arXiv subjects

Souradeep Ghosh

Publications and source records attributed to Souradeep Ghosh.

8 recordsLinked to original sources

arXiv2025

Randomization Times under Quantum Chaotic Hamiltonian Evolution

Randomness generation through quantum-chaotic evolution underpins foundational questions in statistical mechanics and applications across quantum information science, including benchmarking, tomography, metrology, and demonstrations of quantum computational advantage. While statistical mechanics successfully captures the temporal averages of local observables, understanding randomness at the level of higher statistical moments remains a daunting challenge, with analytic progress largely confined to random quantum circuit models or fine-tuned systems exhibiting space-time duality. Here we study how much randomness can be dynamically generated by generic quantum-chaotic evolution under physical, non-random Hamiltonians. Combining theoretical insights with numerical simulations, we show that for broad classes of initially unentangled states, the dynamics become effectively Haar-random well before the system can ergodically explore the physically accessible Hilbert space. Both local and highly nonlocal observables, including entanglement measures, equilibrate to their Haar expectation values and fluctuations on polynomial timescales with remarkably high numerical precision, and with the fastest randomization occurring in regions of parameter space previously identified as maximally chaotic. Interestingly, this effective randomization can occur on timescales linear in system size, suggesting that the sub-ballistic growth of Renyi entropies typically observed in systems with conservation laws can be bypassed in non-random Hamiltonians with an appropriate choice of initial conditions.

cond-mat.stat-mech↗

arXiv2025

Interplay of distinct modes of charge regulation on poly-acid ionization and conformation

We adapt the Edwards-Muthukumar theoretical framework for a single polymer chain to investigate the interplay between proton binding and counterion condensation for poly-acids. We find that changes to pH enable non-monotonic transitions between anti- and conventional polyelectrolyte behaviors. In the former, the net charge and the overall dimensions increase with increasing salt concentration, while the converse is true for conventional polyelectrolytes. The polymeric nature and local solvent polarization drive significant pKa shifts when compared to the values of reference monoacids. These pKa shifts are enhanced in semi-flexible chains.

cond-mat.soft↗

arXiv2025

Performance Analysis of Multi-IRS Aided Multiple Operator Systems at mmWave Frequencies

Intelligent reflecting surfaces (IRSs) are envisioned to enhance the performance of mmWave wireless systems. In practice, multiple mobile operators (MO) coexist in an area and provide simultaneous and independent services to user-equipments (UEs) on different frequency bands. Then, if each MO deploys an IRS to enhance its performance, the IRSs also alter the channels of UEs of other MOs. In this context, this paper addresses the following questions: can an MO still continue to control its IRS independently of other MOs and IRSs? Is joint optimization of IRSs deployed by different MOs and inter-MO cooperation needed? To that end, by considering the mmWave bands, we first derive the ergodic sum spectral efficiency (SE) in a $2$-MO system for the following schemes: 1) joint optimization of an overall phase angle of the IRSs with MO cooperation, 2) MO cooperation via time-sharing, and 3) no cooperation between the MOs. We find that even with no cooperation between the MOs, the performance of a given MO is not degraded by the presence of an out-of-band (OOB) MO deploying and independently controlling its own IRS. On the other hand, the SE gain obtained at a given MO using joint optimization and cooperation over the no-cooperation scheme decreases inversely with the number of elements in the IRS deployed by the other MO. We generalize our results to a multiple MO setup and show that the gain in the sum-SE over the no-cooperation case increases at least linearly with the number of OOB MOs. Finally, we numerically verify our findings and conclude that every MO can independently operate and tune its IRS; cooperation via optimizing an overall phase only brings marginal benefits in practice.

eess.SP↗

arXiv2024

Late-time ensembles of quantum states in quantum chaotic systems

We study the universal structure of late-time ensembles obtained from unitary dynamics in quantum chaotic systems with symmetries, such as charge or energy conservation. We find that although quantum states do not ergodically explore the entire Hilbert space at late times, the late-time ensemble typically becomes indistinguishable from Haar-random states in the thermodynamic limit at the level of finite statistical moments. Importantly, our results apply to initial states easy to prepare in ongoing experiments -- specifically, product states -- that lie in the middle of the spectrum of quantum chaotic systems. We show that these states typically exhibit not only the same late-time ensemble average as Haar-random states, but also the same state-to-state fluctuations and higher statistical moments. In other words, there is no measurement -- whether local or nonlocal -- at the level of finite statistical moments that can tell that the states are not exploring the entire Hilbert space. Interestingly, within the class of low-entanglement initial states, we also find atypical initial conditions in the middle of the spectrum of Hamiltonians known to be "maximally chaotic". Such atypical states have smaller variance of the symmetry operator than Haar-random states and evolve into non-universal ensembles that can be distinguished from the Haar ensemble by simple measurements or subsystem properties. In the limiting case of initial states with negligible variance of the symmetry operator (e.g., states with fixed particle number or energy eigenstates), the late-time ensemble has universal behavior captured by constrained random-state ensembles. Our results reveal that an extremely high level of quantum state randomness can still be achieved even when dynamics is constrained by symmetries.

cond-mat.stat-mech↗

arXiv2024

Thermodynamic Insights into Polyelectrolyte Complexation: A Theoretical Framework

In this study, we propose a theoretical framework to investigate the interactions between flexible polymer chains, specifically polyelectrolytes (PEs). By calculating the system's free energy while considering position-dependent mutual interactions and chain conformations, we gain insights into the local dielectricity as PEs overlap. Our analysis reveals that the thermodynamic drive for complex coacervation is influenced by factors such as the number of ions bound to the polymer backbone and the entropy associated with free ions, challenging earlier assumptions about the relationship between entropy gain and electrostatic temperature. We demonstrate that global thermodynamic behavior is strongly influenced by local factors like dielectric constant, providing clarity on discrepancies between experimental and computational studies. Additionally, we found that entropy gain is inversely proportional to the local dielectric constant, assuming a constant electrostatic temperature. Our findings highlight the importance of considering polymer-specific parameters when exploring the thermodynamic behavior of charged polymer complexation.

cond-mat.soft↗

arXiv2024

Slow relaxation of quasi-periodically driven integrable quantum many-body systems

We study the emergence and stability of a prethermal phase in an integrable many-body system subjected to a Fibonacci drive. Despite not being periodic, Fibonacci drives have been shown to introduce dynamical constraints due to their self-similar structure, unlike random driving protocols. From perturbative analysis, this has been argued to result in an exponentially long prethermal phase in the high frequency limit of driving. Examining higher order terms in the perturbative expansion, we show that the perturbative description breaks down eventually in such systems at a finite universal order, which depends solely on features of the Fibonacci sequence. This leads to an onset of energy absorption at long time scales for intermediate and low driving frequencies. Interestingly, in spite of the breakdown of an effective Hamiltonian in the perturbative analysis, we still observe slow logarithmic heating time-scales, unlike purely random drives.

cond-mat.stat-mech↗

arXiv2023

Effect of Counterion Size on Polyelectrolyte Conformations and Thermodynamics

We present a theoretical model to study the effect of counterion size on the effective charge, size, and thermodynamic behavior of a single, isolated, and flexible polyelectrolyte (PE) chain. We analyze how altering counterion size modifies the energy and entropy contributions to the system, including the ion-pair free energy, excluded volume interactions, entropy of free and condensed ions, and dipolar attraction among monomer-counterion pairs, which result in competing effects challenging intuitive predictions. The PE self energy is calculated using Edwards-Muthukumar Hamiltonian, considering a Gaussian monomer distribution for the PE. The condensed ions are assumed confined within a cylindrical volume around the PE backbone. The dipolar and excluded volume interactions are described by the second and third virial coefficients. Assumption of freely-rotating dipoles results in a first-order coil-globule transition of the PE chain. A more realistic weaker dipolar attraction, parameterized in our theory, shifts it to a second-order continuous transition. We calculate the size scaling-exponent of the PE and find exponents according to the relative dominance of the electrostatic, excluded volume, or dipolar effects. We further identify the entropy- and energy-driven regimes of the effective charge and conformation of the PE, highlighting the interplay of free ion entropy and ion-pair energy with varying electrostatic strengths. The crossover strength, dependent on the counterion size, indicates that diminishing sizes favor counterion condensation at the expense of free ion entropy. The predictions of the model are consistent with trends in simulations, and generalize findings of the point-like counterion theories.

cond-mat.soft↗

arXiv2023

Polymer Complexation: Partially Ionizable Asymmetric Polyelectrolytes

Studies of the thermodynamics of complex coacervation of pairs of symmetric, strongly ionizable, oppositely charged polyelectrolyte chains are abundant. To generalize such understanding to asymmetric chain lengths and variable ionizability (chemical charge density), frequently observed in experiments, we present a theoretical framework to analyze the effective charge and size of the complex and the thermodynamics of complexation of two polyions as a function of such asymmetries. The free energy ensuing from the Edwards' Hamiltonian undergoes variational extremization, and explicitly accounts for the screened Coulomb and non-electrostatic interactions among monomers within individual polyions and between two polyions. Assuming maximal ion-pair formation of the complexed part, the system free energy comprising configurational entropy of the polyions and free-ion entropy of the small ions is minimized. The thermodynamic drive for complexation is found to increase with the ionizability of the symmetric polyions and to be maximum for symmetric chain lengths for equally ionizable polyions. The effective charge and size of the complex increase with asymmetry in charge density, where the size can be substantially larger than a collapsed globule found for symmetric chains. The regimes of enthalpy- and entropy-driven complexation are found, respectively, for low and high Coulomb strengths. The crossover strength is found to be strongly dependent on the dielectric environment and salt, but marginally dependent on the charge density, thus implying an entropy-driven process at moderate strengths. The key results match the trends in simulations and experiments, and are expected to provide insight for asymmetric complexation in real systems.

cond-mat.soft↗