# Weekly Papers on Quantum Foundations (38)

This is a list of this week’s papers on quantum foundations published in various journals or uploaded to preprint servers such as arxiv.org and PhilSci Archive.

Information Content of Gravitational Radiation and the Vacuum. (arXiv:1607.03122v2 [hep-th] UPDATED)

on 2016-9-17 8:11am GMT

Authors: Raphael BoussoIllan HalpernJason Koeller

Known entropy bounds, and the Generalized Second Law, were recently shown to imply bounds on the information arriving at future null infinity. We complete this derivation by including the contribution from gravitons. We test the bounds in classical settings with gravity and no matter. In Minkowski space, the bounds vanish on any subregion of the future boundary, independently of coordinate choices. More generally, the bounds vanish in regions where no gravitational radiation arrives. In regions that do contain Bondi news, the bounds are compatible with the presence of information, including the information stored in gravitational memory. All of our results are consistent with the equivalence principle, which states that empty Riemann-flat spacetime regions contain no classical information. We also discuss the possibility that Minkowski space has an infinite vacuum degeneracy labeled by a choice of Bondi coordinates (a classical parameter, if physical). We argue that this degeneracy cannot have any observational consequences if the equivalence principle holds. Our bounds are consistent with this conclusion.

SpaceTime from Hilbert Space: Decompositions of Hilbert Space as Instances of Time. (arXiv:1609.01295v2 [hep-th] UPDATED)

on 2016-9-17 8:11am GMT

Authors: Mahdiyar Noorbala

There has been recent interest in identifying entanglement as the fundamental concept from which space may emerge. We note that the particular way that a Hilbert space is decomposed into tensor factors is important in what the resulting geometry looks like. We then propose that time may be regarded as a variable that parameterizes a family of such decompositions, thus giving rise to a family of spatial geometries. As a proof of concept, this idea is demonstrated in two toy models based on Kitaev’s toric code, which feature a dynamical change of dimension and topology.

Classical Universe emerging from quantum cosmology without horizon and flatness problems. (arXiv:1609.04488v1 [gr-qc])

on 2016-9-17 8:11am GMT

We apply the complex de Broglie-Bohm formulation of quantum mechanics [1] to a spatially closed homogeneous and isotropic early Universe whose matter content are radiation and dust perfect fluids. We then show that an expanding classical Universe can emerge from an oscillating (with complex scale factor) quantum Universe without singularity. Furthermore, the Universe obtained in this process has no horizon or flatness problems.

Perturbations and quantum relaxation. (arXiv:1609.04485v1 [quant-ph])

on 2016-9-17 8:11am GMT

We investigate whether small perturbations can cause relaxation to quantum equilibrium over very long timescales. We consider in particular a two-dimensional harmonic oscillator, which can serve as a model of a field mode on expanding space. We assume an initial wave function with small perturbations to the ground state. We present evidence that the trajectories are highly confined so as to preclude relaxation to equilibrium even over very long timescales. Cosmological implications are briefly discussed.

Hamiltonian approach to GR – Part 2: covariant theory of quantum gravity. (arXiv:1609.04428v1 [gr-qc])

on 2016-9-17 8:11am GMT

A non-perturbative quantum field theory of General Relativity is presented which leads to a new realization of the theory of Covariant Quantum-Gravity (CQG-theory). The treatment is founded on the recently-identified Hamiltonian structure associated with the classical space-time, i.e., the corresponding manifestly-covariant Hamilton equations and the related Hamilton-Jacobi theory. As shown here the connection with CQG-theory is achieved via the classical GR Hamilton-Jacobi equation, leading to the realization of the CQG-wave equation in 4-scalar form for the corresponding CQG-state and wave-function. The new quantum wave equation exhibits well-known formal properties characteristic of quantum mechanics, including the validity of quantum hydrodynamic equations and suitably-generalized Heisenberg inequalities. In addition, it recovers the classical GR equations in the semiclassical limit, while admitting the possibility of developing further perturbative approximation schemes. Applications of the theory are pointed out with particular reference to the construction of the stationary vacuum CQG-wave equation. The existence of a corresponding discrete energy spectrum is pointed out, which provides a possible estimate for the graviton mass together with a new interpretation about the quantum origin of the cosmological constant.

Synchronizing quantum and classical clocks made of quantum particles. (arXiv:1512.05034v3 [quant-ph] UPDATED)

on 2016-9-17 8:11am GMT

We demonstrate that the quantum corrections to the classical arrival time for a quantum object in a potential free region of space, as computed by Galapon [Phys. Rev. A {\bf 80}, 030102(R) (2009)], can be eliminated up to a given order of $\hbar$ by choosing an appropriate position-dependent phase for the object’s wavefunction. This then implies that we can make the quantum arrival time of the object as close as possible to its corresponding classical arrival time, allowing us to synchronize a classical and quantum clock which tells time using the classical and quantum arrival time of the object, respectively. We provide an example for synchronizing such a clock by making use of a quantum object with a position-dependent phase imprinted on the object’s initial wavefunction with the use of an impulsive potential.

Connecting the quantum and classical worlds. (arXiv:1609.04427v1 [quant-ph])

on 2016-9-17 8:11am GMT

Authors: Barbara Drossel

By considering (non-relativistic) quantum mechanics as it is done in practice in particular in condensed-matter physics, it is argued that a deterministic, unitary time evolution within a chosen Hilbert space always has a limited scope, leaving a lot of room for embedding the quantum-classical transition into our current theories without recurring to difficult-to-accept interpretations of quantum mechanics. Nonunitary projections to initial and final states, the breaking of time-reversal symmetry, a change of Hilbert space, and the introduction of classical concepts such as external potentials or localized atomic nuclei are widespread in quantum mechanical calculations. Furthermore, quantum systems require classical environments that enable the symmetry breaking that is necessary for creating the atomic configurations of molecules and crystals. This paper argues that such classical environments are provided by finite-temperature macroscopic systems in which the range of quantum correlations and entanglement is limited. This leads to classical behavior on larger scales, and to collapse-like events in all dynamical processes that become coupled to the thermalized degrees of freedom.

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

on 2016-9-17 12:11am GMT

Publication date: Available online 12 September 2016
Source:Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Author(s): Marian J.R. Gilton
David Wallace has recently argued that the eigenstate–eigenvalue (E–E) link has no place in serious discussions of quantum mechanics on the grounds that, as he claims, the E–E link is an invention of philosophers rather than the community of practicing physicists. This raises an historical question regarding the origin of the link. This paper aims to answer this question by tracing the historical development of the link through six key textbooks of quantum mechanics. In light of the historical evidence from these textbooks, it is argued that Wallace provides insufficient grounds for dismissing the E–E link from discussions of quantum mechanics.

The Realist Turn in the Philosophy of Science

Philsci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-9-16 5:39pm GMT

Psillos, Stathis (2016) The Realist Turn in the Philosophy of Science. [Preprint]

Chance in the Everett interpretation

Philsci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-9-16 5:33pm GMT

Saunders, Simon (2016) Chance in the Everett interpretation. [Preprint]

Particle path through a nested Mach-Zehnder interferometer

PRA: Fundamental concepts

on 2016-9-16 2:00pm GMT

Author(s): Robert B. Griffiths

Possible paths of a photon passing through a nested Mach-Zehnder interferometer on its way to a detector are analyzed using the consistent histories formulation of quantum mechanics, and confirmed using a set of weak measurements (but not weak values). The results disagree with an analysis by Vaidma…

[Phys. Rev. A 94, 032115] Published Fri Sep 16, 2016

Measurement-dependent locality beyond independent and identically distributed runs

PRA: Fundamental concepts

on 2016-9-16 2:00pm GMT

Author(s): Ernest Y.-Z. Tan, Yu Cai, and Valerio Scarani

When conducting a Bell test, it is normal to assume that the preparation of the quantum state is independent of the measurements performed on it. Remarkably, the violation of local realism by entangled quantum systems can be certified even if this assumption is partially relaxed. Here, we allow such…

[Phys. Rev. A 94, 032117] Published Fri Sep 16, 2016

Grover Search and the No-Signaling Principle

PRL: General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

on 2016-9-14 2:00pm GMT

Author(s): Ning Bao, Adam Bouland, and Stephen P. Jordan

Two of the key properties of quantum physics are the no-signaling principle and the Grover search lower bound. That is, despite admitting stronger-than-classical correlations, quantum mechanics does not imply superluminal signaling, and despite a form of exponential parallelism, quantum mechanics do…

[Phys. Rev. Lett. 117, 120501] Published Wed Sep 14, 2016

Quasiprobability Representations of Quantum Mechanics with Minimal Negativity

PRL: General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

on 2016-9-14 2:00pm GMT

Author(s): Huangjun Zhu

Quasiprobability representations, such as the Wigner function, play an important role in various research areas. The inevitable appearance of negativity in such representations is often regarded as a signature of nonclassicality, which has profound implications for quantum computation. However, litt…

[Phys. Rev. Lett. 117, 120404] Published Wed Sep 14, 2016

Three Aspects of Typicality in Multiverse Cosmology

Philsci-Archive: No conditions. Results ordered -Date Deposited.

on 2016-9-12 6:28pm GMT

Azhar, Feraz (2016) Three Aspects of Typicality in Multiverse Cosmology. [Preprint]

Direct Measurement of the Density Matrix of a Quantum System

PRL: General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

on 2016-9-12 2:00pm GMT

Author(s): G. S. Thekkadath, L. Giner, Y. Chalich, M. J. Horton, J. Banker, and J. S. Lundeen

Researchers have developed a new technique to measure the density matrix—a more general way of characterizing the state of a quantum system than that provided by the wave function.

[Phys. Rev. Lett. 117, 120401] Published Mon Sep 12, 2016

The extended Bloch Representation of Entanglement and Measurement in Quantum Mechanics. (arXiv:1609.02658v1 [quant-ph])

on 2016-9-12 7:48am GMT

The quantum formalism can be completed by assuming that a density operator can also represent a pure state. An ‘extended Bloch representation’ (EBR) then results, in which not only states, but also the measurement-interactions can be represented. The Born rule is obtained as an expression of the subjective lack of knowledge about the measurement-interaction that is each time actualized. Entanglement can also be consistently described in the EBR, as it remains compatible with the principle according to which a composite entity exists only if its components also exist, and therefore are in pure states.

How many bits specify a quantum state?. (arXiv:1609.02587v1 [quant-ph])

on 2016-9-12 7:48am GMT

Authors: Barbara Drossel

Wave functions are reproducible only when they represent long-lived eigenstates in a bound system, or when a well-controlled preparation procedure is available. If we admit that otherwise their precision is limited, we find that the classical and the quantum mechanical description of macroscopic systems can converge, with both of them including particle localization and stochastic effects. This argument is developped using a finite-temperature gas confined in a box as an example.

An analogue of the Coleman-Mandula theorem for quantum field theory in curved spacetimes. (arXiv:1609.02705v1 [math-ph])

on 2016-9-12 7:46am GMT

Authors: Christopher J. Fewster

The Coleman-Mandula (CM) theorem states that the Poincar\’e and internal symmetries of a Minkowski spacetime quantum field theory cannot combine nontrivially in an extended symmetry group. We establish an analogous result for quantum field theory in curved spacetimes, assuming local covariance, the timeslice property, a local dynamical form of Lorentz invariance, and additivity. Unlike the CM theorem, our result is valid in dimensions $n\ge 2$ and for free or interacting theories. It is formulated for theories defined on a category of all globally hyperbolic spacetimes equipped with a global coframe, on which the restricted Lorentz group acts, and makes use of a general analysis of symmetries induced by the action of a group $G$ on the category of spacetimes. Such symmetries are shown to be canonically associated with a cohomology class in the second degree nonabelian cohomology of $G$ with coefficients in the global gauge group of the theory. Our main result proves that the cohomology class is trivial if $G$ is the universal cover $\cal S$ of the restricted Lorentz group. Among other consequences, it follows that the extended symmetry group is a direct product of the global gauge group and $\cal S$, all fields transform in multiplets of $\cal S$, fields of different spin do not mix under the extended group, and the occurrence of noninteger spin is controlled by the centre of the global gauge group. The general analysis is also applied to rigid scale covariance.

Werner Heisenberg and the German Uranium Project 1939 – 1945. Myths and Facts. (arXiv:1609.02775v1 [physics.hist-ph])