Generation and applications of multi-partite multi-level quantum entanglement

In this thesis we present a new multi-level multi-partite quantum protocol which utilizes residual entanglement in order to generate a secure key for cryptographic communication. Furthermore, the multi-level multi-partite quantum state we use for the cryptographic key generation can also be used for a secret sharing protocol. We propose a possible realization of this quantum protocol using the orbital angular momentum degree of freedom of photons.;We also investigate the steady-state entanglement between atoms by describing the system in the dressed state picture which enables us to determine where the entanglement lies in the excitation manifold. This new approach we take to investigate entanglement also shows how the weak field assumption fails regardless of how weak the applied field is when considering the entanglement between the atoms. We demonstrate that two closed two-level atoms inside a cavity can be entangled in the steady state with the use of a non-linear mirror, and two open two-level atoms can be entangled in a cavity through optically pumping population out of the dressed state manifolds.;Finally, we explore a new type of dark state which is a dark state of a collection of atoms. These dark states are entangled, and we offer the simplest example of such a state using a J1/2-- J1/2 system. We demonstrate how one can simplify the problem by exploiting symmetries of the system, and show the general procedure to find these states.