Quantum mechanics is a very successful theory which precisely describes the behavior of microscopic particles. Although the theory provides counter-intuitive predictions, such as that an object can be at two different positions at the same time and the "spooky action at a distance" between two objects, it describes the microscopic world with unprecedental precision. These counter-intuitive quantum behaviors of microscopic particles explainphenomena across disparate scales, from the microscopic scale such as interactions between sub-atomic particles, the atomic spectrum, motion of electrons in solids, and chemical reactions, to macroscopic scale such as behavior of superfluids and superconductors, and even the evolution of stars. So far, no example of conflicts between experiment and theory is known, and the validity of the theory is so broad that one would expect that the quantumtheory can be applied to the macroscopic world, for example, a macroscopic massive object such as a soccer ball can in principle be at two positions at the same time.