Protecting Coherence And Its Application

In this dissertation,we discuss two tasks:(i)One is to control the loss of coher-ence(decoherence)via dipole-dipole interactions and detuning,especially we examine their impacts on the non-Markovianity(memory)and to see how they cause a transi-tion from the Markovian to non-Markovian dynamics and vice versa.(ii)Second,as an application of intrinsic quantum coherence of the quantum walk,we theoretically simulate and experimentally realize the genuine picture of quantum Parrondo effect in one-dimensional quantum walk and also discuss the effect of partial and full decoher-ence on the Parrondo effect.As we know that preserving non-Markovianity and quantum entanglement from decoherence effect,are of theoretical and practical significance in the quantum infor-mation processing technologies.In this context,we study a system S that is initially correlated with an ancilla A,which interacts with the environment E via an amplitude damping channel.We also consider dipole-dipole interactions(DDIs)between the sys-tem and ancilla,which are responsible for strong correlations.We investigate the impact of DDIs and detuning on the non-Markovianity and information exchange in different environments.We show that DDIs are not only better than detuning at protecting the information(without destroying the memory effect)but also induce memory by caus-ing a transition from Markovian to non-Markovian dynamics.In contrast,although detuning also protects the information,it causes a transition from non-Markovian to the Markovian dynamics.In addition,we demonstrate that the non-Markovianity grows with increasing DDI strength and diminishes with increasing detuning.We also show that the effects of negative detuning and DDIs can cancel out each other,causing a certain loss of coherence and information.Next,Parrondo's paradox,where the combination of losing strategies can result in a winning strategy,is a well-known counterintuitive phenomenon.Here,based on our compact large-scale quantum walk platform,we report the first experiment using two different coin operators representing two losing games A and B,respectively,to produce the Parrondo effect.We show that games A and B are losing games when played individually,but can produce a winning expectation when alternated in an ABB sequence.In addition,we also investigate the games in the quantum domain as well as their relations to quantum coherence in a delayed-choice quantum walk scheme that cannot be realized with classical light In the experimental scenario,we have realized the game just for the period of three due to the limitation of the optical elements and for the sake of small statistical errors,but this period is not fixed,we have found different regions of the coins operator where Parrondo effect exists for the particular sequence of different periods.Along with the applications of different kinds of quantum walks,our results potentially motivate the development of new quantum algorithms.