arXiv subjects

J. Bharathi Kannan

Publications and source records attributed to J. Bharathi Kannan.

11 recordsLinked to original sources

arXiv2026

Probing Chaos and Criticality with Observational Entropy and Finite-Resolution Measurements

Coarse-grained measurements offer a scalable alternative to full state tomography for characterizing complex quantum dynamics. We show that observational entropy (OE), an information-theoretic entropy defined directly from finite-resolution measurement outcomes, provides a unified and experimentally accessible framework for quantifying chaos and probing criticality. From probing the insulator-metal crossover in the Aubry-Andre model to tracking the gradual destruction of Kolmogorov-Arnold-Moser tori in the Kicked Rotor, derivatives of OE provide an accurate and unified diagnostic of probing these transitions. In both cases, the critical points extracted from dynamical evolution and eigenstate analyses converge to the exact theoretical values once the observational resolution exceeds a finite threshold. In the chaotic limit, OE exhibits a linear behavior within the Ehrenfest time regime, and its slope defines an observable Lyapunov exponent. Using a Pretty Good Measurement correction to the Husimi phase-space distribution, this entropy-production rate quantitatively reproduces the classical Lyapunov exponent in both the standard and singular kicked rotors. Our results establish OE as a compact information-theoretic bridge between classical instability, quantum criticality, and realistic finite-resolution measurements.

quant-ph↗

arXiv2026

Localization with Hopping Disorder in Quasi-periodic Synthetic Momentum Lattice

Lattice quasi-periodicity is easily realized with ultracold atoms in optical lattices and has been used to study delocalization-localization transition at low dimensions. Models with true disorder, however, remains largely unrealized in experiments. Here, using Bose-Einstein Condensate of ${^{87}{\text{Rb}}}$ atoms, we realize a Generalized Aubry-André (GAA) chain with added hopping disorder in a Momentum Space Lattice (MSL) via multiple Bragg diffractions. Unlike real space lattice simulators, MSL allows simulations of arbitrary disorder configurations and control over spatial disorder correlations. Uncorrelated hopping disorder added to the AA model enhances localization in all phases, smoothening the transition into a crossover between weakly and strongly localized regimes. On the other hand, numerical analysis shows that, spatially correlated hopping disorder induces partial delocalization of localized states in the vicinity of strong hopping bonds. Over a range of disorder strengths and correlations, the experimental results agree quantitatively with the numerical simulation of the dynamics in MSL. Ability of the platform to resolve correlation-dependent dynamical features in dynamics reflects the precision achieved in the realization. Our results demonstrate MSL as a viable platform for studying general disordered quantum systems beyond quasiperiodic systems.

cond-mat.quant-gas↗

arXiv2026

Upper bounds on charging power and tangible advantage in quantum batteries

Quantum battery is expected to outperform its classical counterpart due to quantum effects. Usually, in a quantum battery made of $N$ cells, quantum advantage is demonstrated through super-extensive scaling of the upper bound to the charging power with $N$. In this work, we show that potential quantum advantage as measured by the power bounds need not translate to {\it tangible} advantage in practice. We demonstrate this by considering an all-to-all coupled spin-chain model of a quantum battery with 2-local interactions. It exhibits super-extensive charging when analyzed using the upper bound derived from the uncertainty principle. Unlike the previously studied models, the contribution to this apparent quantum advantage is two-fold -- arising from both the battery and the charger. The model is also experimentally friendly, as it does not require global couplings and yet generates genuine multipartite entanglement. However, we demonstrate that the potential quantum advantage in this scenario is not tangible by employing a tighter upper bound on power. Additionally, we show that even this tighter bound can fail in a range of physical situations and indicate a quantum enhancement that is intangible in practice. Hence, we argue that actual power transferred must be evaluated along with proper characterization of the resources before claiming quantum advantage.

quant-ph↗

arXiv2025

Dependence of Krylov complexity on the initial operator and state

Krylov complexity, a quantum complexity measure which uniquely characterizes the spread of a quantum state or an operator, has recently been studied in the context of quantum chaos. However, the definitiveness of this measure as a chaos quantifier is in question in light of its strong dependence on the initial condition. This article clarifies the connection between the Krylov complexity dynamics and the initial operator or state. We find that the Krylov complexity depends monotonically on the inverse participation ratio (IPR) of the initial condition in the eigenbasis of the Hamiltonian. We explain the reversal of the complexity saturation levels observed in \href{https://doi.org/10.1103/PhysRevE.107.024217}{ Phys.Rev.E.107,024217, 2023} using the initial spread of the operator in the Hamiltonian eigenbasis. IPR dependence is present even in the fully chaotic regime, where popular quantifiers of chaos, such as out-of-time-ordered correlators and entanglement generation, show similar behavior regardless of the initial condition. Krylov complexity averaged over many initial conditions still does not characterize chaos.

quant-ph↗

arXiv2025

Probing the localization effects in Krylov basis

Krylov complexity (K-complexity) is a measure of quantum state complexity that minimizes wavefunction spreading across all the possible bases. It serves as a key indicator of operator growth and quantum chaos. In this work, K-complexity and Arnoldi coefficients are applied to probe a variety of localization phenomena in the quantum kicked rotor system. We analyze four distinct localization scenarios -- ranging from compact localization effect arising from quantum anti-resonance to a weaker form of power-law localization -- each one exhibiting distinct K-complexity signatures and Arnoldi coefficient variations. In general, K-complexity not only indicates the degree of localization, but surprisingly also of the nature of localization. In particular, the long-time behaviour of K-complexity and the wavefunction evolution on Krylov chain can distinguish various types of observed localization in QKR. In particular, the time-averaged K-complexity and scaling of the variance of Arnoldi coefficients with effective Planck's constant can distinguish the localization effects induced by the classical regular phase structures and the dynamical localization arising from quantum interferences. Further, the Arnoldi coefficient is shown to capture the transition from integrability to chaos as well. This work shows how localization dynamics manifests in the Krylov basis.

quant-ph↗

arXiv2025

Dichotomy in the effect of chaos on ergotropy

The maximum unitarily extractable work from a quantum system -- ergotropy -- is the basic principle behind quantum batteries, a rapidly emerging field. This work studies ergotropy in two quantum chaotic systems, the quantum kicked top and the kicked Ising spin chain, to illustrate the effects of chaotic dynamics. In an ancilla-assisted scenario, chaos enhances ergotropy when the state is known, a consequence of large entanglement production in the chaotic regime. When the state is unknown, we need to at least partially characterize the state using coarse-grained measurements for useful extraction of work. In this case, chaos impedes ergotropy by suppressing information gained from coarse-grained measurements, while entanglement with an ancilla still facilitates ergotropy. In this scenario, we study the interplay between chaos and entanglement and find a sweet spot in the chaos parameter for optimal work. Our results point to the potential of quantum chaos-assisted batteries for better work extraction.

quant-ph↗

arXiv2024

Asymmetric dynamical localization and precision measurement of BEC micromotion

We employ a Bose Einstein Condensate (BEC) based atom-optic kicked rotor to generate an asymmetrically localized momentum distribution that depends upon initial velocity of the BEC. Asymmetric features are shown to arise from the early-time dynamics induced by the broken parity symmetry and, asymptotically freeze as the dynamical localization stabilizes. The asymmetry in the momentum distribution critically depends upon the initial launch velocity and is sensitive to very small initial velocities ('micromotion') of the BEC. In this work, we also perform a precise measurement of the 'micromotion'. By utilizing the technique of measuring the early-time asymmetry of momentum distribution, we report measurement of micromotion down to (230 \pm 17 , μ\text{m/s}).

quant-ph↗

arXiv2024

Faster entanglement driven by quantum resonance in many-body kicked rotors

Quantum resonance in the paradigmatic kicked rotor system is a purely quantum effect that ignores the state of underlying classical chaos. In this work, it is shown that quantum resonance leads to superlinear entanglement production. In $N$-interacting kicked rotors set to be at quantum resonance, entanglement growth is super-linear until a crossover timescale $t^*$, beyond which growth slows down to a logarithmic form with superimposed oscillations. By mapping positional interaction to momentum space and analytically assessing the linear entropy, we unravel the mechanism driving these two distinct growth profiles. The analytical results agree with the numerical simulations performed for two- and three-interacting kicked rotors. The late time entanglement oscillation is sensitive to changes in scaled Planck's constant with a high quality factor suitable for high precision measurements. These results are amenable to an experimental realization on atom optics setup.

quant-ph↗

arXiv2024

Tuning for Quantum Speedup in Directed Lackadaisical Quantum Walks

Quantum walks constitute an important tool for designing quantum algorithms and information processing tasks. In a lackadaisical walk, in addition to the possibility of moving out of a node, the walker can remain on the same node with some probability. This is achieved by introducing self-loops, parameterized by self-loop strength $l$, attached to the nodes such that large $l$ implies a higher likelihood for the walker to be trapped at the node. In this work, {\it directed}, lackadaisical quantum walks is studied. Depending on $l$, two regimes are shown to exist -- one in which classical walker dominates and the other dominated by the quantum walker. In the latter case, we also demonstrate the existence of two distinct scaling regimes with $l$ for quantum walker on a line and on a binary tree. Surprisingly, a significant quantum-induced speedup is realized for large $l$. By tuning the initial state, the extent of this speedup can be manipulated.

quant-ph↗

arXiv2024

Chaos and localized phases in a two-body linear kicked rotor system

Despite the periodic kicks, a linear kicked rotor (LKR) is an integrable and exactly solvable model in which the kinetic energy term is linear in momentum. It was recently shown that spatially interacting LKRs are also integrable, and results in dynamical localization in the corresponding quantum regime. Similar localized phases exist in other non-integrable models such as the coupled relativistic kicked rotors. This work, using a two-body LKR, demonstrates two main results; firstly, it is shown that chaos can be induced in the integrable linear kicked rotor through interactions between the momenta of rotors. An analytical estimate of its Lyapunov exponent is obtained. Secondly, the quantum dynamics of this chaotic model, upon variation of kicking and interaction strengths, is shown to exhibit a variety of phases -- classically induced localization, dynamical localization, subdiffusive and diffusive phases. We point out the signatures of these phases from the perspective of entanglement production in this system. By defining an effective Hilbert space dimension, the entanglement growth rate can be understood using appropriate random matrix averages.

quant-ph↗

arXiv2022

Interaction-induced directed transport in quantum chaotic systems

Quantum directed transport can be realized in non-interacting, deterministic, chaotic systems by appropriately breaking the spatio-temporal symmetries in the potential. In this work, the focus is on the class of interacting quantum systems whose classical limit is chaotic. In this limit, one subsystem effectively acts as a source of "noise" to the other leading to temporal symmetry breaking. Thus, the quantum directed currents can be generated with two ingredients -- broken spatial symmetry in the potential and presence of interactions. This is demonstrated in two-body interacting kicked rotor and kicked Harper models. Unlike earlier schemes employed for single-particle ratchet currents, this work provides a minimal framework for realizing quantum directed transport in interacting systems. This can be generalized to many-body quantum chaotic systems.

quant-ph↗