Weekly Papers on Quantum Foundations (48)

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 DimićMarko MilivojevićDragoljub GočaninČaslav 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 MarchewkaZeev 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 ThompsonAndrew J. P. GarnerJohn R. MahoneyJames P. CrutchfieldVlatko VedralMile 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

— Read more on ScientificAmerican.com


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 GondaRavi KunjwalDavid SchmidElie WolfeAna 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 [vimeo.com/52923835 (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]

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