Spontaneous emission is the process by which a quantum system such as an
atom,
molecule,
nanocrystal or
nucleus in an
excited state undergoes a transition to a state with a lower energy (e.g., the
ground state) and emits quanta of energy. Light or luminescence from an atom is a fundamental process that plays an essential role in many phenomena in nature and forms the basis of many applications, such as fluorescent tubes, older television screens (
cathode ray tubes), plasma display panels,
lasers, and
light emitting diodes. Lasers start by spontaneous emission, and then normal continuous operation works by
stimulated emission.