arXiv subjects

Yasusada Nambu

Publications and source records attributed to Yasusada Nambu.

At least 19 recordsLinked to original sources

arXiv2026

Squeezed quantum states and partner modes in the moving mirror model of black hole evaporation

The standard moving mirror model in (1+1)-dimensional spacetime is known to reproduce several quantum aspects of black hole evaporation. A perfectly reflecting, accelerating mirror can emit radiation whose frequency spectrum resembles that of Hawking radiation, although this correspondence holds only under certain approximations. Moreover, the standard in-out formulation does not provide a natural notion of the partner modes associated with the Hawking radiation. In this paper, we reformulate the moving mirror model in terms of Rindler/Milne modes. This formulation not only attributes the origin of the required approximations to mode squeezing effects but also naturally incorporates the notion of partner modes. Furthermore, as a consequence of these mode squeezing effects, the radiation received by an inertial observer at future null infinity exhibits additional nontrivial quantum correlations, even though its frequency spectrum approximately follows a Bose--Einstein or Fermi--Dirac distribution.

gr-qc↗

arXiv2026

Energy Flux as an Entanglement Current in Moving-Mirror Radiation

In this work, we investigate the quantum entanglement properties of analog Hawking radiation produced by a moving mirror. Using two detector modes defined through window functions on a quantum field, we quantify the bipartite entanglement established between these modes. Our results reveal that the amount of entanglement accessible to the detectors increases when the mirror follows trajectories with non-monotonic, time-dependent acceleration, which are accompanied by the emission of negative energy flux. This indicates that the negative energy flux acts as a channel through which information can be returned. To substantiate this perspective, we examine how the recovery or reconstruction of the associated partner modes is related to the negative energy flux emitted by the mirror.

gr-qc↗

arXiv2025

Entanglement Harvesting from Quantum Field: Insights via the Partner Formula

We examine the condition necessary for extracting entanglement from a quantum field through the use of two local modes A and B (detector modes). We show that Simon's entanglement criterion for the bipartite Gaussian state can be reformulated in terms of commutators between the canonical operators of the detector mode B and the partner mode P of the detector mode A. Using the profile representation of detector modes, we identify that harvesting is prohibited under certain specific conditions. According to analyses based on moving mirror models, Hawking radiation originates from the Milne modes at past null infinity, that reflect off at the mirror and ultimately transform into real particle modes. Drawing parallels between the Unruh effect and Hawking radiation, our findings indicate an absence of quantum correlations between ``real particles" emitted as Hawking radiation.

gr-qc↗

arXiv2025

Acceleration-induced radiation from a qudit particle detector model

We nonperturbatively examine the emission rate of acceleration-induced radiation from a uniformly accelerated gapless qudit-type Unruh-DeWitt detector. We find that the emission rate can be written as Larmor's formula multiplied by a factor that depends on the detector's initial state. In particular, certain initial states of integer-spin detectors do not produce radiation. Although the appearance of Larmor's formula may suggest a classical phenomenon, we argue that the resulting radiation is fundamentally distinct from that of structureless classical sources, as it evolves into a multimode coherent state correlated with the detector's internal degree of freedom. Thus, gapless detectors cannot be treated as structureless sources, as previously proposed.

gr-qc↗

arXiv2024

Gravitational entanglement witness through Einstein ring image

We investigate the interplay between quantum theory and gravity by exploring gravitational lensing and Einstein ring images in a weak gravitational field induced by a mass source in spatial quantum superposition. We analyze a quantum massless scalar field propagating in two distinct models of gravity: the first quantized Newtonian gravity (QG) model, which generates quantum entanglement between the mass source and other systems, and the Schr\"odinger-Newton (SN) gravity model, which does not produce entanglement. Visualizing the two-point correlation function of the scalar field, we find that the QG model produces a composition of multiple Einstein rings, reflecting the spatial superposition of the mass source. By contrast, the SN model yields a single deformed ring image, representing a classical spacetime configuration. Furthermore, we introduce a specific quantity named the which-path information indicator and visualize its image. The QG model again reveals multiple Einstein rings, while the image intensity in the SN model notably vanishes. Our findings provide a visual approach to witness gravity-induced entanglement through distinct features in Einstein ring images. This study advances our understanding of quantum effects in general relativistic contexts and establishes a foundation for future studies of other relativistic phenomena.

gr-qc↗

arXiv2024

Second-Order Coherence as an Indicator of Quantum Entanglement of Hawking Radiation in Moving-Mirror Models

The second-order coherence of light is a widely recognized physical quantity used to assess the quantum characteristics of light, and its properties have been extensively investigated in the field of quantum optics. Recently, it has been proposed that second-order coherence can be utilized as an indicator of quantum entanglement. In this study, we evaluated the second-order coherence in the context of the moving-mirror model, which serves as an analog model for Hawking radiation from a black hole. We discuss the relation between entanglement and the second-order coherence of Hawking radiation paying attention to the noise effect due to the thermality of Hawking radiation, which reduces the quantum correlation in the entanglement-harvesting protocol with two-qubit detectors.

quant-ph↗

arXiv2024

Hawking radiation in quantum Hall system with an expanding edge: application of anomaly method

The relationship between gravitational anomalies and Hawking radiation of black holes was revealed by Wilczek and Robinson. In this study, we apply their method to an analogue de Sitter spacetime in the quantum Hall system with an expanding edge. Because this system is chiral, there is no need to impose the condition of ingoing modes near the horizon, which was assumed in the original method. Moreover, this system is structured so that the de Sitter space is sandwiched between two flat spaces, and although the effects of the anomaly would not appear in an ordinal de Sitter spacetime, they manifest themselves as boundary conditions between the de Sitter and the flat regions. By performing calculations under these boundary conditions, we obtain the flux of Hawking radiation in the outer flat region with the Gibbons-Hawking temperature of the de Sitter horizon.

gr-qc↗

arXiv2024

Stochastic inflation and entropy bound in de Sitter spacetime

We investigate the entropy dynamics of de Sitter spacetime during the inflationary phase. The cosmological horizon in de Sitter spacetime, which limits the causally accessible region for an observer, exhibits thermal properties similar to a black hole event horizon. According to holographic principles, the entropy within a causally connected region is bounded by its surface area. However, this entropy bound is violated during the eternal phase of inflation. To address these violations from a quantum information perspective, we adopt a stochastic approach to cosmic inflation. Specifically, we analyze the Shannon entropy of the inflaton field's probability distribution, which mirrors the behavior of the entanglement entropy of a Hubble-sized region in stochastic inflation. Using the volume-weighted probability distribution for the inflaton field, we demonstrate a significant entropy behavior in de Sitter spacetime.

hep-th↗

arXiv2024

The final burst of the moving mirror is unrelated to the partner mode of analog Hawking radiation

Flying mirrors with appropriate trajectories have been recognized as an analog system that mimics black hole Hawking evaporation and have been widely investigated. It has recently been suggested that the partner mode of the analog Hawking radiation emitted from a moving mirror would manifest itself through a final burst when the mirror executes a sudden stop. Here we argue the opposite via the partner formula for the moving mirror model. By expanding the theoretical foundation of the partner formula and augmenting it with numerical analysis, we demonstrate that the supposed final burst is induced by a shock that requires the input of external energy, whereas the Hawking radiation partner mode, which is associated with the zero-point vacuum fluctuations, is not responsible for the burst.

gr-qc↗

arXiv2024

Large violation of Leggett-Garg inequalities with coherent-state projectors for a harmonic oscillator and chiral scalar field

We investigate violations of Leggett-Garg inequalities (LGIs) for a harmonic oscillator and a (1+1)-dimensional chiral scalar field with coherent-state projectors, which is equivalent to a heterodyne-type measurement scheme. For the harmonic oscillator, we found that the vacuum and thermal states violated the LGIs by evaluating the two-time quasi-probability distribution function. In particular, we demonstrate that the value of the two-time quasi-probability reaches -0.123 for a squeezed coherent-state projector, which is equivalent to 98% of the Lüders bound corresponding to the maximal violation of the LGIs. We also find a violation of the LGIs for the local mode of a quantum chiral scalar field by constructing a coherent-state projector similar to the harmonic oscillator case. In contrast to the harmonic oscillator, the periodicity in the time direction of the quasi-probability disappears, which is related to the existence of quantum entanglement between the local mode and its complementary degrees of freedom.

quant-ph↗

arXiv2023

Entanglement partners and monogamy in de Sitter universes

We investigate entanglement of local spatial modes defined by a quantum field in a de Sitter universe. The introduced modes show dis-entanglement behavior when the separation between two regions where local modes are assigned becomes larger than the cosmological horizon. To understand the emergence of separability between these local modes, we apply the monogamy inequality proposed by S. Camalet. We embed the focusing bipartite mode defined by the quantum field in a pure four-mode Gaussian state, and identify its partner modes. Then applying a Gaussian version of the monogamy relation, we show that the external entanglement between the bipartite mode and its partner modes constrains the entanglement of the bipartite mode. Thus the emergence of separability of local modes in the de Sitter universe can be understood from the perspective of entanglement monogamy.

gr-qc↗

arXiv2023

Analog de Sitter universe in quantum Hall systems with an expanding edge

Expanding edges in quantum Hall systems can become a simulator of quantum 1+1 dimensional expanding universes. In these systems, edge exciations are represented as a chiral scalar field in curved spacetimes. We investigate Hawking radiation and entanglement behavior predicted by this model assuming that the expansion law of the edge region corresponds to a de Sitter universe. As observable quantities for the quantum field, local spatial modes associated with detection regions are introduced using window functions for the field, and their correlations are evaluated. We found impact of Hawking radiation caused by the edge expansion on auto-correlation functions of the local modes, and confirmed that entanglement death due to Hawking radiation occurs. This behavior of entanglement is related to ``quantum to classical transition" in cosmic inflations.

gr-qc↗

arXiv2022

Quantumness of gravity in harmonically trapped particles

This study investigates the quantumness of gravity under the setup of the atomic interferometry from the viewpoint of mass-energy equivalence. We evaluated interference visibility considering a particle with internal energy levels in a harmonic trapping potential. As per the result, for a spatially superposed gravitational source mass, interference visibility exhibits collapse and revival behavior, which implies that an initial separable internal state evolves to the entangled state with respect to the degrees of freedom of the center of mass, the internal energy levels, and the external source state. In particular, it does not exhibit revival behavior when gravity is treated as a quantum interaction, while it revives with a finite period for a semiclassical treatment of gravity. We also examined the temporal behavior of entanglement negativity and found that the nonrevival behavior of visibility reflects the creation of the entanglement between the internal energy state and the external source state which is uniquely induced by the quantum interaction of gravity in accordance with the weak equivalence principle.

gr-qc↗

arXiv2022

Entanglement Renyi entropy of two disjoint intervals for large $c$ Liouville field theory

Entanglement entropy (EE) is a quantitative measure of the effective degrees of freedom and the correlation between the sub-systems of a physical system. Using the replica trick, we can obtain the EE by evaluating the entanglement Renyi entropy (ERE). The ERE is a $q$-analogue of the EE and expressed by the $q$ replicated partition function. In the semi-classical approximation, it is apparently easy to calculate the EE because the classical action represents the partition function by the saddle point approximation and we do not need to perform the path integral for the evaluation of the partition function. In previous studies, it has been assumed that only the minimal-valued saddle point contributes to the EE. In this paper, we propose that all the saddle points contribute equally to the EE by dealing carefully with the semi-classical limit and then the $q \to 1$ limit. For example, we numerically evaluate the ERE of two disjoint intervals for the large $c$ Liouville field theory with $q \sim 1$. We exploit the BPZ equation with the four twist operators, whose solution is given by the Heun function. We determine the ERE by tuning the behavior of the Heun function such that it becomes consistent with the geometry of the replica manifold. We find the same two saddle points as previous studies for $q \sim 1$ in the above system. Then, we provide the ERE for the large but finite $c$ and the $q \sim 1$ in case that all the saddle points contribute equally to the ERE. Based on this work, it shall be of interest to reconsider EE in other semi-classical physical systems with multiple saddle points.

hep-th↗

arXiv2022

Deep Learning Metric Detectors in General Relativity

We consider conceptual issues of deep learning (DL) for metric detectors using test particle geodesics in curved spacetimes. Advantages of DL metric detectors are emphasized from a view point of general coordinate transformations. Two given metrics (two spacetimes) are defined to be conneted by a DL isometry if their geodesic image data cannot be discriminated by any DL metric detector at any time. The fundamental question of when the DL isometry appears is extensively explored. If the two spacetimes connected by the DL isometry are in superposition of quantum gravity theory, the post-measurement state may be still in the same superposition even after DL metric detectors observe the superposed state. We also demonstrate metric-detection DL's in 2+1 dimensional anti-de Sitter (AdS) spacetimes to estimate the cosmological constants and Brown-Henneaux charges. In the AdS/CFT correspondence dictionary, it may be expected that such metric detectors in the AdS bulk region correspond to quantum measurement devices in the CFT at the AdS boundary.

gr-qc↗

arXiv2022

Holographic entanglement entropy of two disjoint intervals in AdS$_3$/CFT$_2$

The Ryu-Takayanagi conjecture predicts a holographic dual of the entanglement entropy of a CFT. It proposes that the entanglement entropy is given by the area of the minimal surface in the dual spacetime. In the semi-classical limit, this conjecture is supported by the saddle point approximation. If there are multiple classical solutions, it is assumed that only the minimal action contributes to the entanglement entropy. However, we will point out that these saddles equally contribute to the entanglement entropy in some cases. Therefore, the derivation of the conjecture is incomplete if there are multiple extremal surfaces that extend from a sub-system on the AdS boundary. We will consider two disjoint intervals in CFT$_{1+1}$ as the simplest but non-trivial example, and propose another candidate for a holographic dual of the entanglement entropy of this system, which is the sum of all the signed areas of extremal surfaces in the dual spacetime. After that, we will derive it from the CFT calculations and propose the corresponding gravity side action.

hep-th↗

arXiv2022

Leggett-Garg inequalities for testing quantumness of gravity

In this study, we determine a violation of the Leggett-Garg inequalities due to gravitational interaction in a hybrid system consisting of a harmonic oscillator and a spatially localized superposed particle. The violation of the Leggett-Garg inequalities is discussed using the two-time quasiprobability in connection with the entanglement negativity generated by gravitational interaction. It is demonstrated that the entanglement suppresses the violation of the Leggett-Garg inequalities when one of the two times of the quasiprobability t_1 is chosen as the initial time. Further, it is shown that the Leggett-Garg inequalities are generally violated due to gravitational interaction by properly choosing the configuration of the parameters, including t_1 and t_2, which are the times of the two-time quasiprobability. The feasibility of detecting violations of the Leggett-Garg inequalities in hybrid systems is also discussed.

quant-ph↗

arXiv2022

Particle Creation and Entanglement in Dispersive Model with Step Velocity Profile

We investigate particle creation and entanglement structure in a dispersive model with subliminal dispersion relation. Assuming the step function spatial velocity profile of the background flow, mode functions for a massless scalar field is exactly obtained by the matching method. Power spectrums of created particles are calculated for the subsonic and the transsonic flow cases. For the transsonic case, the sonic horizon exists and created particles show the Planckian distribution for low frequency region but the thermal property disappears for high frequency region near the cutoff frequency introduced by the non-linear dispersion. For the subsonic case, although the sonic horizon does not exist, the effective group velocity horizon appears due to the non-linear dispersion for high frequency region and approximate thermal property of the power spectrum arises. Relation between particle creation and entanglement between each mode is also discussed.

gr-qc↗