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In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect ...
The coherence length is defined as the distance the wave travels in time . [11]: 560, 571–573 The coherence time is not the time duration of the signal; the coherence length differs from the coherence area (see below).
The superconducting coherence length is a measure of the size of a Cooper pair (distance between the two electrons) and is of the order of cm. The electron near or at the Fermi surface moving through the lattice of a metal produces behind itself an attractive potential of range of the order of 3 × 10 − 6 {\displaystyle 3\times 10^{-6}} cm ...
This is in contrast to single-component superconductors, where there is only one coherence length and the superconductor is necessarily either type 1 (>) or type 2 (<) (often a coherence length is defined with extra / factor, with such a definition the corresponding inequalities are > and <).
The Fried parameter has units of length and is typically expressed in centimeters. It is defined as the diameter of a circular area over which the rms wavefront aberration due to passage through the atmosphere is equal to 1 radian , and typical values relevant to astronomy are in the tens of centimeters depending on atmospheric conditions.
Ginzburg–Landau theory introduced the superconducting coherence length ξ in addition to London magnetic field penetration depth λ. According to Ginzburg–Landau theory, in a type-II superconductor / > /. Ginzburg and Landau showed that this leads to negative energy of the interface between superconducting and normal phases.
The coherence time, usually designated τ, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by = where λ is the central wavelength of the source, Δν and Δλ is the spectral width of the source in units of frequency and wavelength respectively, and c is the speed of light in vacuum.
Higher order coherence or n-th order coherence (for any positive integer n>1) extends the concept of coherence to quantum optics and coincidence experiments. [1] It is used to differentiate between optics experiments that require a quantum mechanical description from those for which classical fields are sufficient.