Latest Papers on Quantum Foundations - Updated Daily by IJQF


In the last 20 years or so, since the publication of a seminal paper by Watts and Strogatz (Nature 393(6684):440–442, 1998), an interest in topological explanations (Huneman in Synthese 177:213–245, 2010) has spread like a wild fire over many areas of science, e.g. ecology, evolutionary biology, medicine, and cognitive neuroscience. The topological approach is still very young by all standards, and even within special sciences it still doesn’t have a single methodological programme that is applicable across all areas of science. That is why this special issue is important as a first systematic philosophical study of topological explanations and their relation to a well understood and widespread explanatory strategy, such as mechanistic one.

Ladyman, James and Robertson, Katie (2013) Landauer Defended: Reply to Norton. Studies in History and Philosophy of Modern Physics. ISSN 1355-2198

Authors: Merab Gogberashvili

It is widely believed that quantum particles can cross a black hole event horizons. This conclusion is based on the assumption that by specific singular coordinate transformations it is possible to remove divergences in the geodesic equations. We note that, while the used singular coordinate transformations do not cause problems on the level of the classical geodesic equations, which contain the first derivatives of particle wavefunctions, they usually lead to the appearance of delta-functions in the equations of quantum particles.

In this paper, using physical boundary conditions for the equation of motion of quantum particles close to the black hole event horizon, it is found the real-valued exponentially time-dependent (not harmonic) wavefunctions. This means that quantum particles probably do not enter the Schwarzschild sphere, but are absorbed and some are reflected by it, what potentially can solve the main black hole mysteries.

Authors: Aleksandra Dimić, Marko Milivojević, Dragoljub Gočanin, Časlav Brukner

The realization of indefinite causal order, a theoretical possibility that even the causal order of events in spacetime can be subjected to quantum superposition, apart from its general significance for the fundamental physics research, would also enable quantum information processing that outperforms protocols in which the underlying causal structure is definite. In this paper, we propose a realization of a simple case of indefinite causal order - the "quantum switch" - by entangling two communicating Rindler laboratories. Additionally, we obtain a quantum superposition of "direct-cause" and "common-cause" causal structures. For this we exploit the fact that two Rindler observers cannot communicate with each other if their worldlines belong to space-like separated wedges of a given light cone in Minkowski spacetime. Furthermore, we show that by using the "double Rindler quantum switch" one can realize entangled causal structures.

Authors: Avi Marchewka, Zeev Schuss

We derive rigorously the short-time escape probability of a quantum particle from its compactly supported initial state, which has a discontinuous derivative at the boundary of the support. We show that this probability is liner in time, which seems to be a new result. The novelty of our calculation is the inclusion of the boundary layer of the propagated wave function formed outside the initial support. This result has applications to the decay law of the particle, to the Zeno behavior, quantum absorption, time of arrival, quantum measurements, and more, as will be discussed separately.

Authors: Jayne Thompson, Andrew J. P. Garner, John R. Mahoney, James P. Crutchfield, Vlatko Vedral, Mile Gu

Causal asymmetry is one of the great surprises in predictive modelling: the memory required to predictive the future differs from the memory required to retrodict the past. There is a privileged temporal direction for modelling a stochastic process where memory costs are minimal. Models operating in the other direction incur an unavoidable memory overhead. Here we show that this overhead can vanish when quantum models are allowed. Quantum models forced to run in the less natural temporal direction not only surpass their optimal classical counterparts, but also any classical model running in reverse time. This holds even when the memory overhead is unbounded, resulting in quantum models with unbounded memory advantage.


A Gedanken experiment is presented where an excited and a ground-state atom are positioned such that, within the former’s half-life time, they exchange a photon with 50% probability. A measurement of their energy state will therefore indicate in 50% of the cases that no photon was exchanged. Yet other measurements would reveal that, by the mere possibility of exchange, the two atoms have become entangled. Consequently, the “no exchange” result, apparently precluding entanglement, is non-locally established between the atoms by this very entanglement. This quantum-mechanical version of the ancient Liar Paradox can be realized with already existing transmission schemes, with the addition of Bell’s theorem applied to the no-exchange cases. Under appropriate probabilities, the initially-excited atom, still excited, can be entangled with additional atoms time and again, or alternatively, exert multipartite nonlocal correlations in an interaction free manner. When densely repeated several times, this result also gives rise to the Quantum Zeno effect, again exerted between distant atoms without photon exchange. We discuss these experiments as variants of interaction-free-measurement, now generalized for both spatial and temporal uncertainties. We next employ weak measurements for elucidating the paradox. Interpretational issues are discussed in the conclusion, and a resolution is offered within the Two-State Vector Formalism and its new Heisenberg framework.

ROVELLI, Carlo (2017) "Space is blue and birds fly through it". [Preprint]
Quentin, Ruyant (2017) Can we make sense of relational quantum mechanics? [Preprint]
New “qubit” designs could enable more robust quantum machines

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Author(s): Thomas Callister, A. Sylvia Biscoveanu, Nelson Christensen, Maximiliano Isi, Andrew Matas, Olivier Minazzoli, Tania Regimbau, Mairi Sakellariadou, Jay Tasson, and Eric Thrane

Now that gravitational-wave detection is a reality, measurements of gravitational-wave polarization could provide crucial tests of alternatives to the general theory of relativity. A new analysis provides a way to extract polarizations from the stochastic gravitational-wave background and investigates how additional detectors could provide constraints on theories of gravity.

[Phys. Rev. X 7, 041058] Published Thu Dec 07, 2017

Steeger, Jeremy and Teh, Nicholas (2017) Two Forms of Inconsistency in Quantum Foundations. [Preprint]
Chen, Eddy Keming (2017) Time's Arrow in a Quantum Universe I: On the Simplicity and Uniqueness of the Initial Quantum State. [Preprint]

Authors: Tomáš Gonda, Ravi Kunjwal, David Schmid, Elie Wolfe, Ana Belén Sainz

Ernst Specker considered a particular feature of quantum theory to be especially fundamental, namely that pairwise joint measurability implies global joint measurability for sharp measurements [ (2009)]. To date, it seemed that Specker's principle failed to single out quantum theory from the space of all general probabilistic theories. In particular, consistent exclusivity --- an important consequence of Specker's principle --- is satisfied by both quantum and almost quantum correlations. Here, we identify another statistical implication of Specker's principle besides consistent exclusivity, which possibly holds for almost quantum correlations. However, our main result asserts that Specker's principle cannot be satisfied in any theory that yields almost quantum models.

Authors: Stephen L. Adler

Heating induced by the noise postulated in wave function collapse models leads to a lower bound to the temperature of solid objects. For the noise parameter values $\lambda ={\rm coupling~strength}\sim 10^{-8} {\rm s}^{-1}$ and $r_C ={\rm correlation~length} \sim 10^{-5} {\rm cm}$, which were suggested \cite{adler1} to make latent image formation an indicator of wave function collapse and which are consistent with the recent experiment of Vinante et al. \cite{vin}, the effect may be observable. For metals, where the heat conductivity is proportional to the temperature at low temperatures, the lower bound (specifically for RRR=30 copper) is $\sim 5\times 10^{-11} (L/r_C) $K, with L the size of the object. For the thermal insulator Torlon 4203, the comparable lower bound is $\sim 3 \times 10^{-6} (L/r_c)^{0.63}$ K. We first give a rough estimate for a cubical metal solid, and then give an exact solution of the heat transfer problem for a sphere.

Author(s): Graeme Pleasance and Barry M. Garraway

Quantum Darwinism extends the traditional formalism of decoherence to explain the emergence of classicality in a quantum universe. A classical description emerges when the environment tends to redundantly acquire information about the pointer states of an open system. In light of recent interest, we...

[Phys. Rev. A 96, 062105] Published Tue Dec 05, 2017

Authors: Marian Kupczynski

Recent experiments allowed concluding that Bell-type inequalities are indeed violated thus it is important to understand what it means and how can we explain the existence of strong correlations between outcomes of distant measurements. Do we have to announce that: Einstein was wrong, Nature is nonlocal and nonlocal correlations are produced due to the quantum magic and emerge, somehow, from outside space time? Fortunately such conclusions are unfounded because if supplementary parameters describing measuring instruments are correctly incorporated in a theoretical model then Bell-type inequalities may not be proven .We construct a simple probabilistic model explaining these correlations in a locally causal way. In our model measurement outcomes are neither predetermined nor produced in irreducibly random way. We explain in detail why, contrary to the general belief; an introduction of setting dependent parameters does not restrict experimenters' freedom of choice. Since the violation of Bell-type inequalities does not allow concluding that Nature is nonlocal and that quantum theory is complete thus the Bohr-Einstein quantum debate may not be closed. The continuation of this debate is not only important for a better understanding of Nature but also for various practical applications of quantum phenomena.

Authors: Karen Crowther

Relationships between current theories, and relationships between current theories and the sought theory of quantum gravity (QG), play an essential role in motivating the need for QG, aiding the search for QG, and defining what would count as QG. Correspondence is the broad class of inter-theory relationships intended to demonstrate the necessary compatibility of two theories whose domains of validity overlap, in the overlap regions. The variety of roles that correspondence plays in the search for QG are illustrated, using examples from specific QG approaches. Reduction is argued to be a special case of correspondence, and to form part of the definition of QG. Finally, the appropriate account of emergence in the context of QG is presented, and compared to conceptions of emergence in the broader philosophy literature. It is argued that, while emergence is likely to hold between QG and general relativity, emergence is not part of the definition of QG, and nor can it serve usefully in the development and justification of the new theory.

Nguyen, James and Teh, Nicholas J. and Wells, Laura (2017) Why surplus structure is not superfluous. [Preprint]
Candiotto, Laura (2017) The reality of relations. [Preprint]
Crowther, Karen (2017) Inter-theory Relations in Quantum Gravity: Correspondence, Reduction, and Emergence. [Preprint]
Robertson, Douglas (2017) Brane Theory, Presentism and the “Now” Problem. [Preprint]

Author(s): Justin Dressel, Areeya Chantasri, Andrew N. Jordan, and Alexander N. Korotkov

We investigate the statistical arrow of time for a quantum system being monitored by a sequence of measurements. For a continuous qubit measurement example, we demonstrate that time-reversed evolution is always physically possible, provided that the measurement record is also negated. Despite this r...

[Phys. Rev. Lett. 119, 220507] Published Fri Dec 01, 2017

Authors: Antonio Accioly, Wallace Herdy

The equivalence principle (EP), as well as Schiff's conjecture, are discussed (en passant), and the connection between the EP and quantum mechanics is then briefly analyzed. Two semiclassical violations of the classical equivalence principle (CEP) but not of the weak one (WEP), i.e. Greenberger gravitational Bohr atom and the tree-level scattering of different quantum particles by an external weak higher-order gravitational field, are thoroughly investigated afterwards. Next, two quantum examples of systems that agree with the WEP but not with the CEP, namely COW experiment and free fall in a constant gravitational field of a massive object described by its wave-function $\Psi$, are discussed in detail. Keeping in mind that among the four examples focused on this work only COW experiment is based on an experimental test, some important details related to it, are presented as well.

Authors: Art Hobson

This paper reviews and suggests a resolution of the problem of definite outcomes of measurement. This problem, also known as "Schrodinger's cat," has long posed an apparent paradox because the state resulting from a measurement appears to be a quantum superposition in which the detector is in two macroscopically distinct states (alive and dead in the case of the cat) simultaneously. Many alternative interpretations of the quantum mathematical formalism, and several alternative modifications of the theory, have been proposed to resolve this problem, but no consensus has formed supporting any one of them. Applying standard quantum theory to the measurement state, together with the analysis and results of decades of nonlocality experiments with pairs of entangled systems, this paper shows the entangled measurement state is not a paradoxical macroscopic superposition of states. It is instead a phase-dependent superposition of correlations between states of the subsystems. Thus Schrodinger's cat is a non-paradoxical "macroscopic correlation" in which one of the two correlated systems happens to be a detector. This insight resolves the problem of definite outcomes but it does not entirely resolve the measurement problem because the entangled state is still reversible.

Physics Today, Volume 70, Issue 12, Page 12-13, December 2017.
Jean Perrins proof in the early 20th century of the reality of atoms and molecules is often taken as an exemplary form of robustness reasoning, where an empirical result receives validation if it is generated using multiple experimental approaches. In this paper, I describe in detail Perrins style of reasoning, and locate both qualitative and quantitative forms of argumentation. Particularly, I argue that his quantitative style of reasoning has mistakenly been viewed as a form of robustness reasoning, whereas I believe it is something different, what I call calibration. From this perspective, I re-evaluate recent interpretations of Perrin provided by Stathis Psillos, Peter Achinstein, Alan Chalmers, and Bas van Fraassen, all of whom read Perrin as a robustness reasoner, though not necessarily in the same sort of way. I then argue that by viewing Perrin as a calibration reasoner we gain a better understanding of why he believes himself to have established the reality of atoms and molecules. To conclude, I provide an alternative and more productive understanding of the basis of the dispute between realists and anti-realists.


This paper focuses on estimating real and quantum potentials from financial commodities. The log returns of six common commodities are considered. We find that some phenomena, such as the vertical potential walls and the time scale issue of the variation on returns, also exists in commodity markets. By comparing the quantum and classical potentials, we attempt to demonstrate that the information within these two types of potentials is different. We believe this empirical result is consistent with the theoretical assumption that quantum potentials (when embedded into social science contexts) may contain some social cognitive or market psychological information, while classical potentials mainly reflect ‘hard’ market conditions. We also compare the two potential forces and explore their relationship by simply estimating the Pearson correlation between them. The Medium or weak interaction effect may indicate that the cognitive system among traders may be affected by those ‘hard’ market conditions.


It is well known that the process of quantization—constructing a quantum theory out of a classical theory—is not in general a uniquely determined procedure. There are many inequivalent methods that lead to different choices for what to use as our quantum theory. In this paper, I show that by requiring a condition of continuity between classical and quantum physics, we constrain and inform the quantum theories that we end up with.


Recently the method based on irreducible representations of finite groups has been proposed as a tool for investigating the more sophisticated versions of Bell inequalities (V. Ugǔr Gűney, M. Hillery, Phys. Rev. A90, 062121 ([2014]) and Phys. Rev. A91, 052110 ([2015])). In the present paper an example based on the symmetry group S 4 is considered. The Bell inequality violation due to the symmetry properties of regular tetrahedron is described. A nonlocal game based on the inequalities derived is described and it is shown that the violation of Bell inequality implies that the quantum strategies outperform their classical counterparts.


Peter Mittelstaedt’s contributions to quantum logic and to the foundational problems of quantum theory have significantly realized the most authentic spirit of the International Quantum Structures Association: an original research about hard technical problems, which are often “entangled” with the emergence of important changes in our general world-conceptions. During a time where both the logical and the physical community often showed a skeptical attitude towards Birkhoff and von Neumann’s quantum logic, Mittelstaedt brought into light the deeply innovating features of a quantum logical thinking that allows us to overcome some strong and unrealistic assumptions of classical logical arguments. Later on his intense research on the unsharp approach to quantum theory and to the measurement problem stimulated the increasing interest for unsharp forms of quantum logic, creating a fruitful interaction between the work of quantum logicians and of many-valued logicians. Mittelstaedt’s general views about quantum logic and quantum theory seem to be inspired by a conjecture that is today more and more confirmed: there is something universal in the quantum theoretic formalism that goes beyond the limits of microphysics, giving rise to interesting applications to a number of different fields.


The long-lasting problem of proper mathematical representation of conjunctions and disjunctions in quantum logics is reviewed and three recent proposals of solutions are described.

Maxwell, Nicholas (2917) Could Inelastic Interactions Induce Quantum Probabilistic Transitions? [Preprint]
Fraser, James D. (2017) Renormalization and the Formulation of Scientific Realism. [Preprint]


By assuming a deterministic evolution of quantum systems and taking realism into account, we carefully build a hidden variable theory for Quantum Mechanics (QM) based on the notion of ontological states proposed by ’t Hooft (The cellular automaton interpretation of quantum mechanics, arXiv:1405.1548v3, 2015; Springer Open 185,, 2016). We view these ontological states as the ones embedded with realism and compare them to the (usual) quantum states that represent superpositions, viewing the latter as mere information of the system they describe. Such a deterministic model puts forward conditions for the applicability of Bell’s inequality: the usual inequality cannot be applied to the usual experiments. We build a Bell-like inequality that can be applied to the EPR scenario and show that this inequality is always satisfied by QM. In this way we show that QM can indeed have a local interpretation, and thus meet with the causal structure imposed by the Theory of Special Relativity in a satisfying way.

A 53-qubit quantum simulator is the biggest of its kind, and has already done some calculations that outperform most, if not all, classical computers

Probing many-body dynamics on a 51-atom quantum simulator

Nature 551, 7682 (2017). doi:10.1038/nature24622

Authors: Hannes Bernien, Sylvain Schwartz, Alexander Keesling, Harry Levine, Ahmed Omran, Hannes Pichler, Soonwon Choi, Alexander S. Zibrov, Manuel Endres, Markus Greiner, Vladan Vuletić & Mikhail D. Lukin

Controllable, coherent many-body systems can provide insights into the fundamental properties of quantum matter, enable the realization of new quantum phases and could ultimately lead to computational systems that outperform existing computers based on classical approaches. Here we demonstrate a method for creating controlled many-body

We show that the Bohmian approach in terms of persisting particles that move on continuous trajectories following a deterministic law can be literally applied to quantum field theory. By means of the Dirac sea modelexemplified in the electron sector of the standard model neglecting radiationwe explain how starting from persisting particles, one is led to standard QFT employing creation and annihilation operators when tracking the dynamics with respect to a reference state, the so-called vacuum. Since on the level of wave functions, both formalisms are mathematically equivalent, this proposal provides for an ontology of QFT that includes a dynamics of individual processes, solves the measurement problem, and explains the appearance of creation and annihilation events.
  • 1Bohmian Mechanics from Quantum Mechanics to Quantum Field Theory
  • 2The Dirac Sea Model
  • 3Equilibrium States and the Vacuum
  • 4Excitations of the Vacuum and the Fock Space Formalism
  • 5The Appearance of Particle Creations and Annihilations
  • 6The Merits of the Bohmian Approach
Niestegge, Gerd (2016) Quantum key distribution without the wavefunction. International Journal of Quantum Information, 15 (6).
Kallfelz, William (2015) Ontic Structural Realism and Natural Necessity. [Preprint]

Quantum physics dropwise

Quantum physics dropwise, Published online: 27 November 2017; doi:10.1038/s41567-017-0015-6

Classical wave-driven particles can mimic basic quantum properties, but how far this parallel extends is yet to be seen. Evidence for quantum-like mirages in a system of droplets moving on a fluid surface pushes the analogy into many-body territory.
Publication date: November 2017
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, Volume 60
Author(s): Jaume Navarro, Alexander Blum, Christoph Lehner

Publication date: Available online 22 November 2017
Source:Physics Reports
Author(s): Jose Beltrán Jiménez, Lavinia Heisenberg, Gonzalo J. Olmo, Diego Rubiera-Garcia
General Relativity has shown an outstanding observational success in the scales where it has been directly tested. However, modifications have been intensively explored in the regimes where it seems either incomplete or signals its own limit of validity. In particular, the breakdown of unitarity near the Planck scale strongly suggests that General Relativity needs to be modified at high energies and quantum gravity effects are expected to be important. This is related to the existence of spacetime singularities when the solutions of General Relativity are extrapolated to regimes where curvatures are large. In this sense, Born–Infeld inspired modifications of gravity have shown an extraordinary ability to regularise the gravitational dynamics, leading to non-singular cosmologies and regular black hole spacetimes in a very robust manner and without resorting to quantum gravity effects. This has boosted the interest in these theories in applications to stellar structure, compact objects, inflationary scenarios, cosmological singularities, and black hole and wormhole physics, among others. We review the motivations, various formulations, and main results achieved within these theories, including their observational viability, and provide an overview of current open problems and future research opportunities.

Interfacing fundamentally different quantum systems is key to building future hybrid quantum networks. Such heterogeneous networks offer capabilities superior to those of their homogeneous counterparts, as they merge the individual advantages of disparate quantum nodes in a single network architecture. However, few investigations of optical hybrid interconnections have been carried out, owing to fundamental and technological challenges such as wavelength and bandwidth matching of the interfacing photons. Here we report optical quantum interconnection of two disparate matter quantum systems with photon storage capabilities. We show that a quantum state can be transferred faithfully between a cold atomic ensemble and a rare-earth-doped crystal by means of a single photon at 1,552  nanometre telecommunication wavelength, using cascaded quantum frequency conversion. We demonstrate that quantum correlations between a photon and a single collective spin excitation in the cold atomic ensemble can be transferred to the solid-state system. We also show that single-photon time-bin qubits generated in the cold atomic ensemble can be converted, stored and retrieved from the crystal with a conditional qubit fidelity of more than 85 per cent. Our results open up the prospect of optically connecting quantum nodes with different capabilities and represent an important step towards the realization of large-scale hybrid quantum networks.

Nature 551 485 doi: 10.1038/nature24468

Zalamea, Federico (2017) The Two-fold Role of Observables in Classical and Quantum Kinematics. [Preprint]

Author(s): Shunlong Luo

Born's rule, which postulates the probability of a measurement outcome in a quantum state, is pivotal to interpretations and applications of quantum mechanics. By exploiting the departure of the product of two Hermitian operators in Born's rule from Hermiticity, we prescribe an intrinsic and natural...

[Phys. Rev. A 96, 052126] Published Mon Nov 20, 2017

Authors: Pedro Alberto, Saurya Das, Elias C. Vagenas

The problem of a particle in a box is probably the simplest problem in quantum mechanics which allows for significant insight into the nature of quantum systems and thus is a cornerstone in the teaching of quantum mechanics. In relativistic quantum mechanics this problem allows also to highlight the implications of special relativity for quantum physics, namely the effect that spin has on the quantized energy spectra. To illustrate this point, we solve the problem of a spin zero relativistic particle in a one- and three-dimensional box using the Klein-Gordon equation in the Feshbach-Villars formalism. We compare the solutions and the energy spectra obtained with the corresponding ones from the Dirac equation for a spin one-half relativistic particle. We note the similarities and differences, in particular the spin effects in the relativistic energy spectrum. As expected, the non-relativistic limit is the same for both kinds of particles, since, for a particle in a box, the spin contribution to the energy is a relativistic effect.

Authors: Adil Belhaj, Salah Eddine Ennadifi

Motivated by string theory and standard model physics, we discuss the possibility of other particles-based quantum information. A special attention is put on the consideration of the graviton in light of the gravitational wave detection. This may offer a new take in approaching quantum information using messenger particles. The construction is readily extended to higher dimensional qubits where we speculate on possible connections with open and closed string sectors in terms of quiver and graph theories, respectively. In particular, we reveal that the vectorial qubits could be associated with skeleton diagrams considered as extended quivers.

Authors: Domenico P. L. Castrigiano

Causal systems describe the localizability of relativistic quantum systems complying with the principles of special relativity and elementary causality. At their classification we restrict ourselves to real mass and finite spinor systems. It follows that (up to certain not yet discarded unitarily related systems) the only irreducible causal systems are the Dirac and the Weyl fermions. Their wave-equations are established as a mere consequence of causal localization.

The compact localized Dirac and Weyl wave-functions are studied in detail. One finds that, at the speed of light, the carriers shrink in the past and expand in the future. For every direction in space there is a defnite time at which the change from shrinking to expanding occurs. A late changing time characterizes those states, which shrink to a delta-strip if boosted in the opposite direction. Using a density result for these late-change states one shows that all Dirac and Weyl wave-functions are subjected to Lorentz contraction.

We tackle the question whether a causal system induces a representation of a causal logic and thus provides a localization in proper space-time regions rather than on spacelike hyperplanes. The causal logic generated by the spacelike relation is shown to do not admit representations at all. But the logic generated by the non-timelike relation in general does, and the necessary condition is derived that there is a projection valued measure on every non-timelike non-spacelike hyperplane being the high boost limit of the localization on the spacelike hyperplanes. Dirac and Weyl systems are shown to satisfy this condition and thus to extend to all non-timelike hyperplanes, which implies more profound properties of the causal systems. The compact localized eigenstates of the projections to non-spacelike flat strips are late-change states.

Authors: Cesar R. de Oliveira, Renan G. Romano

We add a confining potential to the Aharonov-Bohm model resulting in no contact of the particle with the solenoid (border); this is characterized by a unique self-adjoint extension of the initial Hamiltonian operator. It is shown that the spectrum of such extension is discrete and the first eigenvalue is found to be a nonconstant 1-periodic function of the magnetic flux circulation with a minimum at integers and maximum at half-integer circulations. This is a rigorous verification of the effect.

Authors: Zhikuan Zhao, Robert Pisarczyk, Jayne Thompson, Mile Gu, Vlatko Vedral, Joseph F. Fitzsimons

The traditional formalism of non-relativistic quantum theory allows the state of a quantum system to extend across space, but only restricts it to a single instant in time, leading to distinction between theoretical treatments of spatial and temporal quantum correlations. Here we unify the geometrical description of two-point quantum correlations in space-time. Our study presents the geometry of correlations between two sequential Pauli measurements on a single qubit undergoing an arbitrary quantum channel evolution together with two-qubit spatial correlations under a common framework. We establish a symmetric structure between quantum correlations in space and time. This symmetry is broken in the presence of non-unital channels, which further reveals a set of temporal correlations that are indistinguishable from correlations found in bipartite entangled states.

Authors: Don N. Page

In ordinary situations involving a small part of the universe, Born's rule seems to work well for calculating probabilities of observations in quantum theory. However, there are a number of reasons for believing that it is not adequate for many cosmological purposes. Here a number of possible generalizations of Born's rule are discussed, explaining why they are consistent with the present statistical support for Born's rule in ordinary situations but can help solve various cosmological problems.

Authors: D. Jaffino Stargen, V. Sreenath, L. Sriramkumar

The perturbations in the early universe are generated as a result of the interplay between quantum field theory and gravitation. Since these primordial perturbations lead to the anisotropies in the cosmic microwave background and eventually to the inhomogeneities in the Large Scale Structure (LSS), they provide a unique opportunity to probe issues which are fundamental to our understanding of quantum physics and gravitation. One such fundamental issue that remains to be satisfactorily addressed is the transition of the primordial perturbations from their quantum origins to the LSS which can be characterized completely in terms of classical quantities. Bouncing universes provide an alternative to the more conventional inflationary paradigm as they can help overcome the horizon problem in a fashion very similar to inflation. While the problem of the quantum-to-classical transition of the primordial perturbations has been investigated extensively in the context of inflation, we find that there has been a rather limited effort towards studying the issue in bouncing universes. In this work, we analyze certain aspects of this problem with the example of tensor perturbations produced in bouncing universes. We investigate the issue mainly from two perspectives. Firstly, we approach the problem by examining the extent of squeezing of a quantum state associated with the tensor perturbations with the help of the Wigner function. Secondly, we analyze this issue from the perspective of the quantum measurement problem. In particular, we study the effects of wave function collapse, using a phenomenological model known as continuous spontaneous localization, on the tensor power spectra. We conclude with a discussion of results.

Authors: S. O. Alexeyev, X. Calmet, B. N. Latosh

We show that the non-locality recently identified in quantum gravity using resummation techniques propagates to the matter sector of the theory. We describe these non-local effects using effective field theory techniques. We derive the complete set of non-local effective operators at order $N G^2$ for theories involving scalar, spinor, and vector fields. We then use recent data from the Large Hadron Collider to set a bound on the scale of space-time non-locality and find $M_\star> 3 \times 10^{-11}$ GeV.

Authors: Giulio Gasbarri, Marko Toroš, Sandro Donadi, Angelo Bassi

Starting from an idea of S.L. Adler~\cite{Adler2015}, we develop a novel model of gravity-induced spontaneous wave-function collapse. The collapse is driven by complex stochastic fluctuations of the spacetime metric. After deriving the fundamental equations, we prove the collapse and amplification mechanism, the two most important features of a consistent collapse model. Under reasonable simplifying assumptions, we constrain the strength $\xi$ of the complex metric fluctuations with available experimental data. We show that $\xi\geq 10^{-26}$ in order for the model to guarantee classicality of macro-objects, and at the same time $\xi \leq 10^{-20}$ in order not to contradict experimental evidence. As a comparison, in the recent discovery of gravitational waves in the frequency range 35 to 250 Hz, the (real) metric fluctuations reach a peak of $\xi \sim 10^{-21}$.

Authors: Leonardo Pedro

One attractive interpretation of quantum mechanics is the ensemble interpretation, where Quantum Mechanics merely describes a statistical ensemble of systems rather than individual systems. However, such interpretation does not address why the wave-function plays a central role in the calculations of probabilities, unlike most other interpretations of quantum mechanics. We first show that for a quantum system defined in a 2-dimensional real Hilbert space, the role of the wave-function is identical to the role of the Euler's formula in engineering, while the collapse of the wave-function is identical to selecting the real part of a complex number. We will then show that the wave-function is merely one possible parametrization of any probability distribution describing an ensemble: a surjective map from an hypersphere to the set of all possible probability distributions. The fact that the hypersphere is a surface of constant radius reflects the fact that the integral of the probability distribution is always 1. Any transformation of a probability distribution is represented by a rotation of the hypersphere. It is thus a very good parametrization which allows us to apply group theory to the hypersphere, despite the fact that a stochastic process is not always a Markov process. The collapse of the wave-function is required to compensate the fact that physical transformations on the probability distribution are not linear transformations.

Authors: Craig Hogan, Ohkyung Kwon

A Lorentz invariant framework is developed to interpret exotic cross-correlations in the signals of two separate interferometers, associated with the emergence of space-time and inertial frames from a Planck scale quantum system. The framework extends our earlier models of exotic autospectra based on invariant causal structure. Space-time relationships between world lines are modeled as antisymmetric cross-correlations on past and future light cones between sequences in proper time with Planck bandwidth, arising from nonlocal entanglement information in geometrical states. These exotic correlations of a flat space-time are normalized to have the same holographic information content as black holes. Simple models of interferometer response are shown to produce a unique signature: a broad band imaginary cross spectrum, with a frequency structure determined by the layout of the apparatus. The framework will be useful for interpreting data in the bent reconfiguration of the Fermilab Holometer, and for conceptual design of future experiments.

Authors: Bob Coecke, Stefano Gogioso, John H. Selby

We consider a very general class of theories, process theories, which capture the underlying structure common to most theories of physics as we understand them today (be they established, toy or speculative theories). Amongst these theories, we will be focusing on those which are `causal', in the sense that they are intrinsically compatible with the causal structure of space-time -- as required by relativity. We demonstrate that there is a sharp contrast between these theories and the corresponding time-reversed theories, where time is taken to flow backwards from the future to the past. While the former typically feature a rich gamut of allowed states, the latter only allow for a single state: eternal noise. We illustrate this result by considering of the time-reverse of quantum theory. We also derive a strengthening of the result in PRL 108, 200403 on signalling in time-reversed theories.

The effort to reconcile general relativity with quantum mechanics always hits one snag: gravity. An experiment could finally tell us if it is a quantum force
Flash Physics: need-to-know updates from the world of physics

Author(s): Tim Thomay, Sergey V. Polyakov, Olivier Gazzano, Elizabeth Goldschmidt, Zachary D. Eldredge, Tobias Huber, Vivien Loo, and Glenn S. Solomon

Advances in quantum optics and quantum information technologies increasingly require a way to fully understand the state of single indistinguishable photons. While previous approaches have needed two or more measurements, a new experiment demonstrates a way to characterize single-photon states with just one measurement.

[Phys. Rev. X 7, 041036] Published Wed Nov 15, 2017

Authors: Alejandro Perez, Daniel Sudarsky

We argue that discreteness at the Planck scale (naturally expected to arise from quantum gravity) might manifest in the form of minute violations of energy-momentum conservation of the matter degrees of freedom when described in terms of (idealized) smooth fields on a smooth spacetime. In the context of applications to cosmology such `energy diffusion' from the low energy matter degrees of freedom to the discrete structures underlying spacetime leads to the emergence of an effective dark energy term in Einstein's equations. We estimate this effect using a (relational) hypothesis about the materialization of discreteness in quantum gravity which is motivated by the strict observational constraints supporting the validity of Lorentz invariance at low energies. The predictions coming from simple dimensional analysis yield a cosmological constant of the order of magnitude of the observed value without fine tuning.

Authors: Saul Rodriguez, Daniel Sudarsky

In this work we consider the Higgs inflation scenario, but in contrast with past works, the present analysis is done in the context of a spontaneous collapse theory for the quantum state of the inflaton field. In particular we will rely on a previously studied adaptation of the Continuous Spontaneous Localization model for the treatment of inflationary cosmology. We will show that with the introduction of the dynamical collapse hypothesis, some of the most serious problems of the Higgs inflation proposal, can be evaded in a natural way.

Authors: Djamil Bouaziz, Tolga Birkandan

The problem of a particle of mass m in the field of the inverse square potential is studied in quantum mechanics with a generalized uncertainty principle, characterized by the existence of a minimal length. Using the coordinate representation, for a specific form of the generalized uncertainty relation, we solve the deformed Schr\"odinger equation analytically in terms of confluent Heun functions. We explicitly show the regularizing effect of the minimal length on the singularity of the potential. We discuss the problem of bound states in detail and we derive an expression for the energy spectrum in a natural way from the square integrability condition; the results are in complete agreement with the literature.

Feintzeig, Benjamin H. (2017) The classical limit of a state on the Weyl algebra. [Preprint]
John Wheeler tried to help his heroes see eye to eye on quantum physics 

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Ruyant, Quentin (2017) Structural Realism or Modal Empiricism? The British Journal for the Philosophy of Science.
Barbar, Ahmed (2017) The Kochen-Specker and Conway-Kochen Theorems. [Preprint]

Authors: Jordan Cotler, Chao-Ming Jian, Xiao-Liang Qi, Frank Wilczek

We introduce superdensity operators as a tool for analyzing quantum information in spacetime. Superdensity operators encode spacetime correlation functions in an operator framework, and support a natural generalization of Hilbert space techniques and Dirac's transformation theory as traditionally applied to standard density operators. Superdensity operators can be measured experimentally, but accessing their full content requires novel procedures. We demonstrate these statements on several examples. The superdensity formalism suggests useful definitions of spacetime entropies and spacetime quantum channels. For example, we show that the von Neumann entropy of a superdensity operator is related to a quantum generalization of the Kolmogorov-Sinai entropy, and compute this for a many-body system. We also suggest experimental protocols for measuring spacetime entropies.

Authors: P. N. Kaloyerou

I argue that quantum optical experiments that purport to refute Bohr's principle of complementarity (BPC) fail in their aim. Some of these experiments try to refute complementarity by refuting the so called particle-wave duality relations, which evolved from the Wootters-Zureck reformulation of BPC (WZPC). I therefore consider it important for my forgoing arguments to first recall the essential tenets of BPC, and to clearly separate BPC from WZPC, which I will argue is a direct contradiction of BPC. This leads to a need to consider the meaning of particle-wave duality relations and to question their fundamental status. I further argue (albeit, in opposition to BPC) that particle and wave complementary concepts are on a different footing than other pairs of complementary concepts.

Authors: P.N.Kaloyerou

Grangier, Roger and Aspect (GRA) performed a beam-splitter experiment to demonstrate the particle behaviour of light and a Mach-Zehnder interferometer experiment to demonstrate the wave behaviour of light. The distinguishing feature of these experiments is the use of a gating system to produce near ideal single photon states. With the demonstration of both wave and particle behaviour (in two mutually exclusive experiments) they claim to have demonstrated the dual particle-wave behaviour of light and hence to have confirmed Bohr's principle of complementarity. The demonstration of the wave behaviour of light is not in dispute. But we want to demonstrate, contrary to the claims of GRA, that their beam-splitter experiment does not conclusively confirm the particle behaviour of light, and hence does not confirm particle-wave duality, nor, more generally, does it confirm complementarity. Our demonstration consists of providing a detailed model based on the Causal Interpretation of Quantum Fields (CIEM), which does not involve the particle concept, of GRA's which-path experiment. We will also give a brief outline of a CIEM model for the second, interference, GRA experiment.

Authors: Kaonan Micadei, John P. S. Peterson, Alexandre M. Souza, Roberto S. Sarthour, Ivan S. Oliveira, Gabriel T. Landi, Tiago B. Batalhão, Roberto M. Serra, Eric Lutz

The second law permits the prediction of the direction of natural processes, thus defining a thermodynamic arrow of time. However, standard thermodynamics presupposes the absence of initial correlations between interacting systems. We here experimentally demonstrate the reversal of the arrow of time for two initially quantum correlated spins-1/2, prepared in local thermal states at different temperatures, employing a Nuclear Magnetic Resonance setup. We observe a spontaneous heat flow from the cold to the hot system. This process is enabled by a trade off between correlations and entropy that we quantify with information-theoretical quantities.

Authors: P.N. Kaloyerou

We consider a Wheeler delayed-choice experiment based on the Mach-Zehnder Interferometer. Since the development of the causal interpretation of relativistic boson fields there have not been any applications for which the equations of motion for the field have been solved explicitly. Here, we provide perhaps the first application of the causal interpretation of boson fields for which the equations of motion are solved. Specifically, we consider the electromagnetic field. Solving the equations of motion allows us to develop a relativistic causal model of the Wheeler delayed-choice Mach-Zehnder Interferometer. We show explicitly that a photon splits at a beam splitter. We also demonstrate the inherent nonlocal nature of a relativistic quantum field. This is particularly revealed in a which-path measurement where a quantum is nonlocally absorbed from both arms of the interferometer. This feature explains how when a photon is split by a beam splitter it nevertheless registers on a detector in one arm of the interferometer. Bohm et al \cite{BDH85} have argued that a causal model of a Wheeler delayed-choice experiment avoids the paradox of creating or changing history, but they did not provide the details of such a model. The relativistic causal model we develop here serves as a detailed example which demonstrates this point, though our model is in terms of a field picture rather than the particle picture of the Bohm-de Broglie nonrelativistic causal interpretation.


Following a bi-cylindrical model of geometrical dynamics, our study shows that a 6D-gravitational equation leads to geodesic description in an extended symmetrical time–space, which fits Hubble-like expansion on a microscopic scale. As a duality, the geodesic solution is mathematically equivalent to the basic Klein–Gordon–Fock equations of free massive elementary particles, in particular, the squared Dirac equations of leptons. The quantum indeterminism is proved to have originated from space–time curvatures. Interpretation of some important issues of quantum mechanical reality is carried out in comparison with the 5D space–time–matter theory. A solution of lepton mass hierarchy is proposed by extending to higher dimensional curvatures of time-like hyper-spherical surfaces than one of the cylindrical dynamical geometry. In a result, the reasonable charged lepton mass ratios have been calculated, which would be tested experimentally.

Publication date: Available online 7 November 2017
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Author(s): Kenji Ito

Publication date: Available online 13 October 2017
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Author(s): Christina Conroy
There is an interpretation of Everettian Quantum Mechanics [EQM] known as the Relative Facts Interpretation [RFI] which is a single-world interpretation of EQM and takes generally all facts about objects to be relational. In this paper I argue that from the perspective of the RFI the best theory of modality for EQM is actualism rather than some version of modal realism, as has been suggested by Alastair Wilson. To argue this I draw a parallel between actualism as it was developed by Alvin Plantinga and actualism as it can be developed from the context of the RFI of EQM. The contention is not that Plantinga-style actualism is the only way one can be an actualist with respect to EQM, but rather that showing how one can be an actualist in at least one way, demonstrates that there are options for a modal metaphysics from the context of EQM.

Publication date: Available online 1 November 2017
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Author(s): Dennis Lehmkuhl
In this paper I describe the genesis of Einstein's early work on the problem of motion in general relativity (GR): the question of whether the motion of matter subject to gravity can be derived directly from the Einstein field equations. In addressing this question, Einstein himself always preferred the vacuum approach to the problem: the attempt to derive geodesic motion of matter from the vacuum Einstein equations. The paper first investigates why Einstein was so skeptical of the energy-momentum tensor and its role in GR. Drawing on hitherto unknown correspondence between Einstein and George Yuri Rainich, I then show step by step how his work on the vacuum approach came about, and how his quest for a unified field theory informed his interpretation of GR. I show that Einstein saw GR as a hybrid theory from very early on: fundamental and correct as far as gravity was concerned but phenomenological and effective in how it accounted for matter. As a result, Einstein saw energy-momentum tensors and singularities in GR as placeholders for a theory of matter not yet delivered. The reason he preferred singularities was that he hoped that their mathematical treatment would give a hint as to the sought after theory of matter, a theory that would do justice to quantum features of matter.

Publication date: Available online 17 October 2017
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Author(s): Vincent Ardourel, Alexandre Guay
The transference theory reduces causation to the transmission (or regular manifestation) of physical conserved quantities, like energy or momenta. Although this theory aims at applying to all fields of physics, we claim that it fails to account for a quantum electrodynamic effect, viz. the Aharonov-Bohm effect. After having argued that the Aharonov-Bohm effect is a genuine counter-example for the transference theory, we offer a new physicalist approach of causation, ontic and modal, in which this effect is embedded.

Dorato, Mauro and Rossanese, Emanuele (2017) The nature of representation in Feynman diagrams. [Preprint]
Flash Physics: need-to-know updates from the world of physics
Carls-Diamante, Sidney (2017) The octopus and the unity of consciousness. [Preprint]


We argue that causal decision theory (CDT) is no worse off than evidential decision theory (EDT) in handling entanglement, regardless of one’s preferred interpretation of quantum mechanics. In recent works, Ahmed (Evidence, decision, and causality, Cambridge University Press, Cambridge, 2014) and Ahmed and Caulton (Synthese, 191(18): 4315–4352, 2014) have claimed the opposite; we argue that they are mistaken. Bell-type experiments are not instances of Newcomb problems, so CDT and EDT do not diverge in their recommendations. We highlight the fact that a Causal Decision Theorist should take all lawlike correlations into account, including potentially acausal entanglement correlations. This paper also provides a brief introduction to CDT with a motivating “small” Newcomb problem. The main point of our argument is that quantum theory does not provide grounds for favouring EDT over CDT.

Jaksland, Rasmus (2017) Probing spacetime with a holographic relation between spacetime and entanglement. [Preprint]
François, Jordan (2017) Artificial vs Substantial Gauge Symmetries: a Criterion and an Application to the Electroweak Model. [Preprint]
The idea that we create reality seems absurd. But an audacious new take on quantum theory suggests the fundamental laws of nature emerge from our own experiences
Physics isn't just hard – it can be uncomfortable, too. From quantum many worlds to the universe's heat death, here are five concepts that spell embarrassment
Esfeld, Michael (2017) Super-Humeanism: the Canberra plan for physics. [Preprint]

Scattering is used to probe matter and its interactions in all areas of physics. In ultracold atomic gases, control over pairwise interactions enables us to investigate scattering in quantum many-body systems. Previous experiments on colliding Bose–Einstein condensates have revealed matter–wave interference, haloes of scattered atoms, four-wave mixing and correlations between counter-propagating pairs. However, a regime with strong stimulation of spontaneous collisions analogous to superradiance has proved elusive. In this regime, the collisions rapidly produce highly correlated states with macroscopic population. Here we find that runaway stimulated collisions in Bose–Einstein condensates with periodically modulated interaction strength cause the collective emission of matter-wave jets that resemble fireworks. Jets appear only above a threshold modulation amplitude and their correlations are invariant even when the number of ejected atoms grows exponentially. Hence, we show that the structures and atom occupancies of the jets stem from the quantum fluctuations of the condensate. Our findings demonstrate the conditions required for runaway stimulated collisions and reveal the quantum nature of matter-wave emission.

Nature doi: 10.1038/nature24272

Author(s): C. Godfrin, A. Ferhat, R. Ballou, S. Klyatskaya, M. Ruben, W. Wernsdorfer, and F. Balestro

Grover’s algorithm, which finds an element in an unsorted list, has been implemented using a nuclear spin in a single-molecule magnet.

[Phys. Rev. Lett. 119, 187702] Published Thu Nov 02, 2017

Publication date: 20 December 2017
Source:Physics Letters A, Volume 381, Issue 47
Author(s): Marko Toroš, Giulio Gasbarri, Angelo Bassi
Matter-wave interferometry is a direct test of the quantum superposition principle for massive systems, and of collapse models. Here we show that the bounds placed by matter-wave interferometry depend weakly on the details of the collapse mechanism. Specifically, we compute the bounds on the CSL model and its variants, provided by the KDTL interferometry experiment of Arndt's group (Eibenberger et al. (2013) [3]), which currently holds the record of largest mass in interferometry. We also show that the CSL family of models emerges naturally by considering a minimal set of assumptions. In particular, we construct the dynamical map for the colored and dissipative Continuous Spontaneous Localization (cdCSL) model, which reduces to the CSL model and variants in the appropriate limits. In addition, we discuss the measure of macroscopicity based on the cdCSL model.


According to quantum mechanics, statements about the future made by sentient beings like us are, in general, neither true nor false; they must satisfy a many-valued logic. I propose that the truth value of such a statement should be identified with the probability that the event it describes will occur. After reviewing the history of related ideas in logic, I argue that it gives an understanding of probability which is particularly satisfactory for use in quantum mechanics. I construct a lattice of future-tense propositions, with truth values in the interval [0, 1], and derive logical properties of these truth values given by the usual quantum-mechanical formula for the probability of a history.

Author(s): Daniel Carney, Laurent Chaurette, Dominik Neuenfeld, and Gordon Walter Semenoff

We discuss information-theoretic properties of low-energy photons and gravitons in the S matrix. Given an incoming n-particle momentum eigenstate, we demonstrate that unobserved soft photons decohere nearly all outgoing momentum superpositions of charged particles, while the universality of gravity ...

[Phys. Rev. Lett. 119, 180502] Published Mon Oct 30, 2017

Author(s): Seyyed M. H. Halataei

I present an explicit classical simulation of arbitrary quantum noise for quantum models in which one qubit interacts with a quantum bath. The classical model simulates the interaction of the bath and the qubit by random unitary evolutions. I show that any arbitrary quantum dynamics, including quant...

[Phys. Rev. A 96, 042338] Published Fri Oct 27, 2017

Author(s): René Schwonnek, Lars Dammeier, and Reinhard F. Werner

Quantifying quantum mechanical uncertainty is vital for the increasing number of experiments that reach the uncertainty limited regime. We present a method for computing tight variance uncertainty relations, i.e., the optimal state-independent lower bound for the sum of the variances for any set of ...

[Phys. Rev. Lett. 119, 170404] Published Fri Oct 27, 2017

Author(s): J. Sperling and I. A. Walmsley

The dynamical behavior of interacting systems plays a fundamental role for determining quantum correlations, such as entanglement. In this Letter, we describe temporal quantum effects of the inseparable evolution of composite quantum states by comparing the trajectories to their classically correlat...

[Phys. Rev. Lett. 119, 170401] Published Tue Oct 24, 2017


Recently I published an article in this journal entitled “Less interpretation and more decoherence in quantum gravity and inflationary cosmology” (Crull in Found Phys 45(9):1019–1045, 2015). This article generated responses from three pairs of authors: Vassallo and Esfeld (Found Phys 45(12):1533–1536, 2015), Okon and Sudarsky (Found Phys 46(7):852–879, 2016) and Fortin and Lombardi (Found Phys, 2017). In what follows, I reply to the criticisms raised by these authors.


The difficult issues related to the interpretation of quantum mechanics and, in particular, the “measurement problem” are revisited using as motivation the process of generation of structure from quantum fluctuations in inflationary cosmology. The unessential mathematical complexity of the particular problem is bypassed, facilitating the discussion of the conceptual issues, by considering, within the paradigm set up by the cosmological problem, another problem where symmetry serves as a focal point: a simplified version of Mott’s problem.

Publication date: 29 November 2017
Source:Physics Letters A, Volume 381, Issue 44
Author(s): Shovon Biswas
Bohr–van Leeuwen theorem has been studied in non-commutative space where the space coordinates do not commute. It has been found that in non-commutative space Bohr–van Leeuwen theorem, in general, is not satisfied and a classical treatment of the partition function of charged particles in a magnetic field gives rise to non zero magnetization.

Author: Anthony Duncan
ISBN: 9780198807650
Binding: Paperback
Publication Date: 17 October 2017
Price: $55.00