The main objective of this chapter is to show that the 1-bit unitary gates (2.46) and the 2-bit QPG (2.47) can be implemented using the methods of, respectively, 2-level atoms and cavity QED. For clarity, we divide the discussion into four sections: background and reference material from atomic physics (Section 3.1), the quantized light field (Section 3.2), photons in cavities (Section 3.3), and the interaction between an atom and zero or one photon in a cavity (Section 3.4). Quantum erasers, deferred from Section 1.3 of Chap. 1, will be studied in Sections 3.5 and 3.6.