arXiv subjects

Sunil K. Mishra

Publications and source records attributed to Sunil K. Mishra.

13 recordsLinked to original sources

arXiv2026

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↗

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

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↗

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↗

arXiv2023

Quantum information diode based on a magnonic crystal

Exploiting the effect of nonreciprocal magnons in a system with no inversion symmetry, we propose a concept of a quantum information diode, {\it i.e.}, a device rectifying the amount of quantum information transmitted in the opposite directions. We control the asymmetric left and right quantum information currents through an applied external electric field and quantify it through the left and right out-of-time-ordered correlation (OTOC). To enhance the efficiency of the quantum information diode, we utilize a magnonic crystal. We excite magnons of different frequencies and let them propagate in opposite directions. Nonreciprocal magnons propagating in opposite directions have different dispersion relations. Magnons propagating in one direction match resonant conditions and scatter on gate magnons. Therefore, magnon flux in one direction is damped in the magnonic crystal leading to an asymmetric transport of quantum information in the quantum information diode. A quantum information diode can be fabricated from an yttrium iron garnet (YIG) film. This is an experimentally feasible concept and implies certain conditions: low temperature and small deviation from the equilibrium to exclude effects of phonons and magnon interactions. We show that rectification of the flaw of quantum information can be controlled efficiently by an external electric field and magnetoelectric effects.

quant-ph↗

arXiv2022

Scrambling and quantum feedback in a nanomechanical system

The question of how swiftly entanglement spreads over a system has attracted vital interest. In this regard, the out-of-time ordered correlator (OTOC) is a quantitative measure of the entanglement spreading process. Particular interest concerns the propagation of quantum correlations in the lattice systems, {\it e.g.}, spin chains. In a seminal paper D. A. Roberts, D. Stanford and L. Susskind, J. High Energy Phys. 03, 051, (2015) the concept of the OTOC's radius was introduced. The radius of the OTOC defines the front line reached by the spread of entanglement. Beyond this radius operators commute. In the present work, we propose a model of two nanomechanical systems coupled with two Nitrogen-vacancy (NV) center spins. Oscillators are coupled to each other directly while NV spins are not. Therefore, the correlation between the NV spins may arise only through the quantum feedback exerted from the first NV spin to the first oscillator and transferred from the first oscillator to the second oscillator via the direct coupling. Thus nonzero OTOC between NV spins quantifies the strength of the quantum feedback. We show that NV spins cannot exert quantum feedback on classical nonlinear oscillators. We also discuss the inherently quantum case with a linear quantum harmonic oscillator indirectly coupling the two spins and verify that in the classical limit of the oscillator, the OTOC vanishes.

quant-ph↗

arXiv2021

Directional scrambling of quantum information in helical multiferroics

Local excitations as carriers of quantum information spread out in the system in ways governed by the underlying interaction and symmetry. Understanding this phenomenon, also called quantum scrambling, is a prerequisite for employing interacting systems for quantum information processing. The character and direction dependence of quantum scrambling can be inferred from the out-of-time-ordered commutators (OTOCs) containing information on correlation buildup and entanglement spreading. Employing OTOC, we study and quantify the directionality of quantum information propagation in oxide-based helical spin systems hosting a spin-driven ferroelectric order. In these systems, magnetoelectricity permits the spin dynamics and associated information content to be controlled by an electric field coupled to the emergent ferroelectric order. We show that topologically nontrivial quantum phases, such as chiral or helical spin ordering, allows for electric-field controlled anisotropic scrambling and a direction-dependent buildup of quantum correlations. Based on general symmetry considerations, we find that starting from a pure state (e.g., the ground state) or a finite temperature state is essential for observing directional asymmetry in scrambling. In the systematic numerical studies of OTOC, we quantify the directional asymmetry of the scrambling and verify the conjectured form of the OTOC around the ballistic wavefront. The obtained direction-dependent butterfly velocity $v_{\mathrm{B}}(\mathbf{n})$ provides information on the speed of the ballistic wavefront. In general, our calculations show an early-time power-law behavior of OTOC, as expected from an analytic expansion for small times. The long-time behavior of OTOC reveals the importance of (non-)integrability of the underlying Hamiltonian as well as the implications of conserved quantities such as the $z$-projection of the total spin.

cond-mat.str-el↗

arXiv2018

Analytical solutions of inhomogeneous transverse field Ising models

The inhomogeneous transverse field Ising models mainly impurity based and the joint chain are analysed analytically using Jordan-Wigner transformations. The effects of inhomogeneities on the phase transition have been discussed in detail. We constructed an ansatz to diagonalize the two models which are taken into consideration. The inhomogeneity is quantified by a coupling parameter, which can be tuned to control the occurrence of quantum phase transition in these models. We have shown a systematic setup using which we can generalise the solution to a system with an arbitrary number of impurity sites and junctions, which are separated by at least two lattice sites. We have analysed the quantum critical point by calculating the correlation functions, transverse magnetization and the gap between the ground state and the first excited state.

quant-ph↗

arXiv2017

Qubit(s) transfer in helical spin chains

Qubit(s) transfer through a helical chain is studied. We consider the transfer of a single state and Bell states across a multiferroic spin chain and the possibility of an electric field control of the fidelity of the single state and the Bell pairs. We analyze pure and imperfect multiferroic spin chains. A scheme for an efficient transfer of spin states through a multiferroic channel relies on kicking by appropriate electric field pulses at regular interval. This electric field pulse sequence undermines the effect of impurity on the fidelity and improves the state transfer through the helical chain.

quant-ph↗

arXiv2014

Three-level spin system under decoherence-minimizing driving fields: Application to nitrogen-vacancy spin dynamics

Within the framework of a general three-level problem, the dynamics of the nitrogen-vacancy (NV) spin is studied for the case of a special type of external driving consisting of a set of continuous fields with decreasing intensities. Such a set has been proposed for minimizing coherence losses. Each new driving field with smaller intensity is designed to protect against the fluctuations induced by the driving field at the preceding step with larger intensity. We show that indeed this particular type of external driving minimizes the loss of coherence, using purity and entropy as quantifiers for this purpose. As an illustration, we study the coherence loss of an NV spin due to a surrounding spin bath of $^{13}$C nuclei.

quant-ph↗

arXiv2014

Protocol using kicked Ising dynamics for generating states with maximal multipartite entanglement

We present a solvable model of iterating cluster state protocols that lead to entanglement production, between contiguous blocks, of 1 ebit per iteration. This continues till the blocks are maximally entangled at which stage an unravelling begins at the same rate till the blocks are unentangled. The model is a variant of the transverse field Ising model and can be implemented with CNOT and single qubit gates. The inter qubit entanglement as measured by the concurrence is shown to be zero for periodic chain realizations while for open boundaries there are very specific instances at which these can develop. Thus we introduce a class of simply produced states with very large multipartite entanglement content of potential use in measurement based quantum computing.

quant-ph↗

arXiv2014

Quantum Otto heat engine based on a multiferroic chain working substance

We study a quantum Otto engine operating on the basis of a helical spin- 1/2 multiferroic chain with strongly coupled magnetic and ferroelectric order parameters. The presence of a finite spin chirality in the working substance enables steering of the cycle by an external electric field that couples to the electric polarization. We observe a direct connection between the chirality, the entanglement and the efficiency of the engine. An electric-field dependent threshold temperature is identified above which the pair correlations in the system, as quantified by the thermal entanglement, diminish. In contrast to the pair correlations, the collective many-body thermal entanglement is less sensitive to the electric field, and in the high temperature limit converges to a constant value. We also discuss the correlations between the threshold temperature of the pair entanglement, the spin chirality and the minimum of the fidelities in relation to the electric and magnetic fields. The efficiency of the quantum Otto cycle shows a saturation plateau with increasing electric field amplitude.

quant-ph↗

arXiv2013

Quenching and generation of random states in a kicked Ising model

The kicked Ising model with both a pulsed transverse and a continuous longitudinal field is studied numerically. Starting from a large transverse field and a state that is nearly an eigenstate, the pulsed transverse field is quenched with a simultaneous enhancement of the longitudinal field. The generation of multipartite entanglement is observed along with a phenomenon akin to quantum resonance when the entanglement does not evolve for certain values of the pulse duration. Away from the resonance, the longitudinal field can drive the entanglement to near maximum values that is shown to agree well with those of random states. Further evidence is presented that the time evolved states obtained do have some statistical properties of such random states. For contrast the case when the fields have a steady value is also discussed.

cond-mat.stat-mech↗