arXiv subjects

Saptarshi Mandal

Publications and source records attributed to Saptarshi Mandal.

At least 19 recordsLinked to original sources

arXiv2026

Locality of deep thermalisation through the lens of entanglement teleportation

Deep thermalisation characterises the emergence of universal quantum state ensembles on subsystems due to projective measurements on their complement. We study the notion of locality, or lack thereof, in this phenomenon by considering a subsystem partitioned into two disjoint subregions which remain causally disconnected at all times under unitary dynamics. We show that the onset of deep thermalisation in this geometry is fundamentally bounded by measurement-induced entanglement teleportation between the subregions. While measurements on the environment generate entanglement across the disconnected partitions -- suggesting an apparent non-locality -- we demonstrate that generic locally interacting systems exhibit an emergent locality. Specifically, the timescales for both deep thermalisation and entanglement teleportation scale logarithmically with the distance separating the subregions. Exceptions to this include special circuits where the randomness of the measurement outcomes is perfectly transmitted to the ensemble of states of the subsystem, conditioned on the outcomes; in such cases the timescale for deep thermalisation is finite leading to genuine non-locality.

quant-ph↗

arXiv2026

Photovoltaic Possibility of Cu2SiSe3 and Cu2SnS3 Ternary Chalcogenides- Single Junction to Tandem Architecture

Cu based ternary chalcogenides are gathering attention for sustainable energy applications due to their reduced complexity compared to quaternary alternatives. We used drift diffusion modeling to evaluate the feasibility of photovoltaics employing ternary chalcogenide absorbers based on Cu2SiSe3 and Cu2SnS3. The device metrics are evaluated by analyzing absorber layer thickness intrinsic carrier concentration defect density and energy band alignment at interfacial junctions. The optimized single junction Cu2SiSe3 based device configuration achieves a power conversion efficiency of 18.13 percent exhibiting a short circuit current density of 38 mA cm^-2 and an open circuit voltage of 0.64 V. The Cu2SnS3 based device achieves an efficiency of 15.59 percent with a short circuit current density of 48.8 mA cm^-2 and an open circuit voltage of 0.42 V. We examined the impact of the buffer layer on device parameters uncovering further avenues for performance improvement. Additionally we simulated a two terminal tandem solar cell using Cu2SiSe3 Eg 1.44 eV in the upper cell to capture photons from the visible spectrum and Cu2SnS3 Eg 0.91 eV in the lower cell to absorb from the infrared spectrum. The simulated tandem architecture, featuring a VOC of 1.24 V a JSC of 24.6 mA cm^-2 a fill factor (FF) of 79.2 percent and an efficiency of 24.1 percent markedly surpassed conventional single junction devices demonstrating the viability of Cu2SiSe3-Cu2SnS3 absorber-based tandem solar cells for next generation high-efficiency solar technologies.

cond-mat.mtrl-sci↗

arXiv2026

GATE: GPU-Accelerated Traffic Engineering for the WAN

Traffic engineering (TE) has become a crucial tool for enforcing routing policy and maintaining operational efficiency in large networks. Existing TE solutions pick an objective function to optimize, aiming to balance (i) allocating traffic optimally with (ii) reacting quickly to demand changes and disruption events. However, as the scale of networks grows, the runtime of the existing optimal solution becomes infeasibly large. The alternative - approximate solvers - result in costly inefficiencies. We present GPU-Accelerated Traffic Engineering (GATE), which achieves the best of both worlds: enabling fast TE runtimes through a highly-parallelizable GPU-compatible decomposition, while iteratively converging to the provably optimal solution. GATE unlocks a unique set of desirable properties: it becomes increasingly parallelizable with network size, supports a wide spectrum of fairness objectives, and offers theoretically guaranteed convergence to the optimal solution and near-optimal convergence within a bounded time. We evaluate GATE on production traces from two large cloud WANs, and show that GATE achieves near-optimal solutions 5-10x faster than state-of-the-art.

cs.NI↗

arXiv2026

Spin Chern phases and persistent spin texture in a quasi 2D SSH model

We construct a quasi-two-dimensional Su Schrieffer-Heeger model (SSH) like model and uncover a rich set of topological phases with nontrivial spin textures in the presence of complex hopping and spin orbit coupling. Despite its simple structure, the combined effect of complex hopping and spin orbit interaction gives rise not only to the conventional quantum anomalous Hall insulating (QAHI) phase, but also to distinct combinations of spin Chern phases, namely quantum anomalous spin Hall insulating (QASHI) phase. Furthermore, we demonstrate that the bulk bands of this model can host persistent spin textures, whose formation and stability are governed by the relative strengths of nearest and next nearest neighbor complex hopping. To elucidate the underlying mechanisms, we develop a low energy continuum theory that captures the emergence of these topological phases and clarifies the origin of the persistent spin textures. Interestingly, the resulting spin textures closely resemble those typically observed in conventional semiconductor systems with topologically trivial band structures. However, in our case, they emerge within a nontrivial topological framework, enabled by carefully engineered hopping patterns that intertwine lattice geometry, complex hopping, and spin orbit coupling

cond-mat.mes-hall↗

arXiv2026

Hat guessing with proper colorings

We initiate the study of the hat guessing number of a graph where the adversary is only allowed to provide a proper coloring of the graph. This is the largest number $q$ for which there is a guessing strategy on each vertex that only depends on its neighborhood, such that for every proper coloring of the graph with $q$ colors at least one vertex guesses its color correctly. In this variation, we prove that the hat guessing number of the complete graphs on $n$ vertices is $2n - 1$, which is roughly twice the classical hat guessing number of the complete graph. Our winning strategy is related to finding perfect matchings between the middle layers of the boolean poset of dimension $2n - 1$. We prove that the hat guessing number of all trees on $n \geq 3$ vertices is equal to $4$. We derive general upper bounds in terms of the number of vertices, chromatic number, and maximum degree, and obtain improved bounds for book graphs. Using our results and an ILP formulation of the problem, we determine the exact hat guessing number for all graphs on at most $4$ vertices, give bounds on graphs on $5$ vertices, and suggest some open problems.

math.CO↗

arXiv2026

Quantum Otto heat-engine with Kitaev-Heisenberg cluster: Possible roles of frustration, magnons, and duality

We study the performance of Kitaev-Heisenberg (KH) clusters as working media realizing a quantum Otto engine (QOE). An external Zeeman field with linear time dependency is used as the driving mechanism. The efficiency strongly depends on Kitaev ($\kappa$) and Heisenberg ($J$) exchange interaction. Interestingly, efficiency is comparable when the relative magnitude of $\kappa$ and $J$ is the same but of opposite signs. The above results are explained due to a subtle interplay of frustration, quantum fluctuation, and duality of eigen-spectra for the KH system when both the signs of $\kappa$ and $J$ are reversed. The maximum efficiency is shown to be dynamically related to eigen-spectra forming discrete narrow bands, where total spin angular momentum becomes a good quantum number. We relate this optimum efficiency to the realization of weakly interacting magnons, where the system reduces to an approximate eigen-system of the external drive. Finally, we extend our study to the large spin Kitaev model and find a quantum advantage in efficiency for $S=1/2$. The results could be of practical interest for materials with KH interactions as a platform for QOE.

cond-mat.mes-hall↗

arXiv2025

Study of Correlated Disorders and interaction in the Hofstadter Butterfly

We investigate the impact of several quasiperiodic disorders and their continuous interpolation with the Aubry-Andre (AA) potential on the Hofstadter butterfly using mean field approximation at zero temperature for a two-dimensional square lattice. Weak disorder mildly smears the fractal spectrum, while strong quasiperiodic potentials destroy the butterfly and generate multiple energy gaps. The AA potential produces the strongest spectral restructuring, creating prominent gaps near half-filling. Interpolating AA with other quasiperiodic potentials reveals competing gap-opening mechanisms, ranging from AA-dominated gaps at small interpolation parameters to a robust half-filling gap generated by the competing disorders at large parameters. Entanglement entropy follows the area law at low and high magnetic fields but shows pronounced deviations at intermediate fields, with opposite trends for strong AA versus other quasiperiodic potentials. Localization analysis using IPR and NPR confirms enhanced localization with increasing disorder; the AA potential yields the largest IPR, with notable field dependence. Interpolation produces smooth crossovers between distinct localization regimes.

cond-mat.str-el↗

arXiv2025

Kaganov-Lifshitz-Tanatarov theory for tilted Dirac-cone materials: anisotropic heating from uniform light

We point out that in the tilted Dirac cone materials the non-equilibrium (hot) electron relaxation with phonons is anisotropic in the Brillouin zone. It means that there is a preferential heating of the lattice degrees of freedom in the specific directions of the Brillouin zone, in particular, in the direction opposite to the tilt velocity in the model considered by us. This observation will have novel consequences: (1) With pump-probe spectroscopy applied to a given tilted Dirac cone material an anisotropic relaxation would lead to a transient anisotropic heating which can further lead to a transient Seebeck effect as transient thermal gradients would exist in the specific directions of the BZ, and (2) this direction of anisotropic heating can be controlled by controlling the direction of the tilt velocity which can be externally tuned by the application of an external pressure. We foresee novel applications of this effect in ultrafast sensor applications involving transient heating effects. This is equivalent to inducing a transient Seebeck effect by just shinning light on a tilted Dirac cone material!

cond-mat.str-el↗

arXiv2025

Information phases of partial projected ensembles generated from random quantum states and scrambling dynamics

The projected ensemble -- an ensemble of pure states on a subsystem conditioned on projective measurement outcomes on its complement -- provides a finer probe of ergodicity and information structure than the reduced density matrix of the subsystem in bipartite quantum states. This framework can be generalised to partial projected ensembles in tripartite settings, where outcomes from part of the measured subsystem are discarded, leading to ensembles of mixed states. We show that information measures defined for such ensembles, in particular the Holevo information, yield a more detailed characterisation of how quantum information is distributed between subsystems compared to conventional entanglement measures. Using exact analytical results supported by numerical results, we uncover a qualitative change in the scaling of the Holevo information with system size in partial projected ensembles generated by Haar-random states, as the relative sizes of the subsystem are varied. In one phase, the Holevo information decays exponentially with system size, while in the other it grows linearly, thereby defining distinct information phases separated by sharp transitions signalled by non-analyticities in the Holevo information. The exponentially decaying phase rigorously establishes the existence of a measurement-invisible quantum-correlated phase -- a manifestation of many-body information scrambling with no bipartite analogue. We contrast this information-phase diagram with the entanglement-phase structure of tripartite Haar-random states obtained from logarithmic negativity, and show that the Holevo information reveals additional fine structure beyond conventional entanglement measures. Finally, we show that these information phases, as characterised by the Holevo information, emerge in the dynamics of chaotic quantum circuits and discuss the associated timescales.

quant-ph↗

arXiv2025

Finite-Time Convergence of Single-Trajectory Chi-Square Robust Q-Learning With Linear Function Approximation

Distributionally robust reinforcement learning seeks policies that remain effective when the deployment environment differs from the one that generated the training data. We study model-free robust Q-learning with $\chi^2$ uncertainty sets and linear function approximation, using data from a single trajectory of an unknown nominal MDP. Evaluating the $\chi^2$ robust Bellman target introduces the square root of a conditional second moment, which cannot be estimated unbiasedly from one transition, while the projected robust Bellman operator need not be contractive. We address these obstacles through a variational reformulation of the robust Bellman target and a blockwise frozen-target scheme, and establish a finite-time error bound relative to the optimal robust Q-function for every $\gamma\in(0,1)$. A neural-network experiment illustrates how the variational target can be used in a continuous-state nonlinear-control task.

cs.LG↗

arXiv2025

Partial projected ensembles and spatiotemporal structure of information scrambling

Thermalisation and information scrambling in out-of-equilibrium quantum many-body systems are deeply intertwined: local subsystems dynamically approach thermal density matrices while their entropies track information spreading. Projected ensembles--ensembles of pure states conditioned on measurement outcomes of complementary subsystems--provide higher-order probes of thermalisation, converging at late times to universal maximum-entropy ensembles. In this work, we introduce the partial projected ensemble (PPE) as a framework to study how the spatiotemporal structure of scrambling is imprinted on projected ensembles. The PPE consists of an ensemble of mixed states induced on a subsystem by measurements on a spatially separated part of its complement, tracing out the remainder, naturally capturing scenarios involving discarded outcomes or noise-induced losses. We show that statistical fluctuations of the PPE faithfully track the causal lightcone of information spreading, revealing how scrambling dynamics are encoded in ensemble structure. In addition, we demonstrate that the probabilities of bit-string probabilities (PoPs) associated with the PPE exhibit distinct dynamical regimes and provide an experimentally accessible probe of scrambling. Both PPE fluctuations and PoPs display exponential sensitivity to the size of the discarded region, reflecting exponential degradation of quantum correlations under erasure. We substantiate these findings using the non-integrable kicked Ising chain, combining numerics in the ergodic regime with exact results at its self-dual point. We extend our analysis to a many-body localised (MBL) regime numerically, along with analytic results for the $\ell$-bit model. The linear and logarithmic lightcones characteristic of ergodic and MBL regimes emerge naturally from PPE dynamics, establishing it as a powerful tool for probing scrambling and deep thermalisation.

quant-ph↗

arXiv2025

Thermal transport characteristics of impinging ferrofluid droplets in the presence of a magnetic field

Droplet interactions with solid surfaces are fundamental to natural phenomena and hold significant commercial relevance across diverse applications. While the impingement dynamics of conventional aqueous droplets on solid substrates are well-characterized, the behavior of non-aqueous droplets, particularly those influenced by external force fields like electric or magnetic fields, remains a less explored domain. This study addresses this gap by investigating the impact of a magnetic field on the impingement dynamics of ferrofluid droplets on a heated solid surface. Ferrofluids are unique colloidal suspensions of magnetic nanoparticles within a non-magnetic carrier fluid, enabling external manipulation of their dynamic properties through magnetic forces. The application of a magnetic field introduces an attractive force within the ferrofluid, fundamentally altering the droplet's spreading behavior and, consequently, its transport characteristics upon impact. Our findings reveal a substantial increase in both the maximum spreading diameter and the contact time between the droplet and the substrate, directly leading to enhanced thermal transport efficiency. Furthermore, the magnetic force effectively suppresses droplet bounce-off from hydrophobic surfaces. These critical parameters can be precisely controlled by adjusting the strength of the induced magnetic force. Such interactions can be used in the design of thermal switches and thermal management systems. The multi-physics interactions of magnetic fields, fluid flow, interface tracking, and heat transfer within the multiphase system are computationally modelled to examine the effect of Weber number and contact angle on maximum spreading and associated heat transfer characteristics.

physics.flu-dyn↗

arXiv2025

A primer on Kitaev Model: Basic aspects, material realization, and recent experiments

This elementary review article is aimed to the beginning graduate students interested to know basic aspects of Kitaev model. We begin with a very lucid introduction of Kitaev model and present its exact solution, Hilbert space structure, fractionalisation, spin-spin correlation function and topological degeneracy in an elementary way. We then discuss the recent proposal of realizing Kitaev interaction in certain materials. Finally we present some recent experiments done on these materials, mainly magnetization, susceptibility, specific heat and thermal Hall effect to elucidate the recent status of material realization of coveted Kitaev spin-liquid phase. We end with a brief discussion on other theoretical works on Kitaev model from different many-body aspects.

cond-mat.str-el↗

arXiv2025

Extended Haldane model -- a modern gateway to topological insulators

The seminal Haldane model brings up a paradigm beyond the quantum Hall effect to look for a plethora of topological phases in the honeycomb and other lattices. Here we dwell into this model considering a full parameter space in the presence of spin-orbit interaction as well as Zeeman field such that the flavour of Kane-Mele model is invoked. Adopting this extended Haldane model as an example, we elucidate, in a transparent manner, a number of topological features in a pedagogical manner. First, we describe various first order topological insulator phases and their characterizations while explaining various anomalous quantum Hall effects and quantum spin Hall effects in the extended Haldane model. Second, we demonstrate the concepts of higher order topological insulator phases along with the topological invariants in the anisotropic limit of the extended Haldane model. At the end, we discuss various open issues involving \textcolor{black}{emergent or extended} symmetries that might lead to a broader understanding of various topological phases and the associated criteria behind their emergence.

cond-mat.mes-hall↗

arXiv2025

Deciphering competing interactions of Kitaev-Heisenberg-$\Gamma$ system in clusters: part II -- dynamics of Majorana fermions

We perform a systematic and exact study of Majorana fermion dynamics in the Kitaev-Heisenberg-$\Gamma$ model in a few finite-size clusters increasing in size up to twelve sites. We employ exact Jordan-Wigner transformations to evaluate certain measures of Majorana fermion correlation functions, which effectively capture matter and gauge Majorana fermion dynamics in different parameter regimes. An external magnetic field is shown to produce a profound effect on gauge fermion dynamics. Depending on certain non-zero choices of other non-Kitaev interactions, it can stabilise it to its non-interacting Kitaev limit. For all the parameter regimes, gauge fermions are seen to have slower dynamics, which could help build approximate decoupling schemes for appropriate mean-field theory. The probability of Majorana fermions returning to their original starting site shows that the Kitaev model in small clusters can be used as a test bed for the quantum speed limit.

cond-mat.mes-hall↗

arXiv2025

Finite-Horizon Single-Pull Restless Bandits: An Efficient Index Policy For Scarce Resource Allocation

Restless multi-armed bandits (RMABs) have been highly successful in optimizing sequential resource allocation across many domains. However, in many practical settings with highly scarce resources, where each agent can only receive at most one resource, such as healthcare intervention programs, the standard RMAB framework falls short. To tackle such scenarios, we introduce Finite-Horizon Single-Pull RMABs (SPRMABs), a novel variant in which each arm can only be pulled once. This single-pull constraint introduces additional complexity, rendering many existing RMAB solutions suboptimal or ineffective. %To address this, we propose using dummy states to duplicate the system, ensuring that once an arm is activated, it transitions exclusively within the dummy states. To address this shortcoming, we propose using \textit{dummy states} that expand the system and enforce the one-pull constraint. We then design a lightweight index policy for this expanded system. For the first time, we demonstrate that our index policy achieves a sub-linearly decaying average optimality gap of $\tilde{\mathcal{O}}\left(\frac{1}{\rho^{1/2}}\right)$ for a finite number of arms, where $\rho$ is the scaling factor for each arm cluster. Extensive simulations validate the proposed method, showing robust performance across various domains compared to existing benchmarks.

cs.MA↗

arXiv2024

A Theoretical Analysis of Soft-Label vs Hard-Label Training in Neural Networks

Knowledge distillation, where a small student model learns from a pre-trained large teacher model, has achieved substantial empirical success since the seminal work of \citep{hinton2015distilling}. Despite prior theoretical studies exploring the benefits of knowledge distillation, an important question remains unanswered: why does soft-label training from the teacher require significantly fewer neurons than directly training a small neural network with hard labels? To address this, we first present motivating experimental results using simple neural network models on a binary classification problem. These results demonstrate that soft-label training consistently outperforms hard-label training in accuracy, with the performance gap becoming more pronounced as the dataset becomes increasingly difficult to classify. We then substantiate these observations with a theoretical contribution based on two-layer neural network models. Specifically, we show that soft-label training using gradient descent requires only $O\left(\frac{1}{\gamma^2 \epsilon}\right)$ neurons to achieve a classification loss averaged over epochs smaller than some $\epsilon > 0$, where $\gamma$ is the separation margin of the limiting kernel. In contrast, hard-label training requires $O\left(\frac{1}{\gamma^4} \cdot \ln\left(\frac{1}{\epsilon}\right)\right)$ neurons, as derived from an adapted version of the gradient descent analysis in \citep{ji2020polylogarithmic}. This implies that when $\gamma \leq \epsilon$, i.e., when the dataset is challenging to classify, the neuron requirement for soft-label training can be significantly lower than that for hard-label training. Finally, we present experimental results on deep neural networks, further validating these theoretical findings.

cs.LG↗

arXiv2024

Emergent topological phases in an extended Su-Schrieffer-Heeger model with Rashba spin-orbit interaction, higher order hopping and domain wall

We theoretically investigate emergent topological phases in an extended spin-full Su-Schrieffer-Heeger (SSH) model considering Rashba spin-orbit interaction, all possible complex next to next nearest neighbor (NNNN) hopping preserving Chiral symmetry. Our analysis finds exact condition for which the topological phases of both the spin sectors could be independently varied. We show that it necessarily depends on complex NNNN only. We elaborate in detail the emergent topological phases, its criteria through analytic determination of non-trivial gap-closing condition due to the presence of $\cos 2k$ term. We also find that the profile of topological edge modes for finite winding numbers depend non-monotonously on the value of NNNN hopping elucidating competing effect of model parameters. We extend our study to few coupled chains and show explicitly that depending on the parameters all possible winding number ranging from zero to $2N$ could be obtained, where $N$ is the number of chains considered. Finally we incorporate the study of domain wall and remarkably we find that the location of mid-gap zero energy state by changing the values of model parameters. Our study could be of immensely useful for future applications in quantum technology.

cond-mat.mes-hall↗