Spontaneous emission
Spontaneous emission is the process in which a quantum mechanical system (such as a molecule, an atom or a subatomic particle) transits from an excited energy state to a lower energy state (e.g., its ground state) and emits a quantized amount of energy in the form of a photon. Spontaneous emission is ultimately responsible for most of the light we see all around us; it is so ubiquitous that there are many names given to what is essentially the same process. If atoms (or molecules) are excited by some means other than heating, the spontaneous emission is called  luminescence. For example, fireflies are luminescent. And there are different forms of luminescence depending on how excited atoms are produced (electroluminescence, chemiluminescence etc.). If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission. Spontaneous emission cannot be explained by classical electromagnetic theory and is fundamentally a quantum process. The first  person to derive the rate of spontaneous emission accurately from first principles was Dirac in his quantum theory of radiation, the precursor to the theory which he later called quantum electrodynamics. Contemporary physicists, when asked to give a physical explanation for spontaneous emission, generally invoke the zero-point energy of the electromagnetic field. In 1963, the Jaynes–Cummings model was developed describing the system of a two-level atom interacting with a quantized field mode (i.e. the vacuum) within an optical cavity. It gave the nonintuitive prediction that the rate of spontaneous emission could be controlled depending on the boundary  conditions of the surrounding vacuum field. These experiments gave rise to cavity quantum electrodynamics (CQED), the study of effects of mirrors and cavities on radiative corrections. Introduction. If a light source ('the atom') is in an excited state with energy formula_1, it may spontaneously decay to a lower lying level (e.g., the ground state) with energy formula_2, releasing the difference in energy between the two states as a photon. The photon will have angular frequency formula_3 and an energy formula_4: where formula_6 is the reduced Planck constant. Note: formula_7, where formula_8 is the Planck constant and formula_9 is the linear  frequency. The phase of the photon in spontaneous emission is random as is the direction in which the photon propagates. This is not true for stimulated emission. An energy level diagram illustrating the process of spontaneous emission is shown below: If the number of light sources in the excited state at time formula_10 is given by formula_11, the rate at which formula_12 decays is: where formula_14 is the rate of spontaneous emission. In the rate-equation formula_14 is a proportionality constant for this particular transition in this particular light source. The constant is referred to as the "Einstein A coefficient , and has units s. The above equation can be solved to give: where formula_17 is the initial number of light sources in the excited state, formula_10 is the time and formula_19 is the radiative decay rate of the transition. The number of excited states formula_12 thus decays exponentially with time, similar to radioactive decay. After one lifetime, the number of excited states decays to 36.8% of its original value (formula_21-time). The radiative decay rate formula_22 is inversely proportional to the lifetime formula_23: Theory. Spontaneous transitions were not explainable within the framework of the Schrödinger equation, in which the electronic 	Spontaneous emission
23079059	energy levels were quantized, but the electromagnetic field was not. Given that the eigenstates of an atom are properly diagonalized, the overlap of the wavefunctions between the excited state and the ground state of the atom is zero. Thus, in the absence of a quantized electromagnetic field, the excited state atom cannot decay to the ground state. In order to explain spontaneous transitions, quantum mechanics must be extended to a quantum field theory, wherein the electromagnetic field is quantized at every point in space. The quantum field theory of electrons and electromagnetic fields is known as quantum electrodynamics. In 	Spontaneous emission
23079060	quantum electrodynamics (or QED), the electromagnetic field has a ground state, the QED vacuum, which can mix with the excited stationary states of the atom. As a result of this interaction, the stationary state" of the atom is no longer a true eigenstate of the combined system of the atom plus electromagnetic field. In particular, the electron transition from the excited state to the electronic ground state mixes with the transition of the electromagnetic field from the ground state to an excited state, a field state with one photon in it. Spontaneous emission in free space depends upon vacuum fluctuations  to get started. Although there is only one electronic transition from the excited state to ground state, there are many ways in which the electromagnetic field may go from the ground state to a one-photon state. That is, the electromagnetic field has infinitely more degrees of freedom, corresponding to the different directions in which the photon can be emitted. Equivalently, one might say that the phase space offered by the electromagnetic field is infinitely larger than that offered by the atom. This infinite degree of freedom for the emission of the photon results in the apparent irreversible decay, i.e. , spontaneous emission. In the presence of electromagnetic vacuum modes, the combined atom-vacuum system is explained by the superposition of the wavefunctions of the excited state atom with no photon and the ground state atom with a single emitted photon: where formula_26 and formula_27 are the atomic excited state-electromagnetic vacuum wavefunction and its probability amplitude, formula_28 and formula_29 are the ground state atom with a single photon (of mode formula_30) wavefunction and its probability amplitude, formula_31 is the atomic transition frequency, and formula_32 is the frequency of the photon. The sum is over formula_33 and formula_34, which are the  wavenumber and polarization of the emitted photon, respectively. As mentioned above, the emitted photon has a chance to be emitted with different wavenumbers and polarizations, and the resulting wavefunction is a superposition of these possibilities. To calculate the probability of the atom at the ground state (formula_35), one needs to solve the time evolution of the wavefunction with an appropriate Hamiltonian. To solve for the transition amplitude, one needs to average over (integrate over) all the vacuum modes, since one must consider the probabilities that the emitted photon occupies various parts of phase space equally. The "spontaneously" emitted photon has  infinite different modes to propagate into, thus the probability of the atom re-absorbing the photon and returning to the original state is negligible, making the atomic decay practically irreversible. Such irreversible time evolution of the atom-vacuum system is responsible for the apparent spontaneous decay of an excited atom. If one were to keep track of all the vacuum modes, the combined atom-vacuum system would undergo unitary time evolution, making the decay process reversible. Cavity quantum electrodynamics is one such system where the vacuum modes are modified resulting in the reversible decay process, see also Quantum revival. The  theory of the spontaneous emission under the QED framework was first calculated by Weisskopf and Wigner. Rate of spontaneous emission. The rate of spontaneous emission (i.e., the radiative rate) can be described by Fermi's golden rule. The rate of emission depends on two factors: an 'atomic part', which describes the internal structure of the light source and a 'field part', which describes the density of electromagnetic modes of the environment. The atomic part describes the strength of a transition between two states in terms of transition moments. In a homogeneous medium, such as free space, the rate of spontaneous  emission in the dipole approximation is given by: where formula_3 is the emission frequency, formula_39 is the index of refraction, formula_40 is the transition dipole moment, formula_41 is the vacuum permittivity, formula_6 is the reduced Planck constant, formula_43 is the vacuum speed of light, and formula_44 is the fine-structure constant. The expression formula_45 stands for the definition of the transition dipole moment formula_46 for dipole moment operator formula_47, where formula_48 is the elementary charge and formula_49 stands for position operator. (This approximation breaks down in the case of inner shell electrons in high-Z atoms.) The above equation clearly  shows that the rate of spontaneous emission in free space increases proportionally to formula_50. In contrast with atoms, which have a discrete emission spectrum, quantum dots can be tuned continuously by changing their size. This property has been used to check the formula_50-frequency dependence of the spontaneous emission rate as described by Fermi's golden rule. Radiative and nonradiative decay: the quantum efficiency. In the rate-equation above, it is assumed that decay of the number of excited states formula_12 only occurs under emission of light. In this case one speaks of full radiative decay and this means that  the quantum efficiency is 100%. Besides radiative decay, which occurs under the emission of light, there is a second decay mechanism; nonradiative decay. To determine the total decay rate formula_53, radiative and nonradiative rates should be summed: where formula_53 is the total decay rate, formula_22 is the radiative decay rate and formula_57 the nonradiative decay rate. The quantum efficiency (QE) is defined as the fraction of emission processes in which emission of light is involved: In nonradiative relaxation, the energy is released as phonons, more commonly known as heat. Nonradiative relaxation occurs when the energy difference between the levels is  very small, and these typically occur on a much faster time scale than radiative transitions. For many materials (for instance, semiconductors), electrons move quickly from a high energy level to a meta-stable level via small nonradiative transitions and then make the final move down to the bottom level via an optical or radiative transition. This final transition is the transition over the bandgap in semiconductors. Large nonradiative transitions do not occur frequently because the crystal structure generally cannot support large vibrations without destroying bonds (which generally doesn't happen for relaxation). Meta-stable states form a very important  feature that is exploited in the construction of lasers. Specifically, since electrons decay slowly from them, they can be deliberately piled up in this state without too much loss and then stimulated emission can be used to boost an optical signal.