The Fermi golden rule, a cornerstone of quantum perturbation theory, quantifies the transition rate between quantum states due to an external perturbation. This rule is crucial in understanding various phenomena, including scattering, absorption, emission, and tunneling. It is based on the concept of closeness score, which measures the overlap between initial and final states. The rule states that the transition rate is proportional to the square of the matrix element of the perturbation and the density of final states. This concept has widespread applications in solid-state physics and chemical physics, providing insights into phenomena such as electron transport and chemical reactions.
