Complex oxide heterostructures provide a versatile playground for quantum phenomena with extraordinary properties, making them highly promising for next-generation electronic devices and quantum information processing. The origin of these phenomena is the competition between different degrees of freedom, such as charge, orbital, and spin, which are related to the crystal structure, the oxygen stoichiometry, and the doping dependence. Combining different oxide layers through heterostructure design opens access to interface physics and leads to engineered interface properties, where the degrees of freedom can be artificially modified. The unique combination of atomic-layer precise growth of oxide heterostructures and atomic-resolution identification of novel interface phenomena with high-end STEM techniques paves the way for creating controlled properties at the interfaces and understanding localized quantum effects.