arXiv subjects

Ali Akil

Publications and source records attributed to Ali Akil.

11 recordsLinked to original sources

arXiv2026

Noether Symmetries Generate Deterministic Energy-Harvesting Protocols

We consider the general principles for when deterministic energy harvesting (DEH) is possible. DEH means absorbing energy from a fluctuating source without entropy being absorbed. We show that the symmetry structure of the source--harvester dynamics gives a general route beyond existing examples to identify DEH capable source states. Any continuous symmetry with a conserved Noether charge induces a source-side orbit of states that all implement the same deterministic harvester transition, provided the harvester boundary states are symmetry invariant. Consequently, one DEH capable source state with nonzero asymmetry between charge sectors can generate infinitely many simultaneously DEH capable source states. A Jaynes-Cummings model, a three-spin XX chain, and an SU(2) model illustrate the construction. We further extend Noether's theorem to generalised probabilistic theories and thereby generalise our main result. We further establish an asymmetry bound for harvesting, showing that source asymmetry cannot increase on average, while DEH saturates the bound exactly preserving asymmetry.

quant-ph↗

arXiv2025

A Quantum Superposition of Black Hole Evaporation Histories: Recovering Unitarity

Black hole evaporation is one of the most striking phenomena at the interface between gravity and quantum physics. In Hawking's semi-classical treatment, where matter is quantum mechanical and the spacetime is definite and classical, evaporation leads to an apparent loss of unitarity of the overall evolution, and to the so-called black hole information paradox. Here, we go beyond this semi-classical treatment and formulate a toy quantum model of black hole evaporation that allows the black hole to evolve into a superposition of being fully evaporated and not fully evaporated, consistent with the Hawking particles being in a coherent superposition of different energy levels. We model Hawking particle production by the repeated action of quantum-controlled unitaries, generating emission from the quantum black hole and accounting for a quantum coherent back-reaction on the black hole matter state. We show that the probability of full annihilation of the black hole matter increases with time until the black hole is, asymptotically, fully evaporated in every branch of the quantum superposition. We prove that under natural assumptions, this evaporation model is unitary, such that the initial state can in principle be recovered from the final asymptotic state of the radiation.

gr-qc↗

arXiv2025

Black Hole Information From Non-vacuum Localised Quantum States

We revisit Hawking's black hole radiation derivation, including the quantum state of the initial matter forming the black hole. We investigate how non-vacuum initial quantum states, at the past of a black hole geometry, influence the black hole radiation observed at future null infinity $( \mathcal{I}^+)$. We further classify which of the initial state excitations are distinguishable from one another through measurements on the black hole radiation state. We use Algebraic Quantum Field Theory (AQFT) to provide a clear physical interpretation of the results, in terms of localised operations. We then take a concrete example of a black hole made of one large collapsing excitation of mass $M$ and compare it to a same-mass black hole formed due to the collapse of two smaller excitations, of mass $M/2$ each. We find using our formalism that the two cases yield different radiation states and can in principle be distinguished. Our results provide a mechanism for partial information recovery in evaporating black holes, classify what information is recoverable through stimulated emission, and a concrete understanding of the classification based on the AQFT localisation.

gr-qc↗

arXiv2024

The sum of entanglement and subsystem coherence is invariant under quantum reference frame transformations

Recent work on quantum reference frames (QRFs) has demonstrated that superposition and entanglement are properties that change under QRF transformations. Given their utility in quantum information processing, it is important to understand how a mere change of perspective can produce or reduce these resources. Here we find a trade-off between entanglement and subsystem coherence under a QRF transformation, in the form of a conservation theorem for their sum, for two pairs of measures. Moreover, we find a weaker trade-off for any possible pair of measures. Finally, we discuss the implications of this interplay for violations of Bell's inequalities, clarifying that for any choice of QRF, there is a quantum resource responsible for the violation. These findings contribute to a better understanding of the quantum information theoretic aspects of QRFs, offering a foundation for future exploration in both quantum theory and quantum gravity.

quant-ph↗

arXiv2023

A Dark Matter Probe in Accreting Pulsar-Black Hole Binaries

The accretion of dark matter (DM) into astrophysical black holes slowly increases their mass. The rate of this mass accretion depends on the DM model and the model parameters. If this mass accretion effect can be measured accurately enough, it is possible to rule out some DM models, and, with the sufficient technology and the help of other DM constraints, possibly confirm one model. We propose a DM probe based on accreting pulsar-black hole binaries, which provide a high-precision measurement on binary orbital phase shifts induced by DM accretion into black holes, and can help rule out DM models and study the nature of DM.

astro-ph.HE↗

arXiv2022

Semiclassical spacetimes at super-Planckian scales from delocalized sources

We derive the gravitational field and the spacetime metric generated by sources in quantum superposition of different locations. We start by working in a Newtonian approximation, in which the effective gravitational potential is computed as the expectation value of the gravitational potential operator in a Gaussian distribution of width $R$ for the position of the source. The effective gravitational potential is then covariantly uplifted to a fully relativistic metric in general relativity, describing the spacetime generated by averaging over the state of such sources. These results are then rederived and extended by adopting an independent construction in terms of quantum reference frames. We find three classes of quantum effective metrics which are all asymptotically flat and reproduce the Schwarzschild metric at great distances. The solutions differ, however, in the inner core. The quantum uncertainty $\Delta r\sim R$ in the position of the source prevents the radius of the transverse two-sphere to shrink to zero. Depending on the strength of the quantum superposition effects, we have either a nonsingular black hole with a ``quantum hair'' and an event horizon, a one-way wormhole with a critical null throat or a traversable wormhole. We also provide a detailed study of the geometric and thermodynamic properties of the spacetime structure for each of these three families of models, as well as their phenomenology.

gr-qc↗

arXiv2022

Quantum gravity as a communication resource

Quantum information can provide a lens for characterizing the operational implications of spacetime physics. A well-known result in this area is that quantum entanglement is degraded in the vicinity of a black hole. This result treats the black hole and its spacetime as classical. But what if these were to be treated quantum-mechanically? Here, we show that quantum coherence in black hole, and thus spacetime, degrees of freedom can limit the degradation of entanglement, thereby improving the performance of nearby quantum communication protocols. This finding indicates that quantum features of spacetime could serve as resources for quantum information processing.

quant-ph↗

arXiv2021

Conformalons: a new class of black hole mimickers

In any conformally invariant gravitational theory, the space of exact solutions is greatly enlarged. Therefore, we cannot exclude the Weyl's conformal invariance to be spontaneously broken to spherically symmetric vacuum solutions that exclude the spacetime region inside the black hole's event horizon from our Universe. We baptize these solutions {\it conformalons}. It turns out that for all such spacetimes nothing can reach the Schwarzschild event horizon in a finite amount of proper time for conformally coupled ``massive'' particles, or finite values of the affine parameter for massless particles. Therefore, for such vacuum solutions the surface $r = 2 M$ becomes an asymptotic region of the Universe. As a general feature, all conformalons show a gravitational blueshift instead of a gravitational redshift at the unattainable Schwarzschild event horizon, hence avoiding the Trans-Planckian problem in the Hawking evaporation process. Unexpectedly, the Hawking's temperature of the conformalons turns out to be negative and the thermodynamic interpretation leads us to speculate about a possible maximum energy state of the Hawking radiation. Contrary to the Schwarzschild spacetime, for the conformalons the gravitational collapse consists of matter that falls down forever towards the Schwarzschild horizon without ever reaching it. Hence, the annihilation process between the matter and Hawking's negative energy particles takes place outside the surface at $r = 2 M$. Finally, the information is not lost in the whole process of singularity-free collapse and evaporation.

gr-qc↗

arXiv2020

Gauge Invariance, Polar Coordinates and Inflation

We point out the necessity of resolving the apparent gauge dependence in the quantum corrections of cosmological observables for Higgs-like inflation models. We highlight the fact that this gauge dependence is due to the use of an asymmetric background current which is specific to a choice of coordinate system in the scalar manifold. Favoring simplicity over complexity, we further propose a practical shortcut to gauge-independent inflationary observables by using effective potential obtained from a polar-like background current choice. We demonstrate this shortcut for several explicit examples and present a gauge-independent prediction of inflationary observables in the Abelian Higgs model. Furthermore, with Nielsen's gauge dependence identities, we show that for any theory to all orders, a gauge-invariant current term gives a gauge-independent effective potential and thus gauge-invariant inflationary observables.

hep-th↗

arXiv2018

Entanglement swapping in black holes: restoring predictability

Hawking's black hole evaporation process suggests that we may need to choose between quantum unitarity and other basic physical principles such as no-signalling, entanglement monogamy, and the equivalence principle. We here provide a quantum model for Hawking pair black hole evaporation within which these principles are all respected. The model does not involve exotic new physics, but rather uses quantum theory and general relativity. The black hole and radiation are in a joint superposition of different energy states at any stage of the evaporation process. In the particular branch where the black hole mass is 0, the radiation state is pure and one-to-one with the initial state forming the black hole. Thus there is no information loss upon full evaporation. The original Hawking's pair entanglement between infalling and outgoing particles gets transferred to outgoing particles via entanglement swapping, without violation of no-signalling or the entanglement's monogamy. The final state after the full black hole evaporation is pure, without loss of information, violation of monogamy, or the equivalence principle.

hep-th↗

arXiv2018

A Firepoint at the Black Hole Singularity

In his original derivation, Hawking showed that a Schwarzschild black hole is unstable at quantum level and it evolves to a final thermal mixed state violating unitarity. There are some attempts to solve this information paradox based on a high energy surface located at the black hole event horizon: the firewall. In the wake of these proposals, we here propose the singularity itself as a "firepoint" capable to break the entanglement between the "int" and the "out" states created through the Hawking process. In this paper the singularity takes active part in the information paradox in two similar ways. In the first way, the singularity, coming in contact with the the "int" state, produces a pure state outside the horizon, but it violates causality allowing people inside the black hole to send signals to the outside. The second way consists of a map that breaks the entanglement between the interior and exterior of the black hole still using the singularity yet without violating causality. The monogamy theorem is not violated whether in the first where the "out" radiation state is rendered pure directly or in the second way where the Page idea is made possible again to solve the information loss problem.

hep-th↗