The field of quantum computation and communication has prospered over the last few decades because of multiple advances in our understanding of trapping and controlling physical quantum systems like neutral atoms, superconducting qubits, trapped ions, quantum dots etc. using light. A lot of research has gone into development of techniques which facilitate trapping and manipulation of ensembles of neutral atoms by studying how the atomic properties of the system respond to properties of light used to control them. In this dissertation we shall explore some applications of light-matter interactions in ensembles of lambda three level neutral atoms for the purpose of entanglement generation and distribution.The first part of this dissertation focuses on the study of a protocol that can be used to generate multi-particle entangled quantum states called the Greenberger-Horne-Zeilinger (GHZ) states in an ensemble of N neutral atoms. Schemes for creation of N particle entangled Greenberger-Horne-Zeilinger (GHZ) states are important for understanding multi-particle non-classical correlations. A theoretical protocol for creation of a multi-particle GHZ state implemented on a target ensemble of N, three-level Rydberg atoms and a single Rydberg atom as a control using Stimulated Raman Adiabatic Passage (STIRAP) is presented. We work in the Rydberg blockade regime for the ensemble atoms induced due to excitation of the control atom to a high lying Rydberg level. It is shown that using STIRAP, atoms from one ground state of the ensemble can be adiabatically transferred with high fidelity to the other multi-particle ground state, depending on the state of the control atom. Measurement of the control atom in a specific basis after this conditional transfer facilitates one-step creation of a N particle GHZ state. A thorough analysis of adiabatic conditions associated with STIRAP for this scheme and the influence of radiative decay from the excited Rydberg levels is presented. The most important and novel feature of this scheme is that it is immune to the decay rate of the excited level in ensemble atoms and provides a robust way of creating GHZ states.In the second part of this dissertation, we study atomic ensemble based quantum interfaces used in quantum repeater protocols for entanglement distribution. Quantum interfaces provide a platform where in the flying photonic qubits used for information and entanglement transfer can interact with a physical system which stores, processes and releases this information back as photons. The Duan-Lukin-Cirac-Zoller (DLCZ) quantum repeater protocol, which was proposed to realize long distance quantum communication, requires usage of quantum memories or quantum interfaces. Atomic ensembles interacting with optical beams based on off-resonant Raman scattering serve as convenient on-demand quantum memories. Here a complete, free space, three-dimensional theory of the associated read and write process for this quantum memory is worked out with the aim of understanding intrinsic retrieval efficiency. We develop a formalism to calculate the transverse mode structure for the signal and the idler photons and use the formalism to study the intrinsic retrieval efficiency under various configurations. The effects of atomic density fluctuations and atomic motion are incorporated by numerically simulating this system for a range of realistic experimental parameters. Results describe the variation in the intrinsic retrieval efficiency as a function of the memory storage time for skewed beam configuration at a finite temperature, which provides valuable information for optimization of the retrieval efficiency in experiments.
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Studies of Light-Matter Interactions in Atomic Ensembles for Creation of Entangled States and Quantum Interfaces
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