Spontaneous emission

Spontaneous emission is the transition of an excited quantum system to a lower energy state, releasing a photon without an external trigger.

Einstein coefficients: Coefficients describing the rates of absorption, stimulated emission, and spontaneous emission in transitions between energy levels. The coefficient A gives the rate of spontaneous emission for a transition.
Quantum state: A mathematical description encoding the physical properties and measurement probabilities of a quantum system. The emitting system begins in an excited state and ends in a lower-energy state.
Stimulated emission: Emission in which an incident photon induces an excited system to emit a second photon into a matching mode. Unlike spontaneous emission, this process requires an incident electromagnetic field.
Fluorescence: Light emission following absorption of radiation, typically after rapid relaxation to a lower excited state. Fluorescence commonly ends with spontaneous photon emission.
Cavity quantum electrodynamics: The study of interactions between atoms or other emitters and confined electromagnetic fields in cavities. It reveals how confinement changes spontaneous emission and can produce strong emitter–field coupling.
Quantum vacuum: The lowest-energy state of a quantum field, containing no real particles but possessing field fluctuations. Vacuum field modes enable emission even when no external light is present.
Energy level: A discrete allowed energy value of a quantum system, such as an atom or molecule. The energy difference between levels fixes the emitted photon's frequency.
Absorption (electromagnetic radiation): The transfer of electromagnetic energy to matter, promoting the system to a higher-energy state. Absorption reverses the energy flow associated with photon emission.
Atomic spectral line: A discrete feature in an atom's spectrum produced by a transition between electronic energy levels. Each emitted photon can contribute to a characteristic spectral line.
Photonic crystal: A periodic optical structure that controls electromagnetic modes through a photonic band structure. A photonic band gap can suppress emission at selected frequencies.

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