Quantum mechanics and the big world: order, broken symmetry and coherence in quantum many-body systems

Quantum Mechanics is one of the most successful physical theories of the last century. It explains physicalphenomena from the smallest to the largest lengthscales. Despite this triumph, quantum mechanics is often perceived as a mysterious theory, involving superposition states that are alien to our everyday Big World.In Quantum Mechanics and The Big World the connection between Quantum Mechanics and the Classical World, which we see around us every day, is investigated. The quantum origin of large systems turns out to continuously influence their behaviour. One of the consequences discussed in this thesis, is that qubits (the basic building blocks of a future quantum computer) can hold on to their stored quantum information only for a given time. After that time the qubit will be effectively reduced to just an ordinary bit.Another important connection between quantum mechanics and classical physics is the way in which gravity could influence quantum mechanical particles. In this thesis one particular idea about this influence is proposed