Interfacing Ultracold Atoms and a Cryogenic Micromechanical Oscillator

Why is it that our world, which consists of many microscopic quantum systems, can be described in terms of classical physics? This fundamental question about the transition between these two diametrically opposed theories is an unsolved problem of modern physics. In recent years, tremendous progress in the field of optomechanics paved the way for the observation of a large variety of quantum phenomena far beyond the microcosmos. Additional possibilities arise from the skillful combination of different quantum systems to create a so-called hybrid quantum system. In these systems, new possibilities to prepare and detect non-classical states in macroscopic systems arise. In this thesis the setup of a new hybrid quantum system is described that consists of two entirely different systems. The first building block is a 87Rb Bose-Einstein condensate - an ideal realization of a many-body ground state. The second building block is an optomechanical system in a cryogenic environment, a membrane in an optical cavity.