Cavity quantum electrodynamics (CQED), which uses isolated atoms inside resonant electromagnetic cavities, offers another promising physical platform for implementing and manipulating qubits. The cavity itself acts as a spectral filter, only allowing certain frequencies of electromagnetic waves to be present; other frequencies destructively interfere with themselves and rapidly decay away. This allows only single pairs of atomic energy levels, separated by the allowed optical frequency, to participate in interactions. At the same time, photons bouncing back and forth in the cavities have many opportunities to interact with the atoms, effectively amplifying otherwise weak effects. The presence of the mirrors bounding the cavity can also be thought of as altering the vacuum states with which the atom interacts, which will in turn affect properties like atomic energy levels and decay rates of excited states, thus offering additional control over the atom and its interactions. Over the next few sections, we’ll look first at the electric field inside a resonant cavity, then we’ll add an atom into the cavity. In the last section, we look at implementing quantum information processes using the atom-cavity system.