Spontaneous Emission Context

An excited atom can make a downward transition and emit a photon without an applied radiation field. This is spontaneous emission. Its transition rate is described phenomenologically by an Einstein A coefficient.

An incident electromagnetic field at the transition frequency can induce stimulated emission. The emitted photon matches the stimulating mode in frequency, phase, polarisation, and direction; the transition rate is proportional to the spectral energy density through an Einstein B coefficient. This coherence is central to laser operation.

In quantum electrodynamics, spontaneous emission arises from the interaction between the atom and quantised electromagnetic-field modes, including vacuum fluctuations. It should not be described as an ordinary classical field secretly applied to the atom. The full rate also depends on the available electromagnetic modes and can be modified by the atom’s environment, such as a cavity.

This is archive context rather than part of the maintained blackbody-to-Compton sequence. Return to the Quantum Theory of Light Hub.