Subinay Dasgupta

A small world network of prime numbers

According to Goldbach conjecture, any even number can be broken up as the sum of two prime numbers: n=p+q. We construct a network where each node is a prime number and corresponding to every even number n, we put a link between the component primes p and q. In most cases, an even number can be broken up in many ways, and then we chose one decomposition with a probability |p-q|α. Through computation of average shortest distance and clustering coefficient, we conclude that for α>-1.8 the network is of small world type and for α<-1.8 it is of regular type. We also present a theoretical justification for such behaviour.

Quantum time of arrival distribution in a simple lattice model

Imagine an experiment where a quantum particle inside a box is released at some time in some initial state. A detector is placed at a fixed location inside the box and its clicking signifies arrival of the particle at the detector. What is the time of arrival (TOA) of the particle at the detector ? Within the paradigm of the measurement postulate of quantum mechanics, one can use the idea of projective measurements to define the TOA. We consider a setup where a detector keeps making instantaneous measurements at regular finite time intervals until it detects the particle at time t, which is defined as the TOA. This is a stochastic variable and, for a simple lattice model of a free particle in a one-dimensional box, we find interesting features such as power-law tails in its distribution and in the probability of survival (non-detection). We propose a perturbative calculational approach which yields results that compare very well with exact numerics.

Modular organization enhances the robustness of attractor network dynamics

Modular organization characterizes many complex networks occurring in nature, including the brain. In this paper we show that modular structure may be responsible for increasing the robustness of certain dynamical states of such systems. In a network of threshold-activated binary elements, we observe that the basins of attractors, corresponding to patterns that have been embedded using a learning rule, occupy maximum volume in phase space at an optimal modularity. Simultaneously, the convergence time to these attractors decreases as a result of cooperative dynamics between the modules. The role of modularity in increasing global stability of certain desirable attractors of a system may provide a clue to its evolution and ubiquity in natural systems.

Persistence and dynamics in the ANNNI chain

We investigate both the local and global persistence behaviour in the ANNNI (axial next-nearest-neighbour Ising) model. We find that when the ratio κ of the second neighbour interaction to the first neighbour interaction is less than 1, P(t), the probability of a spin to remain in its original state up to time t shows a stretched exponential decay. For κ > 1, P(t) has an algebraic decay but the exponent is different from that of the nearest-neighbour Ising model. The global persistence behaviour shows similar features. We also conduct some deeper investigations in the dynamics of the ANNNI model and conclude that it has a different dynamical behaviour compared to the nearest-neighbour Ising model.

Monte Carlo simulation of the shape space model of immunology

Abstract The shape space model of de Boer, Segel and Perelson for the immune system is studied with a probabilistic updating rule by Monte Carlo simulation. A suitable mathematical form is chosen for the probability of increase of B-cell concentration depending on the concentration around the mirror image site. The results obtained agree reasonably with the results obtained by deterministic cellular automata.

Detection of a quantum particle on a lattice under repeated projective measurements

We consider a quantum particle, moving on a lattice with a tight-binding Hamiltonian, which is subjected to measurements to detect its arrival at a particular chosen set of sites. The projective measurements are made at regular time intervals tau, and we consider the evolution of the wave function until the time a detection occurs. We study the probabilities of its first detection at some time and, conversely, the probability of it not being detected (i.e., surviving) up to that time. We propose a general perturbative approach for understanding the dynamics which maps the evolution operator, which consists of unitary transformations followed by projections, to one described by a non-Hermitian Hamiltonian. For some examples of a particle moving on one-and two-dimensional lattices with one or more detection sites, we use this approach to find exact expressions for the survival probability and find excellent agreement with direct numerical results. A mean-field model with hopping between all pairs of sites and detection at one site is solved exactly. For the one-and two-dimensional systems, the survival probability is shown to have a power-law decay with time, where the power depends on the initial position of the particle. Finally, we show an interesting and nontrivial connection between the dynamics of the particle in our model and the evolution of a particle under a non-Hermitian Hamiltonian with a large absorbing potential at some sites.

Extreme variability in convergence to structural balance in frustrated dynamical systems

In many complex systems, heterogeneous connections can subject constituent elements to conflicting influences, resulting in frustration. Here we show numerically that an initially frustrated system can achieve structural balance by a link adaptation process inspired by Hebbs principle, with interaction strengths evolving in accordance with the dynamical states of its components. In the presence of fluctuations the time required to converge to the balanced state exhibits large dispersion characterized by a bimodal distribution, pointing to an intriguing problem in the study of evolving energy landscapes.

Transverse Ising chain under periodic instantaneous quenches: Dynamical many-body freezing and emergence of slow solitary oscillations

We study the real-time dynamics of a quantum Ising chain driven periodically by instantaneous quenches of the transverse field (the transverse field varying as rectangular wave symmetric about zero). Two interesting phenomena are reported and analyzed: (1) We observe dynamical many-body freezing or DMF (Phys. Rev. B, vol. 82, 172402, 2010), i.e. strongly non-monotonic freezing of the response (transverse magnetization) with respect to the driving parameters (pulse width and height) resulting from equivocal freezing behavior of all the many-body modes. The freezing occurs due to coherent suppression of dynamics of the many-body modes. For certain combination of the pulse height and period, maximal freezing (freezing peaks) are observed. For those parameter values, a massive collapse of the entire Floquet spectrum occurs. (2) Secondly, we observe emergence of a distinct solitary oscillation with a single frequency, which can be much lower than the driving frequency. This slow oscillation, involving many high-energy modes, dominates the response remarkably in the limit of long observation time. We identify this slow oscillation as the unique survivor of destructive quantum interference between the many-body modes. The oscillation is found to decay algebraically with time to a constant value. All the key features are demonstrated analytically with numerical evaluations for specific results.

Chimera order in spin systems

It has recently been shown that a population of oscillators having identical environments can exhibit a heterogeneous phase topology termed as chimera state. We extend this phenomenon to the broader perspective of order-disorder transitions in physical systems with discrete states. By an exact analytic treatment we show that chimera states can occur in a system of Ising spins in thermal equilibrium. We also numerically establish the existence of chimera ordering in 3-dimensional models of layered magnetic materials (such as manganites) suggesting possible means of experimentally observing it.

Boundary effects in the three-dimensional Ising model

Abstract Simulations of the standard three-dimensional ising model with a variety of boundary conditions, for medium and large systems, are compared with field-theoretical predictions. In particular, setting boundary spins equal to zero is not the best approximation for the Dirichtet boundary condition of zero magnetization at the boundary. The simulations for the specific heat agree semiquantitatively with the field theory for periodic and Dirichtet boundary conditions.