Unit 6: Macroscopic Quantum Mechanics
The fundamentals of quantum mechanics that we met in Unit 5 characteristically appear on microscopic scales. Macroscopic quantum systems in which liquids, or electric currents, flow without friction or resistance have been known since the early part of the previous century: these are the superfluids and superconductors of traditional condensed matter physics that are discussed in Unit 8. In this unit we focus on an entirely new state of matter only recently created in the laboratory: this is the gaseous macroscopic quantum mechanical system known as a Bose-Einstein Condensate, or BEC. These quantum gases show the full panoply of quantum wave behavior as the individual particles of Unit 5, but now on a size scale visible to the naked eye because many millions, to many billions, of atoms occupy exactly the same quantum state, and thus form a coherent quantum whole. The quantum nature of a BEC can be directly visualized as the quantum effects are not hidden within liquids or solids as is the case with the more traditional superfluids and superconductors. Rather, they may be actually photographed, as the gas itself is a naked macroscopic quantum system. This unit starts by introducing the basic principles necessary to understand BECs, then details how the cooling and trapping introduced in Unit 5 led to the creation and subsequent manipulation of these quantum gases. Finally, we will see how atomic gases of ultra-cold fermions have evolved, in direct analogy to the Cooper paring needed to form bosonic pairs of electrons in superconductors, to molecular BECs, formed from pairs of the fermionic atoms.