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4 pages, 167 KiB

Open AccessEditorial

Some Notes on Quantum Information in Spacetime

by Ignazio Licata

Entropy 2020, 22(8), 864; https://doi.org/10.3390/e22080864 - 6 Aug 2020

Viewed by 2264

Abstract

The results obtained since the 70s with the study of Hawking radiation and the Unruh effect have highlighted a new domain of authority of relativistic principles [...] Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

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15 pages, 876 KiB

Open AccessArticle

Quantum Hair on Colliding Black Holes

by Lawrence Crowell and Christian Corda

Entropy 2020, 22(3), 301; https://doi.org/10.3390/e22030301 - 5 Mar 2020

Cited by 2 | Viewed by 3827

Abstract

Black hole (BH) collisions produce gravitational radiation which is generally thought, in a quantum limit, to be gravitons. The stretched horizon of a black hole contains quantum information, or a form of quantum hair, which is a coalescence of black holes participating in [...] Read more.

Black hole (BH) collisions produce gravitational radiation which is generally thought, in a quantum limit, to be gravitons. The stretched horizon of a black hole contains quantum information, or a form of quantum hair, which is a coalescence of black holes participating in the generation of gravitons. This may be facilitated with a Bohr-like approach to black hole (BH) quantum physics with quasi-normal mode (QNM) approach to BH quantum mechanics. Quantum gravity and quantum hair on event horizons is excited to higher energy in BH coalescence. The near horizon condition for two BHs right before collision is a deformed

A d S

spacetime. These excited states of BH quantum hair then relax with the production of gravitons. This is then argued to define RT entropy given by quantum hair on the horizons. These qubits of information from a BH coalescence should then appear in gravitational wave (GW) data. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

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14 pages, 292 KiB

Open AccessArticle

General Relativistic Wormhole Connections from Planck-Scales and the ER = EPR Conjecture

by Fabrizio Tamburini and Ignazio Licata

Entropy 2020, 22(1), 3; https://doi.org/10.3390/e22010003 - 18 Dec 2019

Cited by 14 | Viewed by 5235

Abstract

Einstein’s equations of general relativity (GR) can describe the connection between events within a given hypervolume of size L larger than the Planck length

L_{P}

in terms of wormhole connections where metric fluctuations give rise to an indetermination relationship that involves the [...] Read more.

Einstein’s equations of general relativity (GR) can describe the connection between events within a given hypervolume of size L larger than the Planck length

L_{P}

in terms of wormhole connections where metric fluctuations give rise to an indetermination relationship that involves the Riemann curvature tensor. At low energies (when

L ≫ L_{P}

), these connections behave like an exchange of a virtual graviton with wavelength

λ_{G} = L

as if gravitation were an emergent physical property. Down to Planck scales, wormholes avoid the gravitational collapse and any superposition of events or space–times become indistinguishable. These properties of Einstein’s equations can find connections with the novel picture of quantum gravity (QG) known as the “Einstein–Rosen (ER) = Einstein–Podolski–Rosen (EPR)” (ER = EPR) conjecture proposed by Susskind and Maldacena in Anti-de-Sitter (AdS) space–times in their equivalence with conformal field theories (CFTs). In this scenario, non-traversable wormhole connections of two or more distant events in space–time through Einstein–Rosen (ER) wormholes that are solutions of the equations of GR, are supposed to be equivalent to events connected with non-local Einstein–Podolski–Rosen (EPR) entangled states that instead belong to the language of quantum mechanics. Our findings suggest that if the ER = EPR conjecture is valid, it can be extended to other different types of space–times and that gravity and space–time could be emergent physical quantities if the exchange of a virtual graviton between events can be considered connected by ER wormholes equivalent to entanglement connections. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

29 pages, 377 KiB

Open AccessFeature PaperArticle

The Information Loss Problem: An Analogue Gravity Perspective

by Stefano Liberati, Giovanni Tricella and Andrea Trombettoni

Entropy 2019, 21(10), 940; https://doi.org/10.3390/e21100940 - 25 Sep 2019

Cited by 18 | Viewed by 3160

Abstract

Analogue gravity can be used to reproduce the phenomenology of quantum field theory in curved spacetime and in particular phenomena such as cosmological particle creation and Hawking radiation. In black hole physics, taking into account the backreaction of such effects on the metric [...] Read more.

Analogue gravity can be used to reproduce the phenomenology of quantum field theory in curved spacetime and in particular phenomena such as cosmological particle creation and Hawking radiation. In black hole physics, taking into account the backreaction of such effects on the metric requires an extension to semiclassical gravity and leads to an apparent inconsistency in the theory: the black hole evaporation induces a breakdown of the unitary quantum evolution leading to the so-called information loss problem. Here, we show that analogue gravity can provide an interesting perspective on the resolution of this problem, albeit the backreaction in analogue systems is not described by semiclassical Einstein equations. In particular, by looking at the simpler problem of cosmological particle creation, we show, in the context of Bose–Einstein condensates analogue gravity, that the emerging analogue geometry and quasi-particles have correlations due to the quantum nature of the atomic degrees of freedom underlying the emergent spacetime. The quantum evolution is, of course, always unitary, but on the whole Hilbert space, which cannot be exactly factorized a posteriori in geometry and quasi-particle components. In analogy, in a black hole evaporation one should expect a continuous process creating correlations between the Hawking quanta and the microscopic quantum degrees of freedom of spacetime, implying that only a full quantum gravity treatment would be able to resolve the information loss problem by proving the unitary evolution on the full Hilbert space. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

25 pages, 454 KiB

Open AccessArticle

A Physically-Motivated Quantisation of the Electromagnetic Field on Curved Spacetimes

by Ben Maybee, Daniel Hodgson, Almut Beige and Robert Purdy

Entropy 2019, 21(9), 844; https://doi.org/10.3390/e21090844 - 30 Aug 2019

Cited by 7 | Viewed by 4002

Abstract

Recently, Bennett et al. (Eur. J. Phys. 37:014001, 2016) presented a physically-motivated and explicitly gauge-independent scheme for the quantisation of the electromagnetic field in flat Minkowski space. In this paper we generalise this field quantisation scheme to curved spacetimes. Working within the [...] Read more.

Recently, Bennett et al. (Eur. J. Phys. 37:014001, 2016) presented a physically-motivated and explicitly gauge-independent scheme for the quantisation of the electromagnetic field in flat Minkowski space. In this paper we generalise this field quantisation scheme to curved spacetimes. Working within the standard assumptions of quantum field theory and only postulating the physicality of the photon, we derive the Hamiltonian,

\hat{H}

, and the electric and magnetic field observables,

\hat{E}

and

\hat{B}

, respectively, without having to invoke a specific gauge. As an example, we quantise the electromagnetic field in the spacetime of an accelerated Minkowski observer, Rindler space, and demonstrate consistency with other field quantisation schemes by reproducing the Unruh effect. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

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9 pages, 247 KiB

Open AccessArticle

Summoning, No-Signalling and Relativistic Bit Commitments

by Adrian Kent

Entropy 2019, 21(5), 534; https://doi.org/10.3390/e21050534 - 25 May 2019

Viewed by 3120

Abstract

Summoning is a task between two parties, Alice and Bob, with distributed networks of agents in space-time. Bob gives Alice a random quantum state, known to him but not her, at some point. She is required to return the state at some later [...] Read more.

Summoning is a task between two parties, Alice and Bob, with distributed networks of agents in space-time. Bob gives Alice a random quantum state, known to him but not her, at some point. She is required to return the state at some later point, belonging to a subset defined by communications received from Bob at other points. Many results about summoning, including the impossibility of unrestricted summoning tasks and the necessary conditions for specific types of summoning tasks to be possible, follow directly from the quantum no-cloning theorem and the relativistic no-superluminal-signalling principle. The impossibility of cloning devices can be derived from the impossibility of superluminal signalling and the projection postulate, together with assumptions about the devices’ location-independent functioning. In this qualified sense, known summoning results follow from the causal structure of space-time and the properties of quantum measurements. Bounds on the fidelity of approximate cloning can be similarly derived. Bit commitment protocols and other cryptographic protocols based on the no-summoning theorem can thus be proven secure against some classes of post-quantum but non-signalling adversaries. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

13 pages, 669 KiB

Open AccessArticle

Simultaneous Classical Communication and Quantum Key Distribution Based on Plug-and-Play Configuration with an Optical Amplifier

by Xiaodong Wu, Yijun Wang, Qin Liao, Hai Zhong and Ying Guo

Entropy 2019, 21(4), 333; https://doi.org/10.3390/e21040333 - 27 Mar 2019

Cited by 11 | Viewed by 3809

Abstract

We propose a simultaneous classical communication and quantum key distribution (SCCQ) protocol based on plug-and-play configuration with an optical amplifier. Such a protocol could be attractive in practice since the single plug-and-play system is taken advantage of for multiple purposes. The plug-and-play scheme [...] Read more.

We propose a simultaneous classical communication and quantum key distribution (SCCQ) protocol based on plug-and-play configuration with an optical amplifier. Such a protocol could be attractive in practice since the single plug-and-play system is taken advantage of for multiple purposes. The plug-and-play scheme waives the necessity of using two independent frequency-locked laser sources to perform coherent detection, thus the phase noise existing in our protocol is small which can be tolerated by the SCCQ protocol. To further improve its capabilities, we place an optical amplifier inside Alice’s apparatus. Simulation results show that the modified protocol can well improve the secret key rate compared with the original protocol whether in asymptotic limit or finite-size regime. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

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13 pages, 249 KiB

Open AccessArticle

On the Thermodynamic Origin of Gravitational Force by Applying Spacetime Entanglement Entropy and the Unruh Effect

by Shujuan Liu and Hongwei Xiong

Entropy 2019, 21(3), 296; https://doi.org/10.3390/e21030296 - 19 Mar 2019

Viewed by 3557

Abstract

We consider the thermodynamic origin of the gravitational force of matter by applying the spacetime entanglement entropy and the Unruh effect originating from vacuum quantum fluctuations. By analyzing both the local thermal equilibrium and quasi-static processes of a system, we may get both [...] Read more.

We consider the thermodynamic origin of the gravitational force of matter by applying the spacetime entanglement entropy and the Unruh effect originating from vacuum quantum fluctuations. By analyzing both the local thermal equilibrium and quasi-static processes of a system, we may get both the magnitude and direction of Newton’s gravitational force in our theoretical model. Our work shows the possibility that the elusive Unruh effect has already shown its manifestation through gravitational force. Full article

(This article belongs to the Special Issue Relativistic Quantum Information)

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