Spin Decoherence from Hamiltonian dynamics in Quantum Dots

The dynamics of a spin-1/2 particle coupled to a nuclear spin bath through an isotropic Heisenberg interaction is studied, as a model for the spin decoherence in quantum dots. The time-dependent polarization of the central spin is calculated as a function of the bath-spin distribution and the polarizations of the initial bath state. For short times, the polarization of the central spin shows a gaussian decay, and at later times it revives displaying nonmonotonic time dependence. The decoherence time scale dep ends on moments of the bath-spin distribuition, and also on the polarization strengths in various bath-spin channels. The bath polarizations have a tendency to increase the decoherence time scale. The effective dynamics of the central spin polarization is shown to be describ ed by a master equation with non-markovian features.Comment: 11 pages, 6 figures Accepted for publication in Phys.Rev

A Game Theoretic Approach to Modelling Jamming Attacks in Delay Tolerant Networks

Cyberspace plays a prominent role in our social, economic and civic welfare and cyber security issues are of paramount importance today. Growing reliance of the intertwined military and civilian applications on wireless computer networks makes these networks highly vulnerable to attacks of which jamming attacks are a vital and exigent problem. In this paper, we study defence against jamming attacks as game in a delay tolerant network, with two adversarial players: the jammer playing against the transmitter. The transmitters seek to choose an optimal time to schedule his transmission securely, so as to maximize the probability of successful delivery before his session expires, while these transmissions are subject to inference from the jammer, who attempts to minimize this probability . We design strategies for the transmitters that offset transmission period based inference of network traffic by the jammer. We model these interactions and decisions as a game and use simulation as a tool to evaluate the games. Probability distribution functions over finite set of strategies are proposed to compute the expected payoff of both the players. Simulation results are used to evaluate the expected payoff along with the resulting equilibrium in cases where players are biased and unbiased. These results are used to strategically decide on the optimal time for both the players, and evaluate the efficiency of the strategies used by the transmitters against jammer attacks.

A Game Theoretic Software Test-bed for Cyber Security Analysis of Critical Infrastructure

National critical infrastructures are vital to the functioning of modern societies and economies. The dependence on these infrastructures is so succinct that their incapacitation or destruction has a debilitating and cascading effect on national security. Critical infrastructure sectors ranging from financial services to power and transportation to communications and health care, all depend on massive information communication technology networks. Cyberspace is composed of numerous interconnected computers, servers and databases that hold critical data and allow critical infrastructures to function. Securing critical data in a cyberspace that holds against growing and evolving cyber threats is an important focus area for most countries across the world. A novel approach is proposed to assess the vulnerabilities of own networks against adversarial attackers, where the adversary’s perception of strengths and vulnerabilities are modelled using game theoretic techniques. The proposed game theoretic framework models the uncertainties of information with the players (attackers and defenders) in terms of their information sets and their behaviour is modelled and assessed using a probability and belief function framework. The attack-defence scenarios are exercised on a virtual cyber warfare test-bed to assess and evaluate vulnerability of cyber systems. Optimal strategies for attack and defence are computed for the players which are validated using simulation experiments on the cyber war-games testbed, the results of which are used for security analyses