arXiv subjects

Dripto Biswas

Publications and source records attributed to Dripto Biswas.

11 recordsLinked to original sources

arXiv2026

Graviton-mediated entanglement due to light bending from a quantum rotor

One of the key tests of the quantum nature of gravity is to test whether the virtual mediator of gravity between matter and photon gives rise to the quantum light-bending phenomenon. The off-shell degrees of freedom, involving the spin-2 and spin-0 components of graviton, reproduce the classical deviation of light rays, as well as have been predicted to generate entanglement between matter and photon. This paper explores the generation of entanglement due to the quantum gravitational interaction in an optomechanical setup with a quantum rotor and photon. The virtual exchange of a graviton provides entanglement between the photon degrees of freedom and the spatial position of the quantum rotor, with the rotational state affecting its magnitude. We analyze the case of a high spinning rotor, in an approximately classical state of angular momentum, and quantify its effect on the gravitationally induced entanglement between the photon and the position of the quantum rotor. We show that the difference in the linear entanglement entropies, of prograde-and-retrograde motion of the photon with respect to the quantum rotor, provide tangible observable consequences.

quant-ph↗

arXiv2025

The Spectrum-generating Algebra for Bosonic Strings in a Linear Dilaton Background

We extend the systematic construction of bosonic DDF operators to the light-like linear dilaton background to investigate how higher-spin string states behave beyond flat spacetime. Using previous results, we show that the spectrum-generating algebra is isomorphic to the flat spacetime case up to a few subtleties. This extension provides a controlled setting to explore higher-spin massive string interactions in a nontrivial yet exactly solvable string background. Most of the derivations lead to expressions very similar to those in the flat background, with the expected modifications appearing quite naturally in the spectrum and the deformed momentum-(non)conserving delta function.

hep-th↗

arXiv2024

DDF amplitudes are lightcone amplitudes and the naturalness of Mandelstam map

We show that on shell DDF amplitudes are on shell lightcone amplitudes and that Mandelstam maps emerge naturally with a precise normalization and are intrinsic to the DDF states. Off shell DDF and Mandelstam amplitudes \`a la Kaku-Kikkawa differ. Underway we give a very explicit formula for the conformal transformation of a generic vertex in the form of a compact generating function for free theories.

hep-th↗

arXiv2024

The Reggeon Vertex for DDF States

We provide a compact expression for the generating function of correlators involving an arbitrary number of bosonic open string DDF states. The explicit correlators for $M$ DDF states can then be obtained by differentiating this generating function with respect to the DDF polarization tensors. The generating function depends on single and double complex integrals centred around the punctures corresponding to the insertion points of the DDF vertices on the real axis in the upper half-plane. We have explicitly evaluated these integrals for an arbitrary number of DDF states. To check our results, we computed some massive scalars and spin-2 amplitudes for $M=3$ and $M=4$, verifying that these amplitudes have the form expected from Lorentz invariance. Moreover, for the first excited levels, we also show that the DDF amplitudes with generic polarizations can be reassembled into Lorentz-covariant amplitudes, with emergent covariant polarizations that match the expressions obtained by comparing the string states in both the DDF and covariant formalisms.

hep-th↗

arXiv2024

Framed DDF operators and the general solution to Virasoro constraints

We define the framed DDF operators by introducing the concept of local frames in the usual formulation of DDF operators. In doing so it is possible to completely decouple the DDF operators from the associated tachyon and show that they are good zero-dimensional conformal operators. This allows for an explicit formulation of the general solution of the Virasoro constraints both on-shell and off-shell. We then make precise the realization of the intuitive idea that DDF operators can be used to embed light-cone states in the covariant formulation. This embedding is not unique, but depends on a coset. This coset is the little group of the embedding of the light-cone and is associated with a frame. The frame allows us to embed the $SO(D-2)$ light-cone physical polarizations into the $SO(1,D-1)$ covariant ones in the most general way. The solution to the Virasoro constraints is not in the gauge that is usually used. This happens since the states obtained from DDF operators are generically the sum of terms which are partially transverse due to the presence of a projector but not traceless and terms which are partially traceless but not transverse. To check the identification, we verify the matching of the expectation value of the second Casimir of the Poincar'e group for some light-cone states with the corresponding covariant states built using the framed DDFs.

hep-th↗

arXiv2022

Gravitational Optomechanics: Photon-Matter Entanglement via Graviton Exchange

The deflection of light in the gravitational field of the Sun is one of the most fundamental consequences for general relativity as well as one of its classical tests first performed by Eddington a century ago. However, despite its center stage role in modern physics, no experiment has tested it in an ostensibly quantum regime where both matter and light exhibit non-classical features. This paper shows that the interaction which gives rise to the light-bending also induces photon-matter entanglement as long as gravity and matter are treated at par with quantum mechanics. The quantum light-bending interaction within the framework of perturbative quantum gravity highlights this point by showing that the entangled states can be generated already with coherent states of light and matter exploiting the non-linear coupling induced by graviton exchange. Furthermore, the quantum light-bending interaction is capable of discerning between the spin-2 and spin-0 gravitons thus also providing a test for alternative theories of gravity at short distances and at the quantum level. We will conclude by estimating the order of magnitude of the entanglement generated by employing the linear entropy. In particular, we find that a half-ring cavity of radius $0.25$ m placed around a $10$ kg mechanical oscillator operating at $150$ Hz, could be used to generate linear entropy of order unity using a petawatt laser source at optical wavelengths. While the proposed scheme is beyond the current experimental realities it nonetheless initiates the discussion about testing the spin of the gravitational interaction at the quantum level.

gr-qc↗

arXiv2021

Quantum Back Flow Across a Black Hole Horizon in a Toy Model Approach

Quantum Back Flow (QBF), discovered quite a few years back, is a generic purely quantum phenomenon, in which the probability of finding a particle in a direction is non-zero (and increasing for a certain period of time) even when the particle has with certainty a velocity in the opposite direction. In this paper, we study QBF of a quantum particle across the event horizon of a Schwarzschild Black Hole. In a toy model approach, we consider a superposition of two ingoing solutions and observe the probability density and probability current. We explicitly demonstrate a non-vanishing quantum backflow in a small region around the event horizon. This is in contrast to the classical black hole picture, that once an excitation crosses the horizon, it is lost forever from the outside world. Deeper implications of this phenomenon are speculated. We also study quantum backflow for another spacetime with a horizon, the Rindler spacetime, where the phenomenon can be studied only within the Rindler wedge.

hep-th↗

arXiv2020

Deviation from the First Law of Thermodynamics for Particle-like Quantum Kerr-de Sitter Black Holes

The goal of this paper is to extend the particle-like quantization scheme presented in Pacheco and Silk (2020), to extremal Kerr-de Sitter black holes in four spacetime dimensions, thereby obtaining various quantized parameters, like the black hole mass and angular momentum, consistent with existing results, in the proper limits. Moreover, we show numerically, that for such extremal quantum black holes, there is a root mean square deviation from the First Law of black hole thermodynamics, of the order $\mathcal{O}(\Lambda^{0.45 \pm 0.01})$, where $\Lambda$ denotes the Cosmological Constant.

gr-qc↗

arXiv2020

On Geodesic Congruences and the Raychaudhuri Equations in $\textrm{SAdS}_4$ Spacetime

In this article, we look into geodesics in the Schwarzschild-Anti-de Sitter metric in (3+1) spacetime dimensions. We investigate the class of marginally bound geodesics (timelike and null), while comparing their behavior with the normal Schwarzschild metric. Using $\textit{Mathematica}$, we calculate the shear and rotation tensors, along with other components of the Raychaudhuri equation in this metric and we argue that marginally bound timelike geodesics, in the equatorial plane, always have a turning point, while their null analogues have at least one family of geodesics that are unbound. We also present associated plots for the geodesics and geodesic congruences, in the equatorial plane.

gr-qc↗

arXiv2019

Quantum Mechanics of Particle on a torus knot: Curvature and Torsion Effects

Constraints play an important role in dynamical systems. However, the subtle effect of constraints in quantum mechanics is not very well studied. In the present work we concentrate on the quantum dynamics of a point particle moving on a non-trivial torus knot. We explicitly take into account the role of curvature and torsion, generated by the constraints that keep the particle on the knot. We exploit the "Geometry Induced Potential (GIP) approach" to construct the Schrodinger equation for the dynamical system, obtaining thereby new results in terms of particle energy eigenvalues and eigenfunctions. We compare our results with existing literature that completely ignored the contributions of curvature and torsion. In particular, we explicitly show how the "knottedness" of the path influences the results. In the process we have revealed a (possibly un-noticed) "topological invariant".

hep-th↗

arXiv2018

Investigating the perturbed geometries of vortex rings in free fall through another liquid

The goal of this paper is to theoretically investigate the origin of the standing wave-like perturbation observed on the vortex rings falling through another liquid. We simplified the Navier-Stokes equation based on observational evidence from related research, and showed that there exists a steady-state velocity field with the required wave-like characteristics. For better visualisation we also plotted the velocity field.

physics.flu-dyn↗