An Etymological Dictionary of Astronomy and Astrophysics
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Of an atomic energy level, the difference between the principal quantum number and the effective quantum number.

→ quantum; → defect.

In a detector, the ratio of the number of photoelectrons released to the number of incident photons at a specific wavelength.

→ quantum; → efficiency.

The → quantum field theory that describes the properties of → electromagnetic radiation and its interaction with electrically charged matter in the framework of → quantum theory.

→ quantum; → electrodynamics.

A quantum → phenomenon that occurs when two or more particles (→ photons or → atomic particles) that have a common origin remain linked together when they travel apart. A measurement of one of the particles determines not only its → quantum state but the quantum state of the other particle as well. A change in one is instantly reflected in the other. To use a familiar example, it is as if you have a pair of dice entangled in such a way that when you throw them the sum of the two is 7. Any time you cast them, if the first die shows 2, 5, 3, etc. the other will show 5, 2, 4, etc., respectively. Quantum entanglement is rooted in the → superposition principle. But, in contrast to → quantum coherence, the states in a superposition are the shared states of two entangled particles rather than those of the two split waves of a single particle. There are several ways for entangling atomic particles. Photons can be entangled using → cascade transitions, as was done by Alain Aspect and colleagues in the early 1980s (→ Aspect experiment). Calcium atoms are put into a highly-excited energy level where the electron is forbidden to return to the → ground state by emitting a single photon. As a result, the atoms → decay by emitting two photons which are entangled. Like quantum coherence, quantum entanglement plays an essential role in quantum technologies, such as quantum teleportation, quantum cryptography, and super dense coding. See also → EPR paradox.

→ quantum; → entanglement.

The quantum mechanical theory based on the assumption that the interactions between particles and fields are mediated by messenger particles. Accordingly, particles are → quanta of a field, just s photons are quanta of light. All fields display a granular structure in interaction. QFT is the framework in which quantum mechanics and → special relativity are successfully reconciled (→ Dirac equation). It forms the basis of today's particle physics.

→ quantum; → field; → theory.

The temporary variation in a → quantum field due to the → uncertainty principle.

→ quantum; → fluctuation.

A theory of gravity, yet to be developed, that would properly include quantum mechanics. Because of the tensor nature of general relativity, it is not renormalizable as a field theory in perturbation from flat space. So far various attempts to quantize general relativity have been unsuccessful.

→ quantum; → gravity.

The science concerned with the transmission, storage, and processing of information using quantum mechanical systems. It exploits the notion of → quantum entanglement between systems and joins several fields of knowledge, mainly quantum physics, information, computation, and probability.

→ quantum; → information.

The transition of a quantum system from one stationary state to another, accompanied by absorption or emission of energy.

→ quantum; → jump.

A → physical system that is specified by a → wave function.

→ quantum; → mechanical; → system.

A development of Newtonian mechanics based on the discrete character of energy (Planck 1900) and the wave motion of material particles (de Broglie 1924). It is relies on the consideration that energy state of a quantum mechanical system can be derived at a given instant by a function whose square of the modulus gives the probability distribution of the coordinates of the system. Quantum mechanics is essential for the treatment of all atomic processes. It holds also for ordinary large scale processes although in this case the deviations from Newtonian mechanics are negligible.

→ quantum; → mechanics.

A random variation of signal due to fluctuations in the average rate of incidence of quanta on a detector. Quantum noise is described by the → Poisson distribution. Same as → photon noise and → shot effect.

→ quantum; → noise.

A number used in quantum mechanics, specifying the state of an electron bound in an atomic system. The quantum numbers are integers or half integers and specify the number of units of energy, momentum, spin, etc. possessed by an electron.

→ quantum; → number.

Since → Planck's constant has the dimension of → energy × → time, its sometimes called the quantum of → action.

→ quantum; → action.

A phase transitions that occurs at zero temperature as a function of a non-thermal parameter like → pressure, → magnetic field, or → chemical composition. In contrast to ordinary → phase transitions, which are associated with passage through a critical temperature, quantum phase transitions are associated with → quantum fluctuations, a consequence of → Heisenberg's uncertainty principle. For example, see → Bose-Einstein condensation.

→ quantum; → phase; → transition.

In → quantum mechanics, the state of a system as described by a set of → quantum numbers and represented by an → eigenfunction.

→ quantum; → state.

The theoretical basis of modern physics which describes the behavior and interactions of elementary particles or energy states based on the assumptions that energy is subdivided into discrete amounts and that matter possesses wave properties. → quantum mechanics; → quantum field theory.

→ quantum; → theory.

A property in → quantum mechanics whereby in a quantum system the sum of all probabilities of all possible outcomes must be 1. Quantum unitarity makes the modulous of a → quantum state invariant with time.

→ quantum. → unitary.

A generalization of the classical concept of → random walk using quantum mechanical laws such as the → superposition principle and → interference of quantum amplitudes. In the classical version the particle moves in the position space with a certain probability. In contrast, in the quantum counterpart the particle moves by exploring multiple possible paths simultaneously with the amplitudes corresponding to the influence of different paths. The concept of quantum walk is studied in two standard forms: → continuous-time quantum walk and → discrete-time quantum walk. Quantum walk was first introduced by Aharonov et al. (1993, Phys. Rev. A, 48, 1687).

→ continuous; → walk.

In photochemistry, the number of defined events which occur per photon absorbed by the system.

→ quantum; → theory.