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Parveen Kumar

Publications and source records attributed to Parveen Kumar.

At least 19 recordsLinked to original sources
arXiv2026

A partial data coefficient identification inverse problem for a semilinear damped wave operator

This manuscript deals with a coefficient identification inverse problem for a semilinear damped wave operator in a bounded domain of $\mathbb{R}^{1+d}\ (d\geq 2)$. We establish the unique recovery of the damping coefficient, zeroth-order linear term, and the coefficient of the power-type nonlinearity from the partial Dirichlet-to-Neumann map. We investigate the corresponding uniqueness problem under the assumption that the coefficients are known in a neighborhood of the boundary, while the Neumann boundary data are prescribed only on an arbitrarily small open subset of the boundary. The analysis is largely based on the unique continuation principle, Fourier Analysis and the higher-order linearization technique.

math.AP↗
arXiv2026

Stable determination of time-dependent coefficients in a reaction-diffusion-convection system

In this manuscript, we investigate an inverse boundary value problem for a reaction-diffusion-convection system in a bounded domain of $\mathbb{R}^{1+n}$, $n\geq 2$. We aim to obtain a stability estimate for determining the time-dependent convection coefficient and matrix-valued potential from boundary measurements represented by the Dirichlet-to-Neumann map. We consider a partial data setting in which the measurements are available only on a subset of the lateral boundary that slightly exceeds one-half of the boundary. We first establish the well-posedness of the associated initial-boundary value problem. Subsequently, by combining Carleman estimates with suitable geometric optics solutions, we derive stability estimates for the unknown coefficients. More precisely, we prove a double logarithmic ($\log$-$\log$) stability estimate for the time-dependent convection coefficient from the knowledge of the partial Dirichlet-to-Neumann map. This stability result is then employed to recover the matrix-valued potential, yielding a triple logarithmic ($\log$-$\log$-$\log$) stability estimate for the zeroth-order coefficient.

math.AP↗
arXiv2026

Reconstruction of time-dependent coefficients in a semilinear dynamical Schr{\"o}dinger equation

In the present manuscript, we study an inverse problem related to a semilinear dynamical Schr{\"o}dinger equation with lower order terms, in a bounded domain of $\Rb^{1+n},n\geq 2$. Our focus is on determination of the time-dependent coefficients appearing in the aforementioned equation, from the boundary measurements of the solutions. More precisely, we establish the {pointwise reconstruction} formulae for determining the time-dependent coefficients of linear and nonlinear terms from the knowledge of Dirichlet-to-Neumann map. Since the concerned non-linear Schr\"odinger equation possesses a trivial solution, we linearize the equation around the trivial solution and use the asymptotic solutions (\textit{with concentrated amplitudes}) of the linearized problem for reconstructing the aforementioned coefficients. To be more specific, we use first-order linearization to reconstruct vector and scalar potentials associated with the coefficients of linear terms and the higher-order linearization technique is used to reconstruct coefficients of nonlinearity. The nonlinear equation considered in this manuscript can be seen as a generalization of the Gross-Pitaevskii equation (GPE), which is employed to describe the dynamics of dilute Bose-Einstein condensates (BEC).

math.AP↗
arXiv2026

Interference of local-measurement histories

The evolution of a quantum system comprises two fundamental processes--continuous unitary dynamics and stochastic measurement-induced jumps. The latter are often viewed as a source of decoherence. Can two histories of such an evolution, made up of local measurements, interfere with each other? Here, we answer this question in the affirmative. A manifestation of this interference is the generation of entanglement between two parts of the system that are individually coupled to distinct detectors. Specifically, we develop a protocol in which two alternative local measurement processes act on a pair of qubits, and show how interference of histories is generated under coherent control, leading to entanglement. Furthermore, we find that averaging over the detectors' readouts still results in an entangled (albeit not maximally entangled) state. Our results extend the notion of quantum interference beyond unitary evolution to genuinely measurement-driven dynamics, and identify limits on the generation of quantum correlations using interference of measurement histories.

quant-ph↗
arXiv2026

Violations of the Leggett-Garg inequality in Hybrid Liouvillian Dynamics: The Nonlinear Role of Detector Efficiency

Violations of the Leggett-Garg inequality (LGI) up to its algebraic bound under non-Hermitian dynamics are well established theoretically. Here, we demonstrate that such extreme violations are intrinsically fragile when realistic measurement processes are taken into account. We consider an open two-level system described by a time-local hybrid Liouvillian, with a continuous parameter $q \in [0,1]$, representing detector efficiency, i.e., the fraction of quantum jump trajectories that are retained in the ensemble. This parameter interpolates between trace-preserving Lindblad dynamics ($q=1$) and non-Hermitian ``no-jump" evolution ($q=0$). While $K_3$ approaches its algebraic maximum of 3 in the null-efficiency limit, even an infinitesimal increase in detector efficiency induces a rapid, highly nonlinear suppression toward the classical bound. This logarithmic sensitivity reveals that maximal LGI violations are not robust physical features but rather singular limits of idealized measurement conditions. Our results have direct experimental implications: achieving algebraic LGI violations in systems undergoing continuous time evolution requires near-perfect suppression of detected quantum jumps (i.e., effective post-selection), placing stringent constraints on detector performance. In contrast to discrete protocols based on time-non-divisible dynamics, our framework shows that extreme violations arising within continuous, divisible quantum trajectory evolution constitute a fundamentally fragile regime.

quant-ph↗
arXiv2026

Gradually opening Schr\"odinger's box reveals a cascade of sharp dynamical transitions

Quantum mechanics predicts that unobserved systems may exist in a superposition of states, yet measurement produces definite outcomes, a tension at the heart of the quantum-to-classical boundary. How the transformation between these opposing regimes unfolds as observation strength increases has remained experimentally unexplored. Here, by continuously tuning the measurement strength on a superconducting qubit, we reveal that measurement-dominated dynamics emerge not gradually but through three distinct transitions: coherent oscillations abruptly halt; the system then freezes near a stable quantum state; and finally enters the quantum Zeno regime, where stronger observation paradoxically slows relaxation. Decoherence, rather than washing out this structure, reorganizes it, inverting the order in which transitions appear and decoupling signatures that coincide in idealized models. These results establish that the route from quantum dynamics to measurement-dominated behavior unfolds in sharp transitions governed by the interplay between observation and environment.

quant-ph↗
arXiv2026

Local Magnetometry from Measurement-Induced Dissipation

Magnetic phases are commonly identified through macroscopic magnetization, yet many ordered states, including antiferromagnets and altermagnets, possess a vanishing net moment despite distinct local spin structure. We show that such an order can be accessed through the measurement-induced steady state of a single primary qubit locally coupled to a spin lattice. Using a controlled primary-ancillary qubit protocol, we derive analytically that the steady state \emph{encodes} a locally weighted exchange field in a signed observable that is linear in the weak-coupling regime. Numerical simulations demonstrate lattice-scale resolution of antiferromagnetic and altermagnetic textures and robustness against short-correlated noise. Our results establish measurement-induced dissipation as a resource for detecting magnetic order through microscopic structure rather than through global moments.

cond-mat.mes-hall↗
arXiv2025

Best Proximity Point Results for Cyclic Orbital Contraction Mappings in $CAT_p(0)$ Metric Spaces

In this paper, we introduce the concept of cyclic orbital contraction mappings which generalizes the concept of cyclic contraction mappings. We establish the existence of best proximity point of these mappings in the framework of $CAT_p(0)$ metric spaces. Also, we study the existence of best proximity point theorems for cyclic orbital contraction mappings in uniformly convex Banach spaces.

math.FA↗
arXiv2025

Optimal Thermalization under Indefinite Causal Order with Identical and Asymmetric Baths

Indefinite causal order (ICO), in which the order of quantum operations is placed in a coherent superposition, has been demonstrated to enhance various information-processing tasks. Here, we investigate its impact on the thermodynamic processes generated by thermalizing quantum channels. We consider a two-level system interacting with two thermal baths under a quantum SWITCH, with the channel order controlled coherently by an ancillary qubit. We derive closed-form expressions for the effective inverse temperature $\beta_f$ of the postselected system state for both identical and distinct bath temperatures, and identify the control-qubit parameters that maximize heating or cooling. Our analysis reveals how the diagonal and coherent components of the control-qubit state contribute separately to the temperature shift, and how their interplay enables departures from the thermal response attainable under protocols with a definite causal order within the thermodynamic setting considered here. Bath asymmetry enhances these effects, while reduced purity of the control qubit state suppresses them. These results provide a systematic framework for assessing SWITCH-based thermalization in the setting of indefinite causal order, and identify control-qubit coherence as a tunable resource.

quant-ph↗
arXiv2025

Intra-night optical variability and radio characteristics of extremely radio-loud narrow-line Seyfert 1 galaxies

Narrow-line Seyfert 1 galaxies (NLS1s) are generally known to be radio-quiet Active Galactic Nuclei (AGN), but a tiny subset of them are found to be extremely radio-loud with radio loudness parameter ($R_{\rm 1.4~GHz}$) $>$ 100. Given their rarity we investigated intra-night optical variability (INOV) and radio characteristics of a sample of 16 extremely radio-loud NLS1s. For all but four sample sources we report intra-night photometric monitoring for the first time with at least one monitoring session per source lasting for a minimum of 3.0 hours duration. In our sample, we detect INOV with a high duty cycle (up to 25 per cent) and large average amplitude ($\overline{\psi}$ $\sim$ 0.16) similar to that found in blazars. Using 3.0 GHz Very Large Array Sky Survey (VLASS) and auxiliary multi-frequency radio data we find that our RL-NLS1s are luminous ($L_{\rm 3.0~GHz}$ $\geq$ 10$^{24}$ W~Hz$^{-1}$), compact (less than a few kpc), variable, flat spectrum (${\alpha}_{\rm radio}$ $>$ -0.5) radio sources. The INOV, radio characteristics, and radio luminosity ($L_{\rm 1.4~GHz}$) versus super-massive black hole mass ($M_{\rm SMBH}$) plot infer that extremely radio-loud NLS1s are low-$z$ and low-luminosity analogs of flat spectrum radio quasars wherein the former are powered by, on average, one order-of-magnitude less massive SMBHs.

astro-ph.GA↗
arXiv2025

General theory of slow non-Hermitian evolution

Non-Hermitian systems are widespread in both classical and quantum physics. The dynamics of such systems has recently become a focal point of research, showcasing surprising behaviors that include apparent violation of the adiabatic theorem and chiral topological conversion related to encircling exceptional points (EPs). These have both fundamental interest and potential practical applications. Yet the current literature features a number of apparently irreconcilable results. Here we develop a general theory for slow evolution of non-Hermitian systems and resolve these contradictions. We prove an analog of the adiabatic theorem for non-Hermitian systems and generalize it in the presence of uncontrolled environmental fluctuations (noise). The effect of noise turns out to be crucial due to inherent exponential instabilities present in non-Hermitian systems. Disproving common wisdom, the end state of the system is determined by the final Hamiltonian only, and is insensitive to other details of the evolution trajectory in parameter space. Our quantitative theory, leading to transparent physical intuition, is amenable to experimental tests. It provides efficient tools to predict the outcome of the system's evolution, avoiding the need to follow costly time-evolution simulations. Our approach may be useful for designing devices based on non-Hermitian physics and may stimulate analyses of classical and quantum non-Hermitian-Hamiltonian dynamics, as well as that of quantum Lindbladian and hybrid-Liouvillian systems.

quant-ph↗
arXiv2023

Quantum state engineering by steering in the presence of errors

Quantum state engineering plays a vital role in various applications in the field of quantum information. Different strategies, including drive-and-dissipation, adiabatic cooling, and measurement-based steering, have been proposed in the past for state generation and manipulation, each with its upsides and downsides. Here, we address a class of measurement-based state engineering protocols where a sequence of generalized measurements is employed to steer a quantum system toward a desired target state. Previously studied measurement-based protocols relied on idealized procedures and avoided exploration of the effects of various errors stemming from imperfections of experimental realizations and external noise. We employ the quantum trajectory formalism to provide a detailed analysis of the robustness of these steering protocols against various errors. We study a set of errors that can be classified as dynamic or static, depending on whether they remain unchanged while running the protocol. More specifically, we investigate the impact of erroneous choice of system-detector coupling, re-initialization of the detector state following a measurement step, fluctuating steering directions, and environmentally induced errors in the system-detector interaction. We show that the protocol remains fully robust against the erroneous choice of system-detector coupling parameters and presents reasonable robustness against other errors. We employ various quantifiers such as fidelity, trace distance, and linear entropy to characterize the protocol's robustness and provide analytical results. Subsequently, we demonstrate the commutation between the classical expectation value and the time-ordering operator of the exponential of a Hamiltonian with multiplicative white noise, as well as the commutation of the expectation value and the partial trace with respect to detector outcomes.

quant-ph↗
arXiv2023

TAPS Responsibility Matrix: A tool for responsible data science by design

Data science is an interdisciplinary research area where scientists are typically working with data coming from different fields. When using and analyzing data, the scientists implicitly agree to follow standards, procedures, and rules set in these fields. However, guidance on the responsibilities of the data scientists and the other involved actors in a data science project is typically missing. While literature shows that novel frameworks and tools are being proposed in support of open-science, data reuse, and research data management, there are currently no frameworks that can fully express responsibilities of a data science project. In this paper, we describe the Transparency, Accountability, Privacy, and Societal Responsibility Matrix (TAPS-RM) as framework to explore social, legal, and ethical aspects of data science projects. TAPS-RM acts as a tool to provide users with a holistic view of their project beyond key outcomes and clarifies the responsibilities of actors. We map the developed model of TAPS-RM with well-known initiatives for open data (such as FACT, FAIR and Datasheets for datasets). We conclude that TAPS-RM is a tool to reflect on responsibilities at a data science project level and can be used to advance responsible data science by design.

cs.CY↗
arXiv2022

A Theoretical Study on Band-Gap Engineering of CsCaI3 by Si Doping for Photo Voltaic Applications

Density functional theory based First Principles calculations were used to study the effect of Silicon (Si) doping on the structural, electronic and optical properties of CsCaI3. From our calculations, we predict that CsCaI3 can form stable perovskite structure. It is also observed that after substitutional doping of Si in CsCaI3, the material still can stay in perovskite form. Band structure studies showed that with Si doping, the band gap can be varied from 4.76eV for un-doped CsCaI3 to 0.639eV for 75 percent Si doped CsCaI3. The optical absorption spectra of the materials showed that Si doping can induce light absorption in the visible region in CsCaI3. Electron Localization Function (ELF) and effective mass calculations show that Si doping can improve the electronic conduction in this material to meet the requirements of photo Voltaic applications.

cond-mat.mtrl-sci↗
arXiv2021

Near-unit efficiency of chiral state conversion via hybrid-Liouvillian dynamics

Following the evolution under a non-Hermitian Hamiltonian (nHH) involves significant probability loss. This makes various nHH effects impractical in the quantum realm. In contrast, Lindbladian evolution conserves probability, facilitating observation and application of exotic effects characteristic of open quantum systems. Here we are concerned with the effect of chiral state conversion: encircling an exceptional point, multiple system states are converted into a single system eigenmode. While for nHH the possible converted-into eigenmodes are pure states, for Lindbladians these are typically mixed states. We consider hybrid-Liouvillian evolution, which interpolates between a Lindbladian and a nHH and enables combining the best of the two worlds. We design adiabatic evolution protocols that give rise to chiral state conversion with $\textit{pure}$ final states, no probability loss, and high fidelity. Furthermore, extending beyond continuous adiabatic evolution, we design a protocol that facilitates conversion to pure states with fidelity 1 and, at the same time, no probability loss. Employing recently developed experimental techniques, our proposal can be implemented with superconducting qubit platforms.

quant-ph↗
arXiv2021

Optimized Steering: Quantum State Engineering and Exceptional Points

The state of a quantum system may be steered towards a predesignated target state, employing a sequence of weak $\textit{blind}$ measurements (where the detector's readouts are traced out). Here we analyze the steering of a two-level system using the interplay of a system Hamiltonian and weak measurements, and show that $\textit{any}$ pure or mixed state can be targeted. We show that the optimization of such a steering protocol is underlain by the presence of Liouvillian exceptional points. More specifically, for high purity target states, optimal steering implies purely relaxational dynamics marked by a second-order exceptional point, while for low purity target states, it implies an oscillatory approach to the target state. The dynamical phase transition between these two regimes is characterized by a third-order exceptional point.

quant-ph↗