Minimizing Decoherence in Optical Fiber for Long Distance Quantum Communication

Abstract

In this research work, I have derived analytical models for decoherence of quantum states of light and developed techniques based on Dynamical Decoupling (DD) to preserve the quantum state of polarization qubit and Orbital Angular Momentum (OAM) qudit in single-mode, multi-mode and specialized optical fibers. In subsequent work, I have derived the analytical model for decoherence of an entangled state in an optical fiber to show that such a decoherence causes loss of entanglement. I also showed that such states can be preserved with DD. In Chapter 1, I have introduced the subject of a quantum computer and its relation to quantum communication. I give a brief overview of fundamental concepts and tools required to understand the subject matter. In Chapter 2, I discuss optical fibers from the perspective of the electromagnetic wave in a waveguide and explain the modes in an optical fiber. Later, I explain about the sources of noise or refractive index fluctuation in an optical fiber and show how to numerically reproduce a model of an optical fiber. In Chapter 3, I introduce the topic of quantum decoherence and discuss in detail an open-loop control technique called Dynamical Decoupling, that is applied to the system to minimize decoherence. I discuss ideal and nonideal pulse sequences used for suppressing decoherence. In Chapter 4, I derive the analytical model for decoherence of a polarization qubit in a single-mode, multi-mode fiber, and decoherence OAM qudit in specialized multi-mode fiber. In Chapter 5, I introduce the topic of entanglement for the bipartite system and multi-partite system, and a measure called concurrence to quantify the entanglement. I then derive the analytic model for decoherence of a pure entangled state and show that decoherence causes loss of entanglement for the case of a pure and mixed Werner-like state. \break In Chapter 6, I discuss the method of numerical simulation and show by numerical simulation that polarization qubit, OAM qudit and entanglement of polarization qudit can be preserved with dynamical decoupling in an optical fiber. In Chapter 7, I have summarized the results and discuss the scope of future work. In the appendix, I have included additional research work not directly related to the current topic of decoherence in an optical fiber. To summarize my research, I have derived a model for decoherence and then numerically showed that a polarization qubit can be preserved in a single mode and multi-mode optical fiber. I