  Circuit quantum electrodynamics ('circuit QED') describes the quantum
mechanics and quantum field theory of superconducting electrical circuits
operating in the microwave regime near absolute zero temperature. It is the
analog of cavity QED in quantum optics with the role of the atoms being played
by superconducting qubits. The present lecture notes focus primarily on novel
quantum states that can be produced and measured using the strong coupling
between an artificial atom and one or more cavities. Of particular importance
are Schrodinger cat states of photons. Despite long being considered exemplars
of frail quantum superpositions that quickly decohere, such states have
recently been used as the basis for quantum error correction codes which have
reached the long-sought goal of enhancing the lifetime of quantum information
through active quantum error correction.
