Classical and quantum ghosts. (arXiv:1406.4550v4 [hep-th] UPDATED)

on 2014-10-24 10:18am GMT

The aim of these notes is to provide a self-contained review of why it is generically a problem when a solution of a theory possesses ghost fields among the perturbation modes. We define what a ghost field is and we show that its presence is associated to a classical instability whenever the ghost field interacts with standard fields. We then show that the instability is more severe at quantum level, and that perturbative ghosts can exist only in low energy effective theories. However, if we don’t consider very ad-hoc choices, compatibility with observational constraints implies that low energy effective ghosts can exist only at the price of giving up Lorentz-invariance or locality above the cut-off, in which case the cut-off has to be much lower that the energy scales we currently probe in particle colliders. We also comment on the possible role of extra degrees of freedom which break Lorentz-invariance spontaneously.

Weak-value amplification: state of play. (arXiv:1410.6252v1 [quant-ph])

on 2014-10-24 10:18am GMT

Weak values arise in quantum theory when the result of a weak measurement is conditioned on a subsequent strong measurement. The majority of the trials are discarded, leaving only very few successful events. Intriguingly those can display a substantial signal amplification. This raises the question of whether weak values carry potential to improve the performance of quantum sensors, and indeed a number of impressive experimental results suggested this may be the case. By contrast, recent theoretical studies have found the opposite: using weak-values to obtain an amplification generally worsens metrological performance. This survey summarises the implications of those studies, which call for a reappraisal of weak values’ utility and for further work to reconcile theory and experiment.

A quantum world arising from many ordinary ones

on 2014-10-24 12:00am GMT

Radical theory proposes that interactions between classical worlds can explain some quantum phenomena.

Nature News doi: 10.1038/nature.2014.16213

Editorial: Does Research on Foundations of Quantum Mechanics Fit into PRX’s Scope?

Recent Articles in Phys. Rev. X

on 2014-10-23 2:00pm GMT

Author(s):

[Phys. Rev. X 4, 040002] Published Thu Oct 23, 2014

Quantum Phenomena Modeled by Interactions between Many Classical Worlds

Recent Articles in Phys. Rev. X

on 2014-10-23 2:00pm GMT

Author(s): Michael J. W. Hall, Dirk-André Deckert, and Howard M. Wiseman

Researchers propose a new “many-interacting-worlds” theory that could explain quantum mechanics in all its strangeness. Numerical simulations reproduce wave behavior in double-slit experiments assuming as few as 40 worlds.

[Phys. Rev. X 4, 041013] Published Thu Oct 23, 2014

on 2014-10-23 2:15am GMT

Hawking radiation has been demonstrated as quantum tunneling across the event horizon and the spectrum has been proved to be nonthermal. These nonthermality factors emerging due to back reaction effects have been claimed to be responsible for correlations among the emitted quanta. It has been claimed by several authors in literature that these correlations actually carry out information locked in a black hole and hence provide a resolution to the long debated black hole information paradox. This paper demonstrates that this is a fallacious proposition.

First-Person Plural Quantum Mechanics. (arXiv:1410.5916v1 [quant-ph])

physics.hist-ph updates on arXiv.org

on 2014-10-23 2:15am GMT

Doing justice to quantum mechanics calls for a deeper examination of the relations between our experience, its objects, and its subjects than either third-person interpretations or the first-person singular interpretation of the QBist permit. The metaphysical space opened by Bohr’s employment of the “Kantian wedge” between the objective world, about which we can communicate, and the world “in itself” allows quantum mechanics to unfold its metaphysical potential. This in turn makes it possible to go a long way towards bridging the epistemological gap between the empirical and transcendental conceptions of reality.

Solving the EPR paradox with pseudo-classical paths. (arXiv:1306.5492v8 [quant-ph] UPDATED)

on 2014-10-23 2:15am GMT

We propose a novel interpretation of Quantum Mechanics, which can resolve the outstanding conflict between the principles of locality and realism and offers new insight on the so-called weak values of physical observables. The discussion is presented in the context of Bohm’s system of two photons in their singlet polarization state in which the Einstein-Podolski-Rosen paradox is commonly addressed. It is shown that quantum states can be understood as statistical mixtures of non-interfering pseudo-classical paths in a {\it hidden} phase space, in a way that overcomes the implicit assumptions of Bell’s theorem and reproduces all expected values and correlations. The polarization properties of the photons along these paths are gauge-dependent magnitudes, whose actual values get fixed only after a reference direction is set by the observer of either photon A or B. Furthermore, these values are not constrained to fulfill standard classical algebraic relationships. These {\it hidden} paths can be grouped into coarser ones consistent with particular post-selection conditions for a complete set of commuting observables and along which every physical observable gets on average its corresponding weak value. Obviously, different sets of commuting observables lead to different coarse statistical representations of the same quantum state. This interpretation follows from the observation that in the Heisenberg picture of a closed quantum system in state $|\Psi>$ every physical observable ${\cal O}(t)=e^{+i H t} {\cal O} e^{-i H t}$ can be represented by an operator $P_{o(t)}$ within a commutative algebra, such that ${\cal O}(t)|\Psi>=P_{o(t)}|\Psi>$. The formalism presented here may become a useful tool for performing numerical simulations of quantum systems.

The Quantum Information of Cosmological Correlations. (arXiv:1410.5508v1 [hep-th])

on 2014-10-22 2:51am GMT

It has been shown that the primordial perturbations sourced by inflation are driven to classicality by unitary evolution alone. However, their coupling with the environment such as photons and subsequent decoherence renders the cosmological correlations quantum, losing primordial information in the process. We argue that the quantumness of the resulting cosmological correlations is given by quantum discord, which captures non-classical behavior beyond quantum entanglement. By considering the environment as a quantum channel in which primordial information contained in the perturbations is transmitted to us, we can then ask how much of this information is inaccessible. We show that this amount of information is given by the discord of the joint primordial perturbations-environment system. To illustrate these points, we model the joint system as a mixed bi-modal Gaussian state, and show that quantum discord is dependent on the basis which decoherence occurs.

A Note on Black Hole Entropy in Loop Quantum Gravity. (arXiv:1410.5763v1 [gr-qc])

on 2014-10-22 2:51am GMT

Several recent results have hinted that black hole thermodynamics in loop quantum gravity simplifies if one chooses an imaginary Barbero-Immirzi parameter $\gamma=i$. This suggests a connection with $\mathrm{SL}(2,\mathbb{C})$ or $\mathrm{SL}(2,\mathbb{R})$ conformal field theories at the “boundaries” formed by spin network edges intersecting the horizon. I present a bit of background regarding the relevant conformal field theories, along with some speculations about how they might be used to count black hole states. I show, in particular, that a set of unproven but plausible assumptions can lead to a boundary conformal field theory whose density of states matches the Bekenstein-Hawking entropy.

on 2014-10-22 2:50am GMT

In a collision of strongly coupled quantum matter we find that the dynamics of the collision produces regions where a local rest frame cannot be defined because the energy-momentum tensor does not have a real time-like eigenvector. This effect is purely quantum mechanical, since for classical systems, a local rest frame can always be defined. We study the relation with the null and weak energy condition, which are violated in even larger regions, and compare with previously known examples. While no pathologies or instabilities arise, it is interesting that regions without a rest frame are possibly present in heavy ion collisions.

The Dirac equation without zitterbewegung. (arXiv:1410.5418v1 [quant-ph])

on 2014-10-22 2:50am GMT

This paper presents a relativistic symmetrical interpretation of the Dirac equation in 1+1 dimensions which predicts no zitterbewegung for a free spin-1/2 particle. This could resolve the longstanding puzzle of zitterbewegung in relativistic quantum mechanics, and help clarify the physical meaning of the zitterbewegung currently under investigation in many-particle systems. Together with an earlier paper describing a relativistic symmetrical interpretation of the Klein-Gordon equation \cite{Heaney1}, this new interpretation resolves some inconsistencies in the conventional interpretations of both equations. This new interpretation also makes several experimental predictions that differ from the conventional interpretation of the Dirac equation.

Testing macroscopic realism through high-mass interferometry

on 2014-10-21 2:00pm GMT

Author(s): Clive Emary, J. P. Cotter, and Markus Arndt

We define a quantum witness for high-mass matter-wave interferometers that allows us to test fundamental assumptions of macroscopic realism. We propose an experimental realization using absorptive laser gratings and show that such systems can strongly violate a macrorealistic quantum-witness equalit…

[Phys. Rev. A 90, 042114] Published Tue Oct 21, 2014

on 2014-10-21 2:57am GMT

We study the quantum correlation and quantum communication channel of both free scalar and fermionic field in de Sitter space, while the Planckian modification presented by choice of a particular $\alpha$-vacuum has been considered. We show that the occurrence of degradation of quantum entanglement between field modes for an inertial observer in curved space, due to the radiation associated with his cosmological horizon. Comparing with standard Bunch-Davies choice, the possible Planckian physics causes some extra decrement on the quantum correlation, which may provide the means to detect quantum gravitational effects via quantum information methodology in future. Beyond single-mode approximation, we construct proper Unruh modes admitting general $\alpha-$vacua, and find a convergent feature of both bosonic and fermionic entanglement. In particular, we show that the convergent points of fermionic entanglement negativity are dependent on the choice of $\alpha$. Moreover, an one-to-one correspondence between convergent points $H_c$ of negativity and zeros of quantum capacity of quantum channels in de Sitter space has been proved.

on 2014-10-21 2:57am GMT

It is usually stated that quantum mechanics presents problems with the identity of particles, the most radical position -supported by E. Schrodinger- asserting that elementary particles are not individuals. But the subject goes deeper, and it is even possible to obtain states with an unde?ned particle number. In this work we present a set theoretical framework for the description of unde?ned particle number states in quantum mechanics which provides a precise logical meaning for this notion. This construction goes in the line of solving a problem posed by Y. Manin, namely, to incorporate quantum mechanical notions at the foundations of mathematics. We also show that our system is capable of representing quantum superpositions.

Conditions for Anomalous Weak Value. (arXiv:1410.5221v1 [quant-ph])

on 2014-10-21 2:57am GMT

We show that the weak value of any observable in pre- and post-selected states can be expressed as the sum of the average of the observable in the pre-selected state and an anomalous part. We argue that at a fundamental level the anomalous nature of the weak values arises due to the interference between the post-selected state and another quantum state which is orthogonal to the pre-selected state. This provides a necessary and sufficient condition for the anomalous nature of the weak value of a quantum observable. Furthermore, we prove that for two non-commuting observables the product of their anomalous parts cannot be arbitrarily large.

Mathematical and physical meaning of the Bell inequalities. (arXiv:1410.4935v1 [quant-ph])

on 2014-10-21 2:57am GMT

It is shown that the Bell inequalities are closely related to the triangle inequalities involving distance functions amongst pairs of random variables with values {0,1} . A hidden variables model may be defined as a mapping between a set of quantum projection operators and a set of random variables. The model is noncontextual if there is a joint probability distribution. The Bell inequalities are necessary conditions for its existence. The inequalities are most relevant when measurements are performed at space-like separation, a possibility showing a conflict between quantum mechanics and local realism (Bell’s theorem).

On the architecture of spacetime geometry

Classical and Quantum Gravity – latest papers

on 2014-10-21 12:00am GMT

We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein–Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity, the AdS/CFT correspondence and loop quantum gravity, as well as Jacobsonʼs ‘thermodynamic’ perspective of gravity.

Entanglement in curved spacetimes and cosmology

Classical and Quantum Gravity – latest papers

on 2014-10-21 12:00am GMT

We review recent results regarding entanglement in quantum fields in cosmological spacetimes and related phenomena in flat spacetime such as the Unruh effect. We begin with a summary of important results about field entanglement and the mathematics of Bogoliubov transformations that is very often used to describe it. We then discuss the Unruh–DeWitt detector model, which is a useful model of a generic local particle detector. This detector model has been successfully used as a tool to obtain many important results. In this context we discuss two specific types of these detectors: a qubit and a harmonic oscillator. The latter has recently been shown to have important applications when one wants to probe nonperturbative physics of detectors interacting with quantum fields. We then detail several recent advances in the study and application of these ideas, including echoes of the early universe, entanglement harvesting, and a nascent proposal for quantum seismology.

Absence of Black Holes at LHC due to Gravity’s Rainbow. (arXiv:1410.4765v1 [hep-th])

on 2014-10-20 8:53am GMT

In this paper, we investigate the effect of Planckian deformation of quantum gravity on the production of black holes at colliders using the framework of gravity’s rainbow. We demonstrate that a black hole remnant exists for Schwarzschild black holes in higher dimensions using gravity’s rainbow. The mass of this remnant is found to be greater than the energy scale at which experiments were performed at the LHC. We propose this as a possible explanation for the absence of black holes at the LHC. Furthermore, we demonstrate that it is possible for black holes in six (and higher) dimensions to be produced at energy scales that will be accessible in the near future.

An improved derivation of minimum information quantum gravity. (arXiv:1409.0809v2 [gr-qc] UPDATED)

on 2014-10-20 8:53am GMT

Minimum information quantum gravity (MIQG) is a theory of quantum gravity which requires no explicite microscopic quantum structure. In this article, it is shown that the MIQG action can be derived using a more elegant and straight-forward method than in the first existence proof. The required assumptions are dramatically reduced. In particular, former assumptions regarding the existence of quantum boxes, the exact differential of the entropy variation and the role of the boundary can be omitted. Moreover, the open problem of the quantum occupation number per box is solved. Thus, the arguments in favour of MIQG become even more stringent. The remaining assumptions are 1. the principle of optimization of the resulting per imposed degrees of freedom, 2. abstract quantum number conservation, 3. the validity of the laws of thermodynamics, 4. identification of a macroscopic parametrization with space-time and 5. unspecific interactions. Although the requirements are reduced, all former results remain valid. In particular, all well established physics as special cases (Quantum Field Theory, QFT, and General Relativity, GR) follow and all measurable quantities may be computed.

on 2014-10-20 8:52am GMT

We show that the Schr\”odinger-Newton equation, which describes the nonlinear time evolution of self-gravitating quantum matter, can be made compatible with the no-signaling requirement by elevating it to a stochastic differential equation. In the deterministic form of the equation, as studied so far, the nonlinearity would lead to diverging energy corrections for localized wave packets and would create observable correlations admitting faster-than-light communication. By regularizing the divergencies and adding specific random jumps or a specific Brownian noise process, the effect of the nonlinearity vanishes in the stochastic average and gives rise to a linear and Galilean invariant evolution of the density operator.

Increase of entanglement by local PT -symmetric operations. (arXiv:1410.4621v1 [quant-ph])

on 2014-10-20 8:52am GMT

Entanglement plays a central role in the field of quantum information science. It is well known that the degree of entanglement cannot be increased under local operations. Here, we show that the concurrence of a bipartite entangled state can be increased under the local PT -symmetric operation. This violates the property of entanglement monotonicity. We also use the Bell-CHSH and steering inequalities to explore this phenomenon.