An Etymological Dictionary of Astronomy and Astrophysics
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In quantum mechanics, a quantum number that distinguishes the different shapes of the orbitals.

Azimuthal, adj. from → azimuth; → quantum number.

A → quantum walk taking place entirely in the position space. Continuous-time quantum walk was introduced by E. Farhi & S. Gutmann (1998, Phys. Rev. A 58, 915).

→ continuous; → time; → quantum; → walk.

The square of the ratio of the output → signal-to-noise (S/N) to the input S/N.

Detective, adj. of → detect; → quantum; → efficiency.

A → quantum walk involving a probabilistic → operator that changes the direction while leaving the position fixed, and a shift operator that changes the position. Discrete-time quantum walk was introduced by J. Watrous (2001, Journal of Computer and System Sciences 62, 376)

→ discrete; → time; → quantum; → walk.

In atomic physics, a quantum number that denotes the energy levels available within a subshell. Designated by the letter m, it is one of a set of quantum numbers which describe the unique quantum state of an electron.

→ magnetic; → quantum; → number.

In atomic physics, the first of a set of quantum numbers which describe an atomic orbital. Symbolized as n, it characterizes the size and energy of an orbital.

→ principal; → quantum; → number.

The smallest amount of energy that can be absorbed or radiated by matter at a specified frequency (plural quanta). It is a → discrete quantity of energy hν associated with a wave of frequency ν, where h represents the → Planck's constant.

Quantum "a particular amount," from L. quantum "how much," neuter singular of quantus "how great." Introduced in physics by Max Planck (1858-1947) in 1900.

A concept whereby properties of objects vary according to the energy with which they are probed. An atomic system in its → ground state tends to remain as it is if little energy is fed in, betraying no evidence of its internal structure. Only when it is excited into a higher state do complexities emerge. This is the essence of quantum censorship. Thus, below an energy threshold, atoms appear to be impenetrable. Above it, their components can be exposed (F. Wilczek, 2013, Nature 498, 31).

→ quantum; censorship, from censor, from M.Fr. censor and directly from L. censor "a Romain magistrate who kept the register or census of the citizens, and supervised morals," from censere "to appraise, value, judge," from PIE root *kens- "to speak solemnly, announce;" cf. Av. səngh- (sanh-) "to declare, explain;" Pers. soxan "word, speech;" Skt. śams- "to praise, recite."

The → quantum field theory that deals with the → strong interaction and the structure of elementary particles in the framework of → quantum theory. The cohesive attraction between the → quarks, that constitute → hadrons, involves the participation of three particles. Each of these particles is assigned a different → color "charge." The existence of these "charges" requires a multiplicity of different messenger particles to communicate the interaction and glue the quarks together. These messengers are called → gluons and there are eight different types.

→ quantum; → chromodynamics

In quantum physics, a situation where an object's wave property is split in two, and the two waves coherently interfere with each other in such a way as to form a single state that is a superposition of the two states. This phenomenon is based on the fact that atomic particles have wave-like properties. Quantum coherence is in many ways similar to → quantum entanglement, which involves the shared states of two quantum particles instead of two quantum waves of a single particle. Quantum coherence and quantum entanglement are both rooted in the → superposition principle.

→ quantum; → coherence.

A type of computer, as yet hypothetical, that uses quantum mechanical laws, such as the → superposition principle and the → quantum entanglement, to perform calculations based on the behavior of particles at the → subatomic level. A quantum computer would gain enormous processing power through the ability to be in multiple states, and to perform tasks using all possible permutations simultaneously.

→ quantum; → computer.

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.