the future and in the past. We do not impose a predefined causal order between the events, but only require that descriptions from different reference frames obey a global consistency condition. We show that our new formalism is equivalent to the pure process matrix formalism. The latter is known to predict certain multipartite correlations, which are incompatible with the assumption of a causal ordering of the events -- these correlations violate causal inequalities. We show how the causal reference frame description can be used to gain insight into the question of realisability of such strongly non-causal processes in laboratory experiments. As another application, we use causal reference frames to revisit a thought experiment where the gravitational time dilation due to a massive object in a quantum superposition of positions leads to a superposition of the causal ordering of two events.

Authors: Diederik Aerts, Massimiliano Sassoli de Bianchi, Sandro Sozzo, Tomas Veloz

Since its inception, many physicists have seen in quantum mechanics the possibility, if not the necessity, of bringing cognitive aspects into the play, which were instead absent, or unnoticed, in the previous classical theories. In this article, we outline the path that led us to support the hypothesis that our physical reality is fundamentally conceptual-like and cognitivistic-like. However, contrary to the 'abstract ego hypothesis' introduced by John von Neumann and further explored, in more recent times, by Henry Stapp, our approach does not rely on the measurement problem as expressing a possible 'gap in physical causation', which would point to a reality lying beyond the mind-matter distinction. On the contrary, in our approach the measurement problem is considered to be essentially solved, at least for what concerns the origin of quantum probabilities, which we have reasons to believe they would be epistemic. Our conclusion that conceptuality and cognition would be an integral part of all physical processes comes instead from the observation of the striking similarities between the non-spatial behavior of the quantum micro-physical entities and that of the human concepts. This gave birth to a new interpretation of quantum mechanics, called the 'conceptualistic interpretation', currently under investigation within our group in Brussels.

Authors: Uri Peleg, Lev Vaidman

The recent criticism of Vaidman's propsal for the analysis of the past of a particle in the nested interferometer is refuted. It is shown that the definition of the past of the particle adopted by Englert et al. [Phys. Rev. A 96, 022126 (2017)] is applicable only to a tiny fraction of photons in the interferometer which indeed exhibit different behaviour. Their proof that all pre- and postselected particles behave this way, i.e. follow a continuous trajectory, does not hold, because it relies on the assumption that it is intended to prove.

Authors: Adam Bednorz

In the standard quantum theory, one can measure precisely only a subset of the incompatible observables. It results in lack of a formal joint probability defining objective realism even if we accept nonlocal or certain faster-than-light interactions. We propose a construction of such realism extending the usual single-copy description to many copies, partially analogous to familiar many worlds. Failure of the standard single copy can be easily looked for experimentally. The copies should interact weakly at the macroscopic level, leading to effective collapse to a single identical pointer state. Experimental evidence for this conjecture could be obtained by detecting incomplete collapse in sequential measurements or finding deviations from the single-copy Born rule when observing simple quantum systems.

Authors: Lee Smolin

Because of the non-locality of quantum entanglement, realist approaches to completing quantum mechanics have implications for our conception of space. Quantum gravity also is expected to predict phenomena in which the locality of classical spacetime is modified or disordered. It is then possible that the right quantum theory of gravity will also be a completion of quantum mechanics in which the foundational puzzles in both are addressed together. I review here the results of a program, developed with Roberto Mangabeira Unger, Marina Cortes and other collaborators, which aims to do just that. The results so far include energetic causal set models, time asymmetric extensions of general relativity and relational hidden variables theories, including real ensemble approaches to quantum mechanics. These models share two assumptions: that physics is relational and that time and causality are fundamental.

Authors: Samuel S. Cree, Tamara M. Davis, Timothy C. Ralph, Qingdi Wang, Zhen Zhu, William G. Unruh

The cosmological constant problem arises because the magnitude of vacuum energy density predicted by quantum mechanics is $\sim 120$ orders of magnitude larger than the value implied by cosmological observations of accelerating cosmic expansion. Recently some of the current authors proposed that the stochastic nature of the quantum vacuum can resolve this tension [1]. By treating the fluctuations in the vacuum seriously, and applying a high-energy cutoff at which Quantum Field Theory is believed to break down, a parametric resonance occurs that predicts a slow expansion and acceleration. In this work we more thoroughly examine the implications of this proposal by investigating the resulting dynamics. Firstly, we improve upon the numerical calculations and show that convergence issues with the original code had overshadowed some important effects. Some of the conclusions are thus reversed, however, the premise that parametric resonance can explain a very slowly accelerating expansion remains sound. After improving the resolution and efficiency of the numerical tests, we explore a wider range of cutoff energies, and examine the effects of multiple particle fields. We introduce a simple model using Mathieu's equation, a prototypical example of parametric resonance, and find that it closely matches numerical results in regimes where its assumptions are valid. Using this model, we extrapolate to find that in a universe with $28$ bosonic fields and a high-energy cutoff $40$ times higher than the Planck energy, the acceleration would be comparable to what is observed.

Franklin, Alexander (2018) Whence the Effectiveness of Effective Field Theories? The British Journal for the Philosophy of Science.

Physics Today, Volume 71, Issue 6, Page 57-57, June 2018.

Author(s): Florian Fröwis, Pavel Sekatski, Wolfgang Dür, Nicolas Gisin, and Nicolas Sangouard

Schrödinger’s thought experiment of a cat in superposition of being dead and alive.

[Rev. Mod. Phys. 90, 025004] Published Thu May 31, 2018

Walter, Scott A. (2018) Figures of light in the early history of relativity (1905-1914). [Preprint]

El Skaf, Rawad (2018) The function and limit of Galileo’s falling bodies thought experiment: Absolute weight, Specific weight and the Medium’s resistance. [Preprint]

Biener, Zvi (2017) The Certainty, Modality, and Grounding of Newton’s Laws. The Monist, 100 (3). pp. 311-325.

Fraser, James D. (2018) Towards a Realist View of Quantum Field Theory. [Preprint]

Authors: Andrew Coates, Charles Melby-Thompson, Shinji Mukohyama

In the context of Horava gravity, the most promising known scenarios to recover Lorentz invariance at low energy are the possibilities that (1) the renormalization group flow of the system leads to emergent infrared Lorentz invariance, and (2) that supersymmetry protects infrared Lorentz invariance. A third scenario proposes that a classically Lorentz invariant matter sector with controlled quantum corrections may simply co-exist with Horava gravity under certain conditions. However, for non-projectable Horava gravity in 3+1 dimensions it is known that, in the absence of additional structures, this mechanism is spoiled by unexpected power-law divergences. We confirm this same result in the projectable version of the theory by employing the recently found gauge-fixing term that renders the shift and graviton propagators regular. We show that the problem persists for all dimensions $D\geq 3$, and that the degree of fine tuning in squared sound speeds between a U(1) gauge field and a scalar field increases with $D$. In particular, this difference in the zero external momentum limit is proportional to $\Lambda^{D-1}$ for $D\geq 3$, where $\Lambda$ is the ultraviolet momentum cutoff for loop integrals, while the power-law divergences are absent for $D=1$ and $D=2$. These results suggest that not only the gravity sector but also the matter sector should exhibit a transition to Lifshitz scaling above some scale, and that there should not be a large separation between the transition scales in the gravity and matter sectors. We close with a discussion of other more promising scenarios, including emergent Lorentz invariance from supersymmetry/strong dynamics, and pointing out challenges where they exist.

Authors: Nicolas Gisin

It is usual to identify initial conditions of classical dynamical systems with mathematical real numbers. However, almost all real numbers contain an infinite amount of information. Since a finite volume of space can't contain more than a finite amount of information, I argue that the mathematical real numbers are not physically relevant. Moreover, a better terminology for the so-called real numbers is "random numbers", as their series of bits are truly random. I propose an alternative classical mechanics, which is empirically equivalent to classical mechanics, but uses only finite-information numbers. This alternative classical mechanics is non-deterministic, despite the use of deterministic equations, in a way similar to quantum theory. Interestingly, both alternative classical mechanics and quantum theories can be supplemented by additional variables in such a way that the supplemented theory is deterministic. Most physicists straightforwardly supplement classical theory with real numbers to which they attribute physical existence, while most physicists reject Bohmian mechanics as supplemented quantum theory, arguing that Bohmian positions have no physical reality. I argue that it is more economical and natural to accept non-determinism with potentialities as a real mode of existence, both for classical and quantum physics.

Authors: Matteo Carlesso, Andrea Vinante, Angelo Bassi

Recently, a non-thermal excess noise, compatible with the theoretical prediction provided by collapse models, was measured in a millikelvin nanomechanical cantilever experiment [Vinante et al., Phys. Rev. Lett. 119, 110401 (2017)]. We propose a feasible implementation of the cantilever experiment able to probe such a noise. The proposed modification, completely within the grasp of current technology and readily implementable also in other type of mechanical non-interferometric experiments, consists in substituting the homogeneous test mass with one composed of different layers of different materials. This will enhance the action of a possible collapse noise above that given by standard noise sources.

Authors: N. David Mermin, Rüdiger Schack

We review the famous no-hidden-variables theorem in John von Neumann's 1932 book on the mathematical foundations of quantum mechanics. We describe the notorious gap in von Neumann's argument, pointed out by Grete Hermann in 1935 and, more famously, by John Bell in 1966. We disagree with recent papers claiming that Hermann and Bell failed to understand what von Neumann was actually doing.

Authors: Manabendra Nath Bera, Andreas Winter, Maciej Lewenstein

Thermodynamics and information have intricate inter-relations. The justification of the fact that information is physical, is done by inter-linking information and thermodynamics - through Landauer's principle. This modern approach towards information recently has improved our understanding of thermodynamics, both in classical and quantum domains. Here we show thermodynamics as a consequence of information conservation. Our approach can be applied to most general situations, where systems and thermal-baths could be quantum, of arbitrary sizes and even could posses inter-system correlations. The approach does not rely on an a priori predetermined temperature associated to a thermal bath, which is not meaningful for finite-size cases. Hence, the thermal-baths and systems are not different, rather both are treated on an equal footing. This results in a "temperature"-independent formulation of thermodynamics. We exploit the fact that, for a fix amount of coarse-grained information, measured by the von Neumann entropy, any system can be transformed to a state that possesses minimal energy, without changing its entropy. This state is known as a completely passive state, which assumes Boltzmann-Gibb's canonical form with an intrinsic temperature. This leads us to introduce the notions of bound and free energy, which we further use to quantify heat and work respectively. With this guiding principle of information conservation, we develop universal notions of equilibrium, heat and work, Landauer's principle and also universal fundamental laws of thermodynamics. We show that the maximum efficiency of a quantum engine, equipped with a finite baths, is in general lower than that of an ideal Carnot's engine. We also introduce a resource theoretic framework for intrinsic-temperature based thermodynamics, within which we address the problem of work extraction and state transformations.

Authors: Jochen Szangolies

In-principle restrictions on the amount of information that can be gathered about a system have been proposed as a foundational principle in several recent reconstructions of the formalism of quantum mechanics. However, it seems unclear precisely why one should be thus restricted. We investigate the notion of paradoxical self-reference as a possible origin of such epistemic horizons by means of a fixed-point theorem in Cartesian closed categories due to F. W. Lawvere that illuminates and unifies the different perspectives on self-reference.

Authors: Yakir Aharonov, Lev Vaidman

Possibility to communicate between spatially separated regions, without even a single photon passing between the two parties, is an amazing quantum phenomenon. The possibility of transmitting one value of a bit in such a way, the interaction-free measurement, was known for quarter of a century. The protocols of full communication, including transmitting unknown quantum states were proposed only few years ago, but it was shown that in all these protocols the particle was leaving a weak trace in the transmission channel, the trace larger than the trace left by a single particle passing through the channel. This made the claim of counterfactuality of these protocols at best controversial. However, a simple modification of these recent protocols eliminates the trace in the transmission channel and makes all these protocols truly counterfactual.

Myrvold, Wayne C. (2018) Ontology of Relativistic Collapse Theories. [Preprint]

Diósi, Lajos (2018) How to teach and think about spontaneous wave function collapse theories: not like before. Collapse of the Wave Function Models: Ontology, Origin, and Implications. pp. 3-11.

Author(s): S. Bose, D. Home, and S. Mal

Can the most “classical-like” of all quantum states, namely the Schrödinger coherent state of a harmonic oscillator, exhibit nonclassical behavior? We find that for an oscillating object initially in a coherent state, merely by observing at various instants which spatial region the object is in, the...

[Phys. Rev. Lett. 120, 210402] Published Fri May 25, 2018

The inherent difficulty in talking about quantum decoherence in the context of quantum cosmology is that decoherence requires subsystems, and cosmology is the study of the whole Universe. Consistent histories gave a possible answer to this conundrum, by phrasing decoherence as loss of interference between alternative histories of closed systems. When one can apply Boolean logic to a set of histories, it is deemed ‘consistent’. However, the vast majority of the sets of histories that are merely consistent are blatantly nonclassical in other respects, and further constraints than just consistency need to be invoked. In this paper, I attempt to give an alternative answer to the issues faced by consistent histories, by exploring a timeless interpretation of quantum mechanics of closed systems. This is done solely in terms of path integrals in non-relativistic, timeless, configuration space. What prompts a fresh look at such foundational problems in this context is the advent of multiple gravitational models in which Lorentz symmetry is not fundamental, but only emergent. And what allows this approach to overcome previous barriers to a timeless, conditional probabilities interpretation of quantum mechanics is the new notion of records—made possible by an inherent asymmetry of configuration space. I outline and explore consequences of this approach for foundational issues of quantum mechanics, such as the natural emergence of the Born rule, conservation of probabilities, and the Sleeping Beauty paradox.

Franklin, Alexander (2017) Whence the Effectiveness of Effective Field Theories? [Preprint]

Esfeld, Michael and Deckert, Dirk-André (2018) Authors’ Response: the virtues of minimalism in ontology and epistemology. [Preprint]

Romero, Gustavo E. and Perez, Daniela and Lopez Armengol, Federico (2018) Cosmological black holes and the direction of time. Foundations of Science, 23. pp. 415-426.

Author(s): Mohammad H. Amin, Evgeny Andriyash, Jason Rolfe, Bohdan Kulchytskyy, and Roger Melko

A new machine-learning algorithm demonstrates the performance of a quantum Boltzmann machine, a quantum extension of a popular classical neural network, paving the way for quantum approaches to machine learning.

[Phys. Rev. X 8, 021050] Published Wed May 23, 2018

Quantum mechanics imposes a fundamental trade-off between the accuracy of time measurements and the size of the systems used as clocks. When the measurements of different time intervals are combined, the errors due to the finite clock size accumulate, resulting in an overall inaccuracy that grows with the complexity of the set-up. Here, we introduce a method that, in principle, eludes the accumulation of errors by coherently transferring information from a quantum clock to a quantum memory of the smallest possible size. Our method could be used to measure the total duration of a sequence of events with enhanced accuracy, and to reduce the amount of quantum communication needed to stabilize clocks in a quantum network.

Authors: Xavier Calmet, Boris Latosh

We show that quantum gravity, whatever its ultra-violet completion might be, could account for dark matter. Indeed, besides the massless gravitational field recently observed in the form of gravitational waves, the spectrum of quantum gravity contains two massive fields respectively of spin 2 and spin 0. If these fields are long-lived, they could easily account for dark matter. In that case, dark matter would be very light and only gravitationally coupled to the standard model particles.

Authors: Steven B. Giddings

A "quantum-first" approach to gravity is described, where rather than quantizing general relativity, one seeks to formulate the physics of gravity within a quantum-mechanical framework with suitably general postulates. Important guides are the need for appropriate mathematical structure on Hilbert space, and correspondence with general relativity and quantum field theory in weak-gravity situations. A basic physical question is that of "Einstein separability:" how to define mutually independent subsystems, e.g. through localization. Standard answers via tensor products or operator algebras conflict with properties of gravity, as is seen in the correspondence limit; this connects with discussions of "soft hair." Instead, gravitational behavior suggests a networked Hilbert space structure. This structure plus unitarity provide important clues towards a quantum formulation of gravity.

Authors: Meenu Kumari, Shohini Ghose

Quantum-classical correspondence in chaotic systems is a long-standing problem. We describe a method to quantify Bohr's correspondence principle and calculate the size of quantum numbers for which we can expect to observe quantum-classical correspondence near periodic orbits of Floquet systems. Our method shows how the stability of classical periodic orbits affects quantum dynamics. We demonstrate our method by analyzing quantum-classical correspondence in the quantum kicked top (QKT), which exhibits both regular and chaotic behavior. We use our correspondence conditions to identify signatures of classical bifurcations even in a deep quantum regime. Our method can be used to explain the breakdown of quantum-classical correspondence in chaotic systems.

Authors: Scott Aaronson

We show that combining two different hypothetical enhancements to quantum computation---namely, quantum advice and non-collapsing measurements---would let a quantum computer solve any decision problem whatsoever in polynomial time, even though neither enhancement yields extravagant power by itself. This complements a related result due to Raz. The proof uses locally decodable codes.

Authors: Hans De Raedt, Mikhail I. Katsnelson, Dennis Willsch, Kristel Michielsen

It is shown that the quantum theoretical description of statistical data resulting from experiments with a finite number of different outcomes emerges by organizing the data such that the descriptions of the preparation and measurement stage are separated as much as possible. The quantum theoretical description that derives from this elementary principle of separation is void of the usual postulates/interpretations regarding "wave functions", "observables", "quantization rules", "Born's rule", and the like. The separation principle is illustrated by application to the Stern-Gerlach and the Einstein-Podolsky-Rosen-Bohm experiment and generalizes in a trivial manner. The von Neunman equation and therefore also the Schr\"odinger equation are shown to follow directly from the mathematical structure that renders possible separated descriptions.

Authors: John B. DeBrota, Christopher A. Fuchs, Blake C. Stacey

We describe a general procedure for associating a minimal informationally-complete quantum measurement (or MIC) and a set of linearly independent post-measurement quantum states with a purely probabilistic representation of the Born Rule. Such representations are motivated by QBism, where the Born Rule is understood as a consistency condition between probabilities assigned to the outcomes of one experiment in terms of the probabilities assigned to the outcomes of other experiments. In this setting, the difference between quantum and classical physics is the way their physical assumptions augment bare probability theory: Classical physics corresponds to a trivial augmentation---one just applies the Law of Total Probability (LTP) between the scenarios---while quantum theory makes use of the Born Rule expressed in one or another of the forms of our general procedure. To mark the essential difference between quantum and classical, one should seek the representations that minimize the disparity between the expressions. We prove that the representation of the Born Rule obtained from a symmetric informationally-complete measurement (or SIC) minimizes this distinction in at least two senses---the first to do with unitarily invariant distance measures between the rules, and the second to do with available volume in a reference probability simplex (roughly speaking a new kind of uncertainty principle). Both of these arise from a significant majorization result. This work complements recent studies in quantum computation where the deviation of the Born Rule from the LTP is measured in terms of negativity of Wigner functions.

Storing time from a quantum stopwatch with qubits – instead of losing accuracy by stopping and starting it – could give us the ultimate precision in timekeeping

Roy, Sudipto and Nandi, Dipika and Ghosh, Sumana and Das, Apashanka (2018) Time Evolution of Density Parameters for Matter and Dark Energy and their Interaction Term in Brans-Dicke Gravity. [Preprint]

Author(s): L. L. Yan, T. P. Xiong, K. Rehan, F. Zhou, D. F. Liang, L. Chen, J. Q. Zhang, W. L. Yang, Z. H. Ma, and M. Feng

A fundamental limit on the heat produced when erasing a bit of information has been confirmed in a fully quantum system.

[Phys. Rev. Lett. 120, 210601] Published Mon May 21, 2018

In this paper we have two aims: first, to draw attention to the close connexion between interpretation and scientific understanding; second, to give a detailed account of how theories without a spacetime can be interpreted, and so of how they can be understood. In order to do so, we of course need an account of what is meant by a theory ‘without a spacetime’: which we also provide in this paper. We describe three tools, used by physicists, aimed at constructing interpretations which are adequate for the goal of understanding. We analyse examples from high-energy physics illustrating how physicists use these tools to construct interpretations and thereby attain understanding. The examples are: the ’t Hooft approximation of gauge theories, random matrix models, causal sets, loop quantum gravity, and group field theory.

Taylor, Peter The Relation Between Classical and Quantum Mechanics. UNSPECIFIED.

Sengupta, Rakesh (2017) How embodied is time? [Preprint]

The quantum story is all brilliant insights, flawed male scientists – and few female ones. To progress, we may need to dump our prejudices, says a new book

Authors: Abraham Loeb (Harvard)

Does the collision of black hole singularities imprint an observable quantum signature on the resulting gravitational wave signal?

Authors: James B. Hartle (UCSB and SFI)

A brief remembrance of some aspects of the author's scientific interaction with Stephen Hawking. A contribution to Physics Today's March 14, 2018 web page in which Stephen Hawking is remembered by his colleagues.

Authors: John E. Carlstrom, Thomas M. Crawford, Lloyd Knox

Temperature and polarization variations across the microwave sky include the fingerprints of quantum fluctuations in the early universe. They may soon reveal physics at unprecedented energy scales.

Viebahn, Emanuel (2018) Presentism, eternalism and where things are located. [Preprint]

Lazarovici, Dustin and Oldofredi, Andrea and Esfeld, Michael (2018) Observables and unobservables in quantum mechanics: How the no-hidden-variables theorems support the Bohmian particle ontology. [Preprint]

Author(s): M. Bahrami

Collapse models postulate that space is filled with a collapse noise field, inducing quantum Brownian motions, which are dominant during the measurement, thus causing collapse of the wave function. An important manifestation of the collapse noise field, if any, is thermal energy generation, thus dis...

[Phys. Rev. A 97, 052118] Published Fri May 18, 2018

Author(s): Ana Belén Sainz, Yelena Guryanova, Antonio Acín, and Miguel Navascués

To identify which principles characterize quantum correlations, it is essential to understand in which sense this set of correlations differs from that of almost-quantum correlations. We solve this problem by invoking the so-called no-restriction hypothesis, an explicit and natural axiom in many rec...

[Phys. Rev. Lett. 120, 200402] Published Fri May 18, 2018

American Journal of Physics, Volume 86, Issue 6, Page 412-416, June 2018.

ROVELLI, Carlo (2018) Physics Needs Philosophy. Philosophy Needs Physics. Foundations of Physics, 48. pp. 481-491. ISSN 0015-9018

Menon, Tushar (2018) Taking up superspace---the spacetime structure of supersymmetric field theory. [Preprint]