arXiv subjects

Sunandan Gangopadhyay

Publications and source records attributed to Sunandan Gangopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

Spin-curvature effects in slowly rotating black hole spacetime

We investigate the spin-curvature interaction for a massive spin-1/2 quantum particle in a slowly rotating black hole spacetime. In contrast to the flat Minkowski space, the curved spacetime introduces non-trivial spin connections into the Dirac equation due to the direct curvature-spin interaction. The force arising from this non-trivial spin-curvature coupling makes the particle deviate from its geodesics. We first calculate the tetrads in the co-moving frame of the particle in the slowly rotating black hole background. We then compute the components of the relativistic quantum force, arising from the spin-curvature interaction, using these co-moving tetrads up to linear order in the rotation parameter ($a$) of the black hole. The procedure of computing the force is based on the WKB approximation and the Gordon decomposition method for the Dirac probability four current. Our results show that the rotation parameter modifies the magnitude of the force by introducing a term that decays more rapidly with radial distance than the force magnitude in a static spherically symmetric Schwarzschild black hole spacetime.

hep-th↗

arXiv2026

Mixed state entanglement measures for open string geometry

In the gauge/gravity framework flavor degrees of freedom are introduced to the gauge theory side through the insertion of probe flavor $D$ branes in the supergravity background. Scalar, vector or spinor fluctuations on theses flavor branes perceive neither the background supergravity metric nor the worldvolume-induced metric of the flavor branes. Indeed, they follow an effective metric called the open string metric (OSM). Through a proper choice of worldvolume gauge fields, one can induce a horizon structure in these OSMs. Studying holographic information-theoretic quantities in these kinds of geometries is equivalent to studying information-theoretic quantities in the flavor sector on the gauge theory side. In this article, we have studied various information-theoretic measures like mutual information, entanglement wedge cross-section and entanglement negativity for an open string geometry. All the calculations are done by considering strip-like parallel boundary subsystems for three- and four-dimensional OSMs. We have also measured the change in these mixed-state information-theoretic quantities from the relevant quantities in the pure AdS geometry due to the application of a background electric field.

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arXiv2026

New insights on mutual information in the island approach to the Page curve

In this article, we have presented one of the very important observations, regarding the behavior of mutual information of two different sets of subsystems in the after Page time scenario. This provides us with some deep insights about the redistribution of geometrical correlation between the two different sets of subsystems on a Cauchy slice. In our earlier works, we have shown how the saturation of mutual information between two specific subsystems plays a vital role in obtaining the correct Page curve for the eternal black hole. In those works, we have shown that the mutual information between $B_+$ and $B_-$, that is, $I(B_{+}: B_{-})$, vanishes at scrambling time, which leads to the correct Page curve. That means that at scrambling time, there is no correlation between $B_+$ and $B_-$. Remarkably, it is observed that at this particular value of observer's time, the mutual information between $\mathcal{I}$ and $R$, that is, $I(\mathcal{I}:R)$, becomes singular. This indicates that the regions $\mathcal{I}$ and $R$ become maximally entangled. This provides us with a notion of the transfer of geometric correlation between different regions on the Cauchy slice. In this work, we have also provided a way to calculate the tripartite mutual information of regions $\mathcal{I}$,~$R_+$ and $R_-$, that is, $I(\mathcal{I}:R_+:R_-)$ on the Cauchy slice using the earlier results involving the bipartite regions. This is a new result which was missing in the earlier literature.

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arXiv2026

Quantum corrections to the black hole entropy using the brick wall model

It was suggested by 't Hooft that to avoid the divergence of the free energy or entropy of the probe fields, one has to introduce an infrared (IR) cutoff. This introduction of the IR cutoff, namely, the brick wall model, has been studied for the Schwarzschild black hole and has been shown by 't Hooft that to have the entropy of the order of Bekenstein-Hawking entropy, the cutoff must be of the order of Planck length. In this study, we investigated the brick wall model in the context of a Reissner-Nordstróm, quantum corrected Schwarzschild and quantum corrected Reissner-Nordström black holes. In particular, we have shown that the brick wall model can give logarithmic correction to the black hole entropy in the subleading order. The important aspect of our analysis is that we have not considered the near horizon approximation of the lapse function and get more accurate correction to the entropy. This corrected entropy has an important feature that it is larger than the usual Bekenstein-Hawking entropy which aligns with the result of Barrow entropy.

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arXiv2026

Entanglement Entropy and Complexity of Multicomponent Universe from Holography

Recent studies in \cite{Park:2020jio,Paul:2025gpk} have calculated various holographic information-theoretic quantities of the four-dimensional FLRW universe for different matter-dominated eras using the braneworld model of cosmology. These studies are done for a single matter component, which is a good toy model for understanding the entanglement properties of the universe. However, for a more realistic model, one should consider a scenario where our universe has coexisting matter components like radiation-dark matter or radiation-exotic matter, etc. In this work, we have presented a systematic way to study various holographic information-theoretic quantities, namely, entanglement entropy and complexity, of the FLRW universe in the presence of coexisting matter components. We have shown that the black brane geometry in the presence of $p$-brane gas indeed supports the existence of a universe with two-component matter sources. The second Israel junction condition, along with the Ryu-Takayanagi formula, is used to compute the time-dependent holographic entanglement entropy of the universe with coexisting radiation-dark matter and radiation-exotic matter. The expression of the time-dependent volume complexity is also evaluated in these scenarios. For both universes, these information-theoretic quantities show a clear radiation dependence in the early time and matter and exotic matter dominance in the late time, which is consistent with the thermal history of the universe \cite{WMAP:2010qai,WMAP:2010sfg,Planck:2014loa,Planck:2018vyg}.

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arXiv2026

One-point holographic correlator in the expanding universe

In this article, we have calculated the time-dependent thermal one-point function of massive operators within an expanding universe. We employ the Randall-Sundrum II braneworld model combined with a $p$-brane gas in the bulk, enabling us to represent various matter-dominated cosmological scenarios localised on the brane. Applying the geodesic formula introduced by Grinberg and Maldacena in \cite{Grinberg:2020fdj}, we have calculated the thermal one-point functions of massive operators within the universe. The time-dependent one-point functions for different matter-dominated universes, both single-component and multi-component, are derived from the brane's evolving radial position. The time-dependent positions of the branes have been obtained using the second Israel junction condition. Additionally, we have analyzed the early and late-time behaviors of the thermal one-point function across various matter-dominated universes.

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arXiv2026

Post-Newtonian analysis of the quantum signatures of gravity

In a recent work \href{https://doi.org/10.1103/PRXQuantum.2.010325}{PRX QUANTUM 2 (2021) 010325}, a new way of investigating quantum gravity signatures using quantum information theoretic techniques, have been proposed. The primary result of this analysis revealed that non-Gaussianity can arise only through the consideration of a quantum model for the gravity part. Compared to classical gravity, only quantum gravity can result in non-quadratic operators in the Hamiltonian which leads to the non-Gaussian behavior. In our current analysis, we have considered a more realistic scenario taking into effect leading order post-Newtonian corrections in the analysis. We have stayed with the same model of a Bose-Einstein condensate placed inside a harmonic trap potential which indeed works as the detector of the non-Gaussianity generated due to quantum gravitational effects. Bose-Einstein condensates are experimentally well studied; apart from being a single quantum system, they include Feshbach resonances, which helps tuning the strength of the electromagnetic interactions which in principle can be set to zero. This is important since it can help distinguish quantum gravity from electromagnetic interactions without affecting gravitational interactions, and any non-Gaussianity can then be solely attributed to quantum gravity. We observe that the signal to noise ratio gets slightly damped due to the post-Newtonian effects taken under consideration.

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arXiv2026

Effect of non-conformal deformation on the gapped quasi-normal modes and the holographic implications

The spectral curve of quasinormal modes for a massive real scalar field in the background of a non-conformal black brane geometry has been obtained by utilizing a Frobenius type near-horizon expansion. The gauge/gravity duality maps this to the computation of spectral curve of a massive scalar operator $\mathcal{O}_ϕ$ for a large-$N$ conformal field theory with irrelevant type non-conformal deformation. In this context, non-conformality has been holographically introduced by using the Einstein-dilaton theory with Liouville type dilaton potential as the bulk theory. It has been observed that the obtained quasinormal modes are characterized by specific gapped dispersion relations. The pole-skipping points have also been computed and classified based upon different dispersion relations satisfied by them. The effect of non-conformality is evident from these results. The radius of convergence of the derivative expansion in the momentum space is then computed from the critical points of the spectral curve. It has been observed that presence of non-conformality increases the domain of applicability of the derivative expansion in momentum space, as it increases the radius of convergence for a given conformal dimension. The comparison between the convergence radii and the absolute momenta corresponding to lowest order pole-skipping points also leads to some interesting findings.

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arXiv2026

Holographic Brownian dynamics of a heavy particle in a boosted thermal plasma background

In this work, we have performed a detailed holographic analysis of the stochastic dynamics of a heavy particle propagating through a strongly coupled plasma moving with a constant velocity along a fixed spatial direction. To model this scenario within the framework of the AdS/CFT correspondence, we consider a boosted AdS black brane geometry in the bulk. The boost corresponds to the uniform motion of the plasma on the boundary field theory side. The presence of this boost introduces a preferred direction, leading to an anisotropic environment in which the behavior of the Brownian particle differs depending on its direction of motion. Consequently, we examine two distinct cases, namely,Brownian motion parallel to the direction of the boost and motion perpendicular to it. In this work we have computed the diffusion coefficient for both along the boost and perpendicular to the boost directions. We have obtained the diffusion coefficient by following the two different approaches in both the cases. These complementary approaches yield consistent results, thereby reinforcing the reliability of the computations carried out. Additionally, we verify the fluctuation-dissipation theorem within this anisotropic setup, confirming its validity in both longitudinal and transverse to the direction of boost. Our findings provide deeper insight into the non-equilibrium transport properties of strongly coupled plasma and further elucidate the holographic description of Brownian motion in anisotropic backgrounds. Finally, we proceed to holographically compute the Butterfly velocity by using the entanglement wedge subregion duality and express the diffusion coefficients in terms of the chaotic observables.

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arXiv2026

Gravity mediated entanglement of phonons in Bose-Einstein condensates

The eigenstates of two test-masses (where each test-mass is placed inside of a harmonic trap) separated by a distance, can get entangled where gravity acts as the mediator of entanglement and it has been argued in \href{https://doi.org/10.48550/arXiv.2511.07348}{arXiv:2511.07348 [quant-ph]} that this entanglement of masses cannot be generated without the underlying quantum nature of gravity. In this work, we consider two non-relativistic Bose-Einstein condensates (formed inside of harmonic trap potentials with identical trapping frequencies) separated by a distance. We take a linearized quantum gravity model and investigate the generation of entanglement while gravitons serve as the mediator of entanglement. The entanglement is generated between the phonon modes of the two condensates, and we observe that for very low separation distance, the entanglement generated is significantly higher than that observed for the quantum gravity induced entanglement of masses or QGEM protocol; however, the fall of entanglement is faster than the two-particle case for two separated Bose-Einstein condensates. We observe that when the number of particles in the condensate is increased, the degree of entanglement for a smaller separation distance becomes substantially higher compared to the case discussed in \href{https://doi.org/10.1103/PhysRevD.105.106028}{Phys. Rev. D 105 (2022) 106028}, which allows for a more robust experimental proposal using this quantum gravity induced entanglement of phonons or QGEP protocol.

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arXiv2026

Enhancement of an Unruh-DeWitt battery performance through quadratic environmental coupling

We investigate relativistic effects on the performance of a quantum battery in an open quantum framework. We consider an Unruh-DeWitt detector driven by a coherent classical pulse as a quantum battery that is interacting with a massless scalar field through a quadratic coupling. The battery follows a trajectory composed of uniform acceleration along one direction, combined with constant four-velocity components in the orthogonal plane to the acceleration. Accelerated motion degrades the performance of the quantum battery rapidly in the absence of the orthogonal velocity component. We first derive the Lindblad equation for quadratic coupling in detail. We then show that the quadratic scalar field coupling enhances coherence and stability in the presence of orthogonal velocity. We observe that decoherence is mitigated significantly, resulting in remarkable improvement in the battery capacity and efficiency compared to the case of the usual linear field coupling. This opens up the possibility of nonlinear environmental coupling enabling stored energy to be retained over longer durations, leading to more efficient operation of quantum devices.

gr-qc↗

arXiv2026

Quantum regression theorem in the Unruh-DeWitt battery

In this paper, we employ the quantum regression theorem, a powerful tool in the study of open quantum systems, to analytically study the correlation functions of an Unruh-DeWitt detector, which is an uniformly accelerated two-level quantum system, absorbing charges from an external classical coherent pulse. The system can thus be viewed as a relativistic quantum battery that interacts with the environment of its perceived particles, namely, the quanta of a massless scalar field. By considering the relativistic battery moving in Rindler spacetime, under Born-Markov approximation, we derive the Gorini-Kossakowski-Sudarshan-Lindblad master equation governing the evolution of the system's reduced density matrix. Moreover, we perform the Fourier transformation of the Wightman functions and use exponential regularisation to compute the functional forms appearing in the master equation. Next, we derive the evolution equations for the the single-time expectation values of the system's operators. We not only solve these equations to find out the single time averages, but also employ the quantum regression theorem to determine the two-time correlation functions of first and second order. We analyse them to explain the phenomenon of spontaneous emission and show analytically how the acceleration can enhance the associated dissipation. Furthermore, we address a special form of second order correlation function relevant to the context of photon bunching arising in Bose-Einstein statistics. We analysed the results both analytically and graphically. Finally, we derive the spontaneous emission spectrum of the battery detector analytically, which in the long-time limit displays a well-defined Lorentzian line shape in the high frequency regime.

quant-ph↗

arXiv2026

Quasi-normal modes of quantum gravity black hole with perfectly fluid dark matter

In this work, we have studied the motion of a massless scalar photon in the renormalization group (RG) improved Schwarzschild black hole spacetime in the presence of perfectly fluid dark matter (PFDM). Considering the critical orbit conditions and the null geodesics condition in static spherically sym- In metric geometry, we have shown the variation of the radius of the photon sphere $r_{ph}$ with the PFDM parameter $ζ$. Due to perturbations in black hole spacetime, gravitational waves are emitted in the form of quasi-normal radiations, which correspond to quasi-normal modes (QNMs). In this work, we have studied two types of perturbations in RG improved Schwarzschild spacetime: scalar field perturbations and electromagnetic (EM) field perturbations. For both cases, we have studied the effect of the PFDM parameter on the quasi-normal mode frequencies and the shadow of the black hole, which is related to the photon radius.

gr-qc↗

arXiv2026

Null reduction and dynamical realization of Carrollian conformal symmetries

We start from a Lorentzian action in a deformed light-cone background and applying the method of null reduction leads to a Carrollian action in one lower spacetime dimensions. We also identify the correct light-cone definitions of the symmetry generators and their dynamical forms in terms of the fields and take the $c\rightarrow0$ limit. It is observed that these generators produce the known kinematic Carrollian conformal algebraic commutation relations.

hep-th↗

arXiv2025

Transition rates and their applications in accelerated single-qubit for fermionic spinor field coupling

In this work, we investigate the interaction between a uniformly accelerated single qubit and a fermionic spinor field. Here we consider both the massless and the massive fermionic spinor fields. The qubit-field interaction occurs over a finite time and was evolved via perturbation theory. This approach yields the transition probability rates, from which we subsequently evaluate the quantum coherence of an Unruh-DeWitt (UDW) detector initially prepared in a qubit state. Our findings reveal that the UDW detector responds more when coupled with the fermionic field, and consequently, quantum coherence (for the fermionic case) degrades much more rapidly when compared to the case of the qubit linearly coupled with the scalar field. Moreover, the analysis suggests that particle mass plays a protective role against Unruh-induced decoherence as the rest mass energy becomes comparable to the detector's energy-level spacing, the detector's excitation probability and response decreases, which leads to the mitigation of quantum coherence degradation in accelerated quantum systems.

quant-ph↗

arXiv2025

Quantum coherence measures in entangled atomic systems

In this study, we investigate the effect of the Lorentz transformation on the measures of quantum coherence in an entangled atomic system. Here, we consider the effect of this relativistic boosts on two-particle entangled generalized Gaussian wave packets in two scenarios. In the first scenario, we consider that the relativistic boost affects the one particle and other remains unaffected while in the second scenario, we consider that both the particles are affected by the effect of the relativistic boost. The coherence of the wave function as measured by the boosted observer is studied as a function of the boost parameter and the width of the Gaussian wave packets. Using various formulations of coherence, it is shown that in general the coherence decays with increase in the width of the Gaussian wave packet, higher values of boost parameter, and the number of particles on which boost is applied.

quant-ph↗

arXiv2025

Density matrix analysis of systems influenced by periodic Hamiltonians

In this work, we consider simple systems that are influenced by Hamiltonians with time periodicity. Our analysis is mainly focussed on the density matrix approach and aims to solve the Liouville equation of motion from which one can extract the state of the system when the system is in a pure state. We start our analysis with the standard Rabi-oscillation problem. We consider a density matrix corresponding to the entire model system and solve the Liouville equation of motion. We have then made use of the Lewis-Reisenfeld invariant approach and arrive at the exact same result which implies that the density matrix of the system can indeed be identified with the Lewis invariant. Finally, we consider a two-level system with a constant magnetic field in the $z$-direction and a time dependent magnetic field in the $x$-direction. Finally, we solve the Liouville equation of motion for this system and calculate the various coherence measures and plot them to investigate the time dependence and reliability of different coherence measures.

quant-ph↗

arXiv2025

Gauge interactions and the Galilean limit

The gauge invariant minimal couplings for a class of relativistic free matter fields with global symmetry (related to usual charge conservation) have been obtained by incorporating an iterative Noether mechanism. Non-relativistic reduction of both matter and gauge sectors of the obtained interacting theory is then performed simultaneously which in turn yield a set of new effective actions which are invariant under the Galilean relativistic framework. To be precise, we show that one can obtain the Schrödinger field theory coupled to Galilean electromagnetism from the scalar quantum electrodynamics theory. Higher derivative corrections have also been included for which the non-relativistic reductions have been consistently carried out once again. On the other hand, the action for quantum electrodynamics leads to the Galilean Pauli-Schrödinger theory where the gauge field is non-relativistic or Galilean. Further, some novel relations are found (in both the electric and magnetic limits) between various components appearing in the Galilean avatar of electrodynamics.

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