arXiv subjects

Ashutosh Singh

Publications and source records attributed to Ashutosh Singh.

At least 55 records · Page 3Linked to original sources

arXiv2022

Cosmic acceleration and ekpyrotic bounce with Chameleon field

In this article, we explore the homogeneous and isotropic flat Friedmann-Robertson-Walker (FRW) model in Chameleon cosmology. By considering a non-minimal coupling between the scalar field and matter, we present a non-singular bouncing cosmological scenario of the universe. The universe initially exhibits the ekpyrotic phase during the contracting era, undergoes a non-singular bounce, and then in expanding era, it smoothly transits to the decelerating era having matter and radiation dominated phases. Further, this decelerating era is smoothly connected to the late-time dark energy-dominated era of the present epoch. We use numerical solution techniques to solve non-minimally coupled gravity equations for understanding the evolution of scalar field along with other quantities like effective potential in the model. The model thus unifies an ekpyrotic, non-singular, asymmetric bounce with the dark energy era of the present epoch. We study the evolution of bouncing model and confront the model with observational results on the equation of state parameter by constraining the model parameters.

gr-qc↗

arXiv2022

Entanglement protection in higher-dimensional systems

The inevitable dissipative interaction of an entangled quantum system with its environment causes degradation in quantum correlations present in the system. This can lead to a finite-time disappearance of entanglement, which is known as entanglement sudden death (ESD). Here, we consider an initially entangled qubit-qutrit system and a dissipative noise which leads to ESD, and propose a set of local unitary operations, which when applied on the qubit, qutrit, or both subsystems during the decoherence process, cause ESD to be hastened, delayed, or avoided altogether, depending on its time of application. The physical implementation of these local unitaries is discussed in the context of an atomic system. The simulation results of such ESD manipulations are presented for two different classes of initially entangled qubit-qutrit systems. A prescription for generalization of this scheme to a qutrit-qutrit system is given. This technique for entanglement protection in the noisy environment is compared with other related techniques such as weak measurement reversal and dynamic decoupling.

quant-ph↗

arXiv2022

Qualitative study of anisotropic Rastall cosmologies

We perform dynamical analysis of anisotropic Rastall cosmologies and, in particular Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi I and LRS Bianchi III cosmologies. Using dynamical system techniques, a qualitative analysis of cosmological equations yield physically interesting cases which are in agreement with observations such as de Sitter, accelerating, stable attractors are isotropic. Features such as isotropization and cosmological bounce are discussed in detail.

gr-qc↗

arXiv2022

Cosmic dynamics and qualitative study of Rastall model with spatial curvature

We investigate the cosmic dynamics of Rastall gravity in non-flat Friedmann-Robertson-Walker (FRW) space-time with barotropic fluid. In this context, we are concerned about the class of model satisfying the affine equation of state. We derive the autonomous system for the Rastall model with barotropic fluid. We apply the derived system to investigate the critical points for expanding and bouncing cosmologies and their stable nature and cosmological properties. The expanding cosmology yields de Sitter universe at late times with decelerating past having matter and radiation dominated phases. Investigation of autonomous system for bouncing cosmology yields oscillating solutions in positive spatial curvature region. We also investigate the consequences of setting-up a model of non-singular universe by using energy conditions. The distinct features of the Rastall model compared to the standard model have been discussed in detail.

gr-qc↗

arXiv2021

Variation is the Norm: Brain State Dynamics Evoked By Emotional Video Clips

For the last several decades, emotion research has attempted to identify a "biomarker" or consistent pattern of brain activity to characterize a single category of emotion (e.g., fear) that will remain consistent across all instances of that category, regardless of individual and context. In this study, we investigated variation rather than consistency during emotional experiences while people watched video clips chosen to evoke instances of specific emotion categories. Specifically, we developed a sequential probabilistic approach to model the temporal dynamics in a participant's brain activity during video viewing. We characterized brain states during these clips as distinct state occupancy periods between state transitions in blood oxygen level dependent (BOLD) signal patterns. We found substantial variation in the state occupancy probability distributions across individuals watching the same video, supporting the hypothesis that when it comes to the brain correlates of emotional experience, variation may indeed be the norm.

q-bio.NC↗

arXiv2020

New non-binary quantum codes from skew constacyclic codes over the ring $\mathbb{F}_{p^m}+v\mathbb{F}_{p^m}+v^2 \mathbb{F}_{p^m}$

In this article, we construct new non-binary quantum codes from skew constacyclic codes over finite commutative non-chain ring $\mathcal{R}= \mathbb{F}_{p^m}[v]/\langle v^3 =v \rangle$ where $p$ is an odd prime and $m \geq 1$. In order to obtain such quantum codes, first we study the structural properties of skew constacyclic codes and their Euclidean duals over the ring $\mathcal{R}$. Then a necessary and sufficient condition for skew constacyclic codes over $\mathcal{R}$ to contain their Euclidean duals is established. Finally, with the help of CSS construction and using Gray map, many new non-binary quantum codes are obtained over $\mathbb{F}_{p^m}$.

cs.IT↗

arXiv2019

Revisiting comparison between entanglement measures for two-qubit pure states

Given a non-maximally entangled state, an operationally significant question is to quantitatively assess as to what extent the state is away from the maximally entangled state, which is of importance in evaluating the efficacy of the state for its various uses as a resource. It is this question which is examined in this paper for two-qubit pure entangled states in terms of different entanglement measures like Negativity (N), Logarithmic Negativity (LN), and Entanglement of Formation (EOF). Although these entanglement measures are defined differently, to what extent they differ in quantitatively addressing the earlier mentioned question has remained uninvestigated. Theoretical estimate in this paper shows that an appropriately defined parameter characterizing the fractional deviation of any given entangled state from the maximally entangled state in terms of N is quite different from that computed in terms of EOF with their values differing up to ~ 15 % for states further away from the maximally entangled state. Similarly, the values of such fractional deviation parameters estimated using the entanglement measures LN and EOF, respectively, also strikingly differ among themselves with the maximum value of this difference being around 23 %. This analysis is complemented by illustration of these differences in terms of empirical results obtained from a suitably planned experimental study. Thus, such appreciable amount of quantitative non-equivalence between the entanglement measures in addressing the experimentally relevant question considered in the present paper highlights the requirement of an appropriate quantifier for such intent. We indicate directions of study that can be explored towards finding such a quantifier.

quant-ph↗

arXiv2019

A Distributed Model-Free Algorithm for Multi-hop Ride-sharing using Deep Reinforcement Learning

The growth of autonomous vehicles, ridesharing systems, and self driving technology will bring a shift in the way ride hailing platforms plan out their services. However, these advances in technology coupled with road congestion, environmental concerns, fuel usage, vehicles emissions, and the high cost of the vehicle usage have brought more attention to better utilize the use of vehicles and their capacities. In this paper, we propose a novel multi-hop ride-sharing (MHRS) algorithm that uses deep reinforcement learning to learn optimal vehicle dispatch and matching decisions by interacting with the external environment. By allowing customers to transfer between vehicles, i.e., ride with one vehicle for sometime and then transfer to another one, MHRS helps in attaining 30\% lower cost and 20\% more efficient utilization of fleets, as compared to the ride-sharing algorithms. This flexibility of multi-hop feature gives a seamless experience to customers and ride-sharing companies, and thus improves ride-sharing services.

cs.LG↗

arXiv2019

Single photon sources: ubiquitous tools in quantum information processing

Quantum technologies are the next big revolution in information technologies, computing, communication security, sensing as well as metrology. What do you use to explore all these fascinating applications when you work in Optics? Photons of course. In this review, we discuss the different available single photon source technologies, compare and contrast them in terms of applicability and properties, discuss state of the art and conclude that the future is indeed bright!

quant-ph↗

arXiv2019

Effect of chiral anomaly on the circular dichroism and Hall angle in doped and tilted Weyl semimetals

From the Kubo formula for transport in a tilted Weyl semimetal we calculate the absorptive part of the dynamic conductivity for both right and left handed circular polarized light. These depend on the real part of the longitudinal conductivity and the imaginary part of the transverse (Hall) conductivity. We include the effect of the chiral anomaly which pumps charge from negative to positive chirality node when the usual ${\bs E}\cdot{\bs B}$ term is included in the electrodynamics and obtain analytic expressions. To calculate the Hall angle we further provide expressions for the imaginary part of longitudinal and real part of the transverse conductivity and compare results with and without the pumping term. We also consider the case of a non centro symmetric Weyl semimetal in which the chiral nodes are displaced in energy by an amount $\pm \mathcal{Q}_0$. This leads to modification in dichroism and Hall angle which parallel the pumping case.

cond-mat.mes-hall↗

arXiv2019

Giant optical activity and Kerr effect in type-I and type-II Weyl semimetals

We explore optical activity in thin films and bulk of type-I and type II Weyl semimetals (WSM), and demonstrate the existence of a giant Kerr effect in both. In time-reversal symmetry broken WSM thin films, the polarization rotation is caused by the optical Hall conductivity including the anomalous Hall term. The Kerr angle is found to be $\propto Q/ω$, with $Q$ and $ω$ being the Weyl node separation and the optical frequency, respectively. In contrast, the optical activity in the bulk WSM is dominated by axion electrodynamics, which persists even in the Pauli blocked regime of no optical transitions. In bulk WSM, $Q$ acts analogous to the magnetization in magnetic materials, leading to large `polar Kerr effect' (linear in $Q$) when light is incident on WSM surface without Fermi arc states, and the `Voigt effect' (quadratic in $Q$), when light is incident on surface with Fermi arc states.

cond-mat.mes-hall↗

arXiv2019

Terahertz modulated optical sideband generation in graphene

Exploration of optical non-linear response of graphene predominantly relies on ultra-short time domain measurements. Here we propose an alternate technique that uses frequency modulated continuous wavefront optical fields, thereby probing graphene's steady state non-linear response. We predict frequency sideband generation in the reflected field that originates from coherent electron dynamics of the photo-excited carriers. The corresponding threshold in input intensity for optimal sideband generation provides a direct measure of the third order optical non-linearity in graphene. Our formulation yields analytic forms for the generated sideband intensity, is applicable to generic two-band systems and suggests a range of applications that include switching of frequency sidebands using non-linear phase shifts and generation of frequency combs.

cond-mat.mes-hall↗

arXiv2018

Nonlinear and anisotropic polarization rotation in two dimensional Dirac materials

We predict nonlinear optical polarization rotation in two dimensional massless Dirac systems including graphene and 8-$Pmmn$ borophene. When illuminated, a continuous wave optical field leads to a nonlinear steady state of photo-excited carriers in the medium. The photo-excited population inversion and the inter-band coherence gives rise to a finite transverse optical conductivity, $σ_{xy} (ω)$. This in turn leads to definitive signatures in associated Kerr and Faraday polarization rotation, which are measurable in a realistic experimental scenario.

cond-mat.mes-hall↗

arXiv2017

Nonlinear, anisotropic and giant photoconductivity in intrinsic and doped graphene

We present a framework to calculate the anisotropic and non-linear photoconductivity for two band systems with application to graphene. In contrast to the usual perturbative (second order in the optical field strength) techniques, we calculate photoconductivity to all orders in the optical field strength. In particular, for graphene, we find the photoresponse to be giant (at large optical field strengths) and anisotropic. The anisotropic photoresponse in graphene is correlated with polarization of the incident field, with the response being similar to that of a half-wave plate. We predict that the anisotropy in the simultaneous measurement of longitudinal ($σ_{xx}$) and transverse $(σ_{yx})$ photoconductivity, with four probes, offers a unique experimental signature of the photo-voltaic response, distinguishing it from the thermal-Seebeck and bolometric effects in photoresponse.

cond-mat.mes-hall↗

arXiv2017

Manipulation of entanglement sudden death in an all-optical setup

The unavoidable and irreversible interaction between an entangled quantum system and its environment causes decoherence of the individual qubits as well as degradation of the entanglement between them. Entanglement sudden death (ESD) is the phenomenon wherein disentanglement happens in finite time even when individual qubits decohere only asymptotically in time due to noise. Prolonging the entanglement is essential for the practical realization of entanglement-based quantum information and computation protocols. For this purpose, the local NOT operation in the computational basis on one or both qubits has been proposed. Here, we formulate an all-optical experimental set-up involving such NOT operations that can hasten, delay, or completely avert ESD, all depending on when it is applied during the process of decoherence. Analytical expressions for these are derived in terms of parameters of the initial state's density matrix, whether for pure or mixed entangled states. After a discussion of the schematics of the experiment, the problem is theoretically analyzed, and simulation results of such manipulations of ESD are presented.

quant-ph↗

arXiv2017

Approaches to Maintain Bi-connectivity for Resilience in Overlaid Multicasting

Application layer multicast (ALM) also called Overlay Multicast, is an attractive alternative solution to most of the problems associated with IP multicast. In ALM, multicast-related functionalities are moved to end-hosts. Application layer multicast builds a peer-to-peer (P2P) overlay topology consisting of end-to-end unicast connections between end-hosts. The key advantages, overlays offer, are flexibility, adaptability and ease of deployment [1]. The general approach to build an application layer multicast architecture involves tracking network characteristics and building appropriate topologies by allowing the end users to self organize into logical overlay networks for efficient data delivery. The major concern in designing ALM protocol is the mechanism to build and maintain a topology and to route data efficiently and reliably in this topology. We propose here a two-fold dynamic overlay tree construction and maintenance scheme in which a mesh-like topology is first built, where an arriving host connects to two already connected hosts. This ensures that two node and link disjoint paths are always maintained between every possible pair of nodes. Once the mesh is formed, on top of it, a single or multiple data delivery tree(s) are built using a suitable protocol. An algorithm is run in the nodes of the overlay topology to maintain the biconnectivity by inserting new links and deleting the redundant links as a continuous process.

cs.NI↗

arXiv2016

Nonlinear optical conductivity of a generic two band systems, with application to doped and gapped graphene

We present a general formulation to calculate the dynamic optical conductivity, beyond the linear response regime, of any electronic system whose quasiparticle dispersion is described by a two band model. Our phenomenological model is based on the optical Bloch equations. In the steady state regime it yields an analytic solution for the population inversion and the interband coherence, which are nonlinear in the optical field intensity, including finite doping and temperature effects. We explicitly show that the optical nonlinearities are controlled by a single dimensionless parameter which is directly proportional to the incident field strength and inversely proportional to the optical frequency. This identification leads to a unified way to study the dynamical conductivity and the differential transmission spectrum across a wide range of optical frequencies, and optical field strength. We use our formalism to analytically calculate the nonlinear optical conductivity of doped and gapped graphene, deriving the well known universal ac conductivity of $σ_0={e^2}/4\hbar$ in the linear response regime of low optical intensities (or equivalently high frequencies) and non-linear deviations from it which appear at high laser intensities (or low frequencies) including the impact of finite doping and band-gap opening.

cond-mat.mes-hall↗

arXiv2014

Wave packet dynamics in various two-dimensional systems: a unified description

In this article we present an exact and unified description of wave-packet dynamics in various 2D systems in presence of a transverse magnetic field. We consider an initial minimum-uncertainty Gaussian wave-packet, and find that its long term dynamics displays the universal phenomena of spontaneous collapse and quantum revival. We estimate the timescales associated with these phenomena based on very general arguments for various materials, whose carrier dynamics is described either by the Schrödinger equation or by the Dirac equation.

cond-mat.mes-hall↗