arXiv subjects

Rohit Kumar Shukla

Publications and source records attributed to Rohit Kumar Shukla.

16 recordsLinked to original sources

arXiv2026

Quantum memory and scrambling from the perspective of a classical neural network

Entropic uncertainty relations are universal quantifiers of fundamental uncertainties of quantum measurements and are widely discussed in the quantum metrology literature. Quantum memory is a phenomenon related to the specific type of quantum correlations that allows for reducing fundamental uncertainties of quantum measurements. In the present work, the modified concept of quantum memory for time-dependent problems is proposed. We compare the time-dependent formulation of quantum memory with the out-of-time-ordered correlator (OTOC). Quantum memory is a rigorous mathematical concept that requires demanding calculations. Thus, until now, quantum memory has been discussed mainly for simple model systems and stationary problems. In the present work, we demonstrate that quantum memory can also be studied for realistic and physically relevant systems, e.g., the atomic helical spin chain, as well as the emergence and propagation of quantum correlations in time. We found that quantum memory manifests faster oscillations in time than OTOC and does not equilibrate. Furthermore, an artificial neural network is trained and asked to predict results for OTOC and quantum memory. These results show that quantum memory is more sensitive than OTOC in terms of broken inversion symmetry and the nonreciprocal effect.

quant-ph↗

arXiv2026

Many-Body Structural Effects in Periodically Driven Quantum Batteries

While quantum batteries have been widely studied under static driving, their performance under periodic driving in many-body systems has received only limited attention. In this Letter, we uncover structural principles showing that many-body structure fundamentally determines the charging performance of a collective spin-1/2 quantum battery driven by a periodic Ising charger. In particular, interaction range, boundary conditions, system size, and integrability -- capturing graph connectivity, geometry, even-odd effects, and many-body dynamics -- emerge as critical factors for enhancing stored energy and charging power. First, we analyze how connectivity scaling and boundary geometry shape battery performance. We show that long-range interacting chargers exhibit superextensive energy storage, approaching the fundamental upper bound over broad ranges of driving periods and system sizes. In contrast, nearest-neighbor chargers achieve optimal charging only under finely tuned commensurability conditions. Moreover, we find that open boundary conditions (OBC) enhance robustness compared to periodic boundary conditions (PBC). Second, we examine the role of integrability under periodic driving. We demonstrate that nonintegrability enhances energy storage by suppressing conserved quantities and promoting ergodic Floquet dynamics, thereby enabling efficient population of the many-body spectrum. Through systematic structural optimization across multiple parameters, we identify long-range nonintegrability as a central resource for fast, scalable, and robust charging of collective quantum batteries. Our results clarify how structural features of many-body systems, together with periodic driving, can be harnessed to achieve efficient collective charging dynamics.

quant-ph↗

arXiv2026

Collective dynamics versus entanglement in quantum battery performance

We investigate charging dynamics in many-body quantum batteries by examining the relationship between instantaneous charging power and the emergence of bipartite, tripartite, and multipartite quantum correlations across different battery--charger configurations. We find that the charging power reaches its maximum before the correlation measures peak, revealing a temporal separation between rapid energy transfer and the buildup of quantum correlations. We further study the role of interaction structure using local and many-body charging Hamiltonians for both unconstrained and constrained charging protocols, in which the total available charging resources are fixed to scale proportionally with the number of spins, thereby isolating the influence of interaction geometry and particle participation on the charging dynamics. In the unconstrained case, the enhanced charging energy observed for higher-order interactions originates primarily from the larger energy scale of the corresponding charging Hamiltonian. Under constrained conditions, however, increasing the interaction order alone provides no advantage over parallel charging when interaction and local spin-flip contributions are balanced, whereas interaction-dominated protocols significantly enhance charging performance, highlighting the importance of collective many-body dynamics. Fully collective interactions yield the largest enhancement in charging power together with stronger multipartite correlations, while partially collective interactions provide only limited improvements. Finally, extending the interaction range through next-nearest-neighbor couplings suppresses the charging power, demonstrating that, under constrained energetic resources, charging performance is governed primarily by interaction structure, particle participation, and collective many-body dynamics rather than by interaction order alone.

quant-ph↗

arXiv2026

Dynamical onset of quasiprobability negativity in quantum many-body systems

Time-dependent quasiprobability distributions provide a quasiprobabilistic description of sequential measurement statistics generated by quantum dynamics, and can reveal nonclassical features with no classical probabilistic counterpart. Yet the dynamical emergence of their negativity in many-body systems remains largely unexplored. We introduce the \emph{first-time negativity} (FTN) of the Margenau-Hill quasiprobability as a dynamical indicator of when local measurement sequences in an interacting quantum system begin to exhibit genuinely nonclassical behavior. Using the Ising chain, we show that FTN discriminates clearly between interaction-dominated and field-dominated regimes, is systematically reshaped by temperature, and responds sensitively to the breaking of integrability. For spatially separated measurements, FTN appears abruptly near the interaction-field crossover, while measurements at opposite boundaries exhibit a distinct weak-field branch whose onset time grows with chain length and is consistent with ballistic propagation across the system. We further compare the numerical onset of negativity with a recently proposed quantum speed limit (QSL) for quasiprobabilities, which provides a geometric benchmark for the observed dynamics. Our results identify FTN as a practical and experimentally accessible probe of the real-time onset of quasiprobability negativity and contextual sequential measurement statistics, directly suited to current platforms capable of sequential weak and strong measurements.

quant-ph↗

arXiv2025

Krylov complexity in ergodically constrained nonintegrable transverse-field Ising model

The nonintegrable transverse-field Ising model is a common platform for studying ergodic quantum dynamics. In this work, we introduce a simple variant of the model in which this ergodic behaviour is suppressed by introducing a spatial inhomogeneity in the interaction strengths. For this we partition the chain into two equal segments within which the spins interact with different coupling strengths. The ratio of these couplings defines an inhomogeneity parameter, whose variation away from unity leads to constrained dynamics. We characterize this crossover using multiple diagnostics, such as the long-time saturation of out-of-time-ordered correlators, level-spacing statistics, and the spectral form factor. We further examine the consequences for operator growth in Krylov space and for entanglement generation in the system's eigenstates. Together, these results demonstrate that introducing a macroscopic inhomogeneity in coupling strengths provides a minimal, disorder-free route to breaking ergodicity in this specific model of interacting spins.

quant-ph↗

arXiv2025

Dynamics of Majorana zero modes across hybrid Kitaev chain

The Kitaev chain has been extensively explored in the context of uniform couplings, with studies focusing either on purely nearest-neighbor interactions or on systems dominated by long-range superconducting pairing. Building on these investigations, we introduce a hybrid Kitaev chain in which the lattice is partitioned into two segments: the left segment comprises nearest-neighbor couplings, while the right segment incorporates long-range pairing. To probe the role of the interface, we study two scenarios: a decoupled (suppressed hopping) case, where the segments are isolated, and a coupled case, where they are connected via interface hopping that enables tunneling. Using this setup, we investigate the behavior of Majorana zero modes at the interface between the two segments, finding that in the decoupled case, Majorana zero modes remain sharply localized at the left segment chain edges while massive Dirac modes remain in right segment chain edges, with their energies and localization strongly dependent on the long-range pairing exponent. Introducing a finite interface coupling enables transfer of Majorana zero modes from the edges of the left segment to those of the right segment of the chain. We characterize this dynamics by the fidelity of state transfer, dynamical rotation, and inverse participation ratio. We show the signature of Majorana zero mode transfer across the interface by the spatiotemporal profile of the probability distribution of the time evolved state.

quant-ph↗

arXiv2025

Diagnosing chaos in a periodically driven Ising model with a ramping field via out-of-time-order correlation saturation

The dynamic region of out-of-time-ordered correlators (OTOCs) serves as a powerful indicator of chaos in classical and semiclassical systems, capturing the characteristic exponential growth. In contrast, this signature fails to appear in spin systems, where even chaotic dynamics lack such exponential escalation, making this region an unreliable marker of chaos. To address this limitation, we turn to the saturation behavior of OTOCs to differentiate between chaotic and integrable regimes. In integrable systems, the saturation region of OTOCs exhibits oscillatory behavior, while in chaotic systems, it shows a stable saturation. To evaluate this distinction, we investigate a time-dependent Ising spin system subjected to a linearly ramping transverse field, analyzing both integrable (without longitudinal field) and non-integrable (with longitudinal field) scenarios. The ramping introduces a time-dependent increase of the external field, which influences the saturation regime of the OTOC, a region crucial for characterizing the chaotic behavior of the system. To quantify the degree of chaoticity, we compute the normalized Fourier spectrum of the OTOC and observe that increasing the ramping field strength leads to a suppression of oscillation frequencies in the saturation region of the OTOC, thereby enhancing the system's chaotic behaviour. To further support our findings, we investigate the level spacing distribution of time-dependent unitary operators, which effectively distinguishes chaotic from regular regions in our system and corroborates the results obtained from the saturation behavior of the OTOC.

quant-ph↗

arXiv2025

Scrambling in Ising spin systems with periodic transverse magnetic fields

Scrambling of quantum information in both integrable and nonintegrable Floquet spin systems is studied. Our study employs tripartite mutual information (TMI), with negative TMI serving as an indicator of scrambling, where a more negative value suggests a higher degree of scrambling. Both integrable and nonintegrable Floquet systems display scrambling behavior across all periods lying between 0 to π/2, except at self-dual point(π/4). Nonintegrable Floquet systems exhibit more pronounced scrambling compared to integrable ones across all periods. The degree of scrambling increases as we move towards the self-dual point (but not at the self-dual point), regardless of the initial states. TMI demonstrates periodic behavior at the self-dual point, with a period matching the system size in the case of the integrable system while displaying complex patterns in the non-integrable system. The initial growth of scrambling in both integrable and nonintegrable Floquet systems manifests as a power-law increase for small periods, followed by a sudden jump in scrambling near the self-dual point.

quant-ph↗

arXiv2025

Entanglement structure for finite system under dual-unitary dynamics

The dynamics of quantum many-body systems in the chaotic regime are of particular interest due to the associated phenomena of information scrambling and entanglement generation within the system. While these systems are typically intractable using traditional numerical methods, an effective framework can be implemented based on dual-unitary circuits which have emerged as a minimal model for maximally chaotic dynamics. In this work, we investigate how individual two-body operators influence the global dynamics of circuits composed of dual-unitaries. We study their effect on entanglement generation while examining it from both bipartite and multipartite perspectives. Here we also highlight the significant role of local unitaries in the dynamics when paired with operators from the dual-unitary class, showing that systems with identical entangling power can exhibit a range of differing entanglement growth rates. Furthermore, we present calculations establishing time-step-dependent lower bounds, which depend on both the initial state and the entangling power of the constituent operators. Finally, we find that time-evolving an initial state composed of pair products generates a state with nearly maximal multipartite entanglement content, approaching the bounds established by Absolutely Maximally Entangled (AME) states.

quant-ph↗

arXiv2025

Characterizing quantum dynamics using multipartite entanglement generation

Entanglement is a defining feature of many-body quantum systems and is an essential requirement for quantum computing. It is therefore useful to study physical processes which generate entanglement within a large system, as they maybe replicated for applications involving the said requirements in quantum information processing. A possible avenue to maximize entanglement generation is to rely on the phenomena of information scrambling, i.e. transport of initially localized information throughout the system. Here the rationale is that the spread of information carries with it an inherent capacity of entanglement generation. Scrambling greatly depends upon the dynamical nature of the system Hamiltonian, and the interplay between entanglement generation and information scrambling maybe investigated taking a chain of interacting spins on a one dimensional lattice. This system is analogous to an array of qubits and this relative simplicity implies that the resulting unitary dynamics can be efficiently simulated using present-day cloud based NISQ devices. In our present work, we consider such a spin model which is made up of nearest and next nearest neighbor XXZ Model, along with an introduced coupling term lambda. This coupling term serves as a tuning parameter which modifies the dynamical nature of the system from the integrable to the quantum chaotic regime. In order to quantify the entanglement generated within the system we use the more general multipartite metric which computes the average entanglement across all system bipartitions to obtain a global picture of the entanglement structure within the entire system.

quant-ph↗

arXiv2025

Out-of-time-order correlation in the quantum Ising Floquet spin system and magnonic crystals

In recent times out-of-time-order correlators (OTOC) have been established as a tool to understand butterfly effects, quantum information scrambling, and many-body localization. They can also be useful in determining different phases of quantum critical systems. OTOCs can identify the quantum chaos within a system undergoing time evolution; and therefore, they can distinguish between chaotic and regular dynamics. This motivates us to study OTOCs in integrable and nonintegrable periodically kicked quantum spin models. A periodically kicked quantum Ising spin system, known as the quantum Ising Floquet system, is a variant of the transverse Ising model. In place of constant transverse magnetic fields in the transverse Ising system, time-periodic fields are applied in the form of delta pulses in the quantum Ising Floquet spin system. It provides very interesting and peculiar dynamics separate from that of the transverse Ising system.

quant-ph↗

arXiv2025

System versus charger in performance optimization of quantum batteries

Quantum batteries provide a platform for investigating energy storage and extraction in quantum many-body systems. Here, we study a charging protocol in which battery and chargoid roles are assigned to different Hamiltonian components of a standalone many-body spin system. By externally controlling the contribution of the intrinsic battery Hamiltonian during charging, we reveal a tunable competition between intrinsic and charging dynamics. We find that suppressing the intrinsic battery contribution can substantially enhance both the maximum stored energy and charging power, with the magnitude of the enhancement determined by the interaction structure and range. We further investigate the protocol in a Markovian open-system setting that incorporates energy relaxation and pure dephasing. While environmental effects generally degrade charging performance, they can instead enhance energy-storage and energy-extraction dynamics in certain interacting systems. The enhancement associated with the controlled suppression of the intrinsic battery dynamics remains robust in the presence of environmental coupling.

quant-ph↗

arXiv2025

Prethermal Floquet time crystals in chiral multiferroic chains and applications as quantum sensors of AC fields

We study the emergence of prethermal Floquet Time Crystal (pFTC) in disordered chiral multiferroic chains. The model is an extension of the usual periodically driven nearest-neighbor disordered Heisenberg chain, with additional next-nearest-neighbor Heisenberg couplings and DMI interactions due to external magnetic and electric couplings. We derive the phase diagram of the model, characterizing the magnetization, entanglement, and coherence dynamics of the system along the extended interactions. In addition, we explore the application of the pFTC as quantum sensors of AC fields. The sensor performance to estimate small AC fields is quantified through the quantum Fisher information (QFI) measure. The sensor offers several advantages as compared to those composed of non-interacting spins due to its intrinsic robustness, long coherent interrogation time, and many-body correlations. Specifically, the sensor can overcome the standard quantum limit ($\rm{SQL} \sim N t^2$) during the prethermal regime, reaching an optimum performance at the pFTC lifetime $t^*$, where the $\rm{QFI}/Nt^{*^2} \sim N^α$ with $α> 0$, scaling superlinarly with the number of spins. Different from \text{full} FTCs, the prethermal lifetime does not diverge in the thermodynamic limit, nevertheless it can be increasingly long with tuning system parameters.

quant-ph↗

arXiv2022

Characteristic, dynamic, and near saturation regions of Out-of-time-order correlation in Floquet Ising models

We study characteristic, dynamic, and saturation regimes of the out-of-time-order correlation (OTOC) in the constant field Floquet system with and without longitudinal field. In the calculation of OTOC, we take local spins in longitudinal and transverse directions as observables which are local and non-local in terms of Jordan-Wigner fermions, respectively. We use the exact analytical solution of OTOC for the integrable model (without longitudinal field term) with transverse direction spins as observables and numerical solutions for other integrable and nonintegrable cases. OTOCs generated in both cases depart from unity at a kick equal to the separation between the observables when the local spins in the transverse direction and one additional kick is required when the local spins in the longitudinal direction. The number of kicks required to depart from unity depends on the separation between the observables and is independent of the Floquet period and system size. In the dynamic region, OTOCs show power-law growth in both models, the integrable (without longitudinal field) as well as the nonintegrable (with longitudinal field). The exponent of the power-law increases with increasing separation between the observables. Near the saturation region, OTOCs grow linearly with a very small rate.

quant-ph↗

arXiv2022

Out-of-time-order correlation and detection of phase structure in Floquet transverse Ising spin system

We study the out-of-time-order correlation (OTOC) of the Floquet transverse Ising model and use it to verify the phase diagram of the system. First, we present the exact analytical solution of the transverse magnetization OTOC using the Jorden-Wigner transformation. We calculate the speed of correlation propagation and analyze the behavior of the revival time with the separation between the observables. In order to get the phase structure of the Floquet transverse Ising system, we use the longitudinal magnetization OTOC as it is known to serve as an order parameter of the system. We show the phase structure numerically in the transverse Ising Floquet system by using the long time average of the longitudinal magnetization OTOC. In both the open and the closed chain systems, we find distinct phases out of which two are paramagnetic (0-paramagnetic and $π$-paramagnetic), and two are ferromagnetic (0-ferromagnetic and $π$-ferromagnetic) as defined in the literature.

cond-mat.stat-mech↗

arXiv2022

Out-of-time-order correlators of nonlocal block-spin and random observables in integrable and nonintegrable spin chains

Out-of-time-order correlators (OTOC) in the Ising Floquet system, that can be both integrable and nonintegrable is studied. Instead of localized spin observables, we study contiguous symmetric blocks of spins or random operators localized on these blocks as observables. We find only power-law growth of OTOC in both integrable and nonintegrable regimes. In the non-integrable regime, beyond the scrambling time, there is an exponential saturation of the OTOC to values consistent with random matrix theory. This motivates the use of "pre-scrambled" random block operators as observables. A pure exponential saturation of OTOC in both integrable and nonintegrable system is observed, without a scrambling phase. Averaging over random observables from the Gaussian unitary ensemble, the OTOC is found to be exactly same as the operator entanglement entropy, whose exponential saturation has been observed in previous studies of such spin-chains.

quant-ph↗