- Wigner quasi-probability distribution
:"See also

Wigner distribution , a disambiguation page."The**Wigner quasi-probability distribution**(also called the**Wigner function**or the**Wigner-Ville distribution**) is a special type ofquasi-probability distribution . It was introduced byEugene Wigner in 1932 to studyquantum corrections to classical statistical mechanics. The goal was to supplant thewavefunction that appears inSchrödinger's equation with a probability distribution inphase space .It is a

generating function for all spatialautocorrelation functions of a given quantum-mechanical wavefunction ψ(x). Thus, it maps on the quantumdensity matrix in the map between real phase-space functions and Hermitian operators introduced byHermann Weyl in1927 , in a context related to representation theory in mathematics (cf.Weyl quantization in physics). In effect, it is the Weyl-Wigner transform of the density matrix, so the realization of that operator in phase space. It was later rederived by J. Ville in1948 as a quadratic (in signal) representation of the local time-frequency energy of a signal.In

1949 ,José Enrique Moyal , who had also rederived it independently, recognized it as the quantum moment-generating functional, and thus as the basis of an elegant encoding of all quantum expectation values, and hence quantum mechanics, in phase space (cfWeyl quantization ). It has applications instatistical mechanics ,quantum chemistry ,quantum optics , classicaloptics and signal analysis in diverse fields such aselectrical engineering ,seismology ,biology , speech processing, and engine design.**Relation to classical mechanics**A classical particle has a definite position and momentum, and hence it is represented by a point in phase space. Given a collection (

ensemble ) of particles, the probability of finding a particle at a certain position in phase space is specified by a probability distribution, the Liouville density. This strict interpretation fails for a quantum particle, due to theuncertainty principle . Instead, the above quasi-probability Wigner distribution plays an analogous role, but does not satisfy all the properties of a conventional probability distribution; and, conversely, satisfies boundedness properties unavailable to classical distributions.For instance, the Wigner distribution can and normally does go negative for states which have no classical model---and is a convenient indicator of quantum mechanical interference.Smoothing the Wigner distribution through a filter of size larger than $hbar$ (e.g., convolving with aphase-space Gaussian to yield the Hussimi representation, below), results in a positive-semidefinitefunction, i.e., it may be thought to have been coarsened to asemi-classical one.

Regions of such negative value are provable (by convolving them with a small Gaussian) to be "small": they cannot extend to compact regions larger than a few $hbar$,and hence disappear in the classical limit. They are shieldedby the

uncertainty principle , which does not allow precise location within phase-space regions smaller than $hbar$,and thus renders such "negative probabilities" less paradoxical.**Mathematical definition**The Wigner distribution "P"("x", "p") is defined as:

:$P(x,p)=frac\{1\}\{pihbar\}int\_\{-infty\}^\{infty\}dy,\; psi^*(x+y)psi(x-y)e^\{2ipy/hbar\}$

where ψ is the wavefunction and x and p are position and momentum but could be any conjugate variable pair. (ie. real and imaginary parts of the electric field or frequency and time of a signal). It is symmetric in x and p:

:$P(x,p)=frac\{1\}\{pihbar\}int\_\{-infty\}^\{infty\}dq,\; phi^*(p+q)phi(p-q)e^\{-2ixq/hbar\}$

where Φ is the

Fourier transform of ψ.In 3D,:$P(vec\{r\},vec\{p\})\; =\; frac\{1\}\{(2\; pi)^3\}\; int\{\; psi^*(vec\{r\}\; +\; hbarvec\{s\}/2)\; psi\; (vec\{r\}\; -\; hbarvec\{s\}/2)\; e^\{ivec\{p\}\; cdot\; vec\{s\; d^3\; s\}$.

In the general case, which includes mixed states, it is the Wigner transform of the

density matrix ::$P(x,p)=frac\{1\}\{pihbar\}int\_\{-infty\}^\{infty\}dy,\; langle\; x-y|\; hat\{\; ho\}\; |x+y\; angle\; e^\{2ipy/hbar\}.$

This Wigner transformation (or map) is the inverse of the Weyl transform, which maps phase-space functions to Hilbert-space operators, in

Weyl quantization . Thus, the Wigner function is the cornerstone of quantum mechanics inphase space . In 1949,José Enrique Moyal elucidated how the Wigner function provides the integration measure in phase space to yield expectation values from phase-spacec-number functions "g(x,p)" (uniquely associatedto suitably ordered operators $hat\{G\}$ through Weyl's transform (cf.Weyl quantization and property 7 below), in a manner evocative of classical probability theory.Specifically, an operator's $hat\{G\}$ expectation value is a "phase-space average" of the Wigner transform of that operator,

: $langle\; hat\{G\}\; angle\; =int!\; dx\; dp~\; P(x,p)~\; g(x,p)~$.

**Mathematical properties**1. "P"("x", "p") is real

2. The "x" and "p" probability distributions are given by the marginals:

*$int\_\{-infty\}^\{infty\}dp,P(x,p)=langle\; x|hat\{\; ho\}|x\; angle$. If the system can be described by apure state , one gets $int\_\{-infty\}^\{infty\}dp,P(x,p)=\; |psi(x)|^2$

*$int\_\{-infty\}^\{infty\}dx,P(x,p)=langle\; p|hat\{\; ho\}|p\; angle$. If the system can be described by apure state , one gets $int\_\{-infty\}^\{infty\}dx,P(x,p)=|phi(p)|^2$

*$int\_\{-infty\}^\{infty\}dxint\_\{-infty\}^\{infty\}dp,P(x,p)=Tr(hat\{\; ho\})$

*Typically the trace of ρ is equal to 1.

*1. and 2. imply the P(x,p) is negative somewhere, with the exception of thecoherent state (and mixtures of coherent states) and the squeezed vacuum state.3. "P"("x", "p") has the following reflection symmetries:

*Time symmetry: $psi(x)\; ightarrow\; psi(x)^*\; Rightarrow\; P(x,p)\; ightarrow\; P(x,-p)$

*Space symmetry: $psi(x)\; ightarrow\; psi(-x)\; Rightarrow\; P(x,p)\; ightarrow\; P(-x,-p)$4. "P"("x", "p") is Galilei-invariant:

*$psi(x)\; ightarrow\; psi(x+y)\; Rightarrow\; P(x,p)\; ightarrow\; P(x+y,p)$

*It is notLorentz invariant .5. The equation of motion for each point in the phase space is classical in the absence of forces:

*$frac\{partial\; P(x,p)\}\{partial\; t\}=frac\{-p\}\{m\}frac\{partial\; P(x,p)\}\{partial\; x\}$In fact, it is classical even in the presence of harmonic forces.6. State overlap is calculated as:

*$|langle\; psi|\; heta\; angle|^2=2pihbarint\_\{-infty\}^\{infty\}dx,int\_\{-infty\}^\{infty\}dp,P\_\{psi\}(x,p)P\_\{\; heta\}(x,p)$

7. Operator expectation values (averages) are calculated as phase-space averages of the respective Wigner transforms:

*$g(x,p)equivint\_\{-infty\}^\{infty\}dy,\; langle\; x-y/2|\; hat\{G\}\; |x+y/2\; angle\; e^\{ipy/hbar\}~~$,

*$langle\; psi|hat\{G\}|psi\; angle=Tr(hat\{\; ho\}hat\{G\})=int\_\{-infty\}^\{infty\}dx,\; int\_\{-infty\}^\{infty\}dp\; P(x,p)g(x,p)~.$8. In order that "P"("x", "p") represent physical (positive) density matrices:

*$int\_\{-infty\}^\{infty\}dx,\; int\_\{-infty\}^\{infty\}dp,\; P(x,p)P\_\{\; heta\}(x,p)ge\; 0~~$,where |θ> is a pure state.9. By virtue of the Schwarz inequality, for a pure state, it is constrained to be bounded,

*$-frac\; 2\; h\; leq\; P(x,p)\; leq\; frac\; 2\; h\; ~$. This bound disappears in the classical limit, $hbar\; ightarrow\; 0$ . In this limit, "P"("x", "p") reduces to the probability density in coordinate space "x", usually highly localized,multiplied by δ-functions in momentum: the classical limitis "spiky". Thus, this quantum-mechanical bound precludes a Wigner function which is a perfectlylocalized delta function in phase space, as a reflection of the uncertainty principle.

**Uses of the Wigner function outside quantum mechanics*** In the modelling of optical systems such as telescopes or fibre telecommunications devices, the Wigner function is used to bridge the gap between simple ray tracing and the full wave analysis of the system. Here $p\; /\; hbar$ is replaced with "k"=|"k"|sinθ≈|"k"|θ in the small angle (paraxial) approximation. In this context, the Wigner function is the closest one can get to describing the system in terms of rays at position "x" and angle θ while still including the effects of interference. If it becomes negative at any point then simple ray-tracing will not suffice to model the system.

* In signal analysis, a time-varying electrical signal, mechanical vibration, or sound wave are represented by a Wigner function. Here, "x" is replaced with the time and $p\; /\; hbar$ is replaced with the angular frequency ω=2π"f", where f is the regular frequency.* In ultrafast optics, short laser pulses are characterized with the Wigner function using the same f and t substitutions as above. Pulse defects such as chirp (the change in frequency with time) can be visualized with the Wigner function. See Figure 2.

* In quantum optics, "x" and $p\; /\; hbar$ are replaced with the "X" and "P" quadratures, the real and imaginary components of the electric field (see

coherent state ). The plots in Figure 1 are of quantum states of light.**Measurements of the Wigner function***

Tomography

*Homodyne detection

*FROG Frequency-resolved optical gating**The Wigner-Weyl transformation**The

**Weyl-Wigner Transformation**(also called**Wigner transformation**) is a general transformation of an operator $hat\; G$ on a Hilbert-space to a function $g(x,p)$ onphase space and is given by$g(x,p)=int\_\{-infty\}^\{infty\}dslangle\; x-s/2|hat\; G|x+s/2\; angle\; e^\{ips/hbar\}$

The inverse of this transformation is called the

**Weyl transformation**, not to be confused with another definition of theWeyl transformation . The**Wigner function**is the Weyl-Wigner Transform of thedensity matrix .**Other related quasi-probability distributions**"See

Quasi-probability distribution for more detail."The Wigner distribution was the first quasi-probability distribution, but many more followed, formally equivalent and transformable to and from it. As in the case of coordinate systems, on account of varying properties, several such have with various advantages for specific applications:

*Glauber P representation

*Husimi Q representation **Historical note**As indicated, the formula for the Wigner function was independently derived several times in different contexts. In fact, apparently, Wigner was unaware that even within the context of quantum theory, it had been introduced previously by

Heisenberg andDirac , albeit purely formally: these two missed its significance, and that of its negative values, as they merely considered it as an approximation to the full quantum description of a system such as the atom. Incidentally, Dirac would later become Wigner's brother-in-law. Symmetrically, in most of his legendary 18-month correspondence with Moyal in the mid 1940s, Dirac was unaware that Moyal's quantum-moment generating function was effectively the Wigner function, and it was Moyal who finally brought it to his attention.**ee also***

Heisenberg group

*Weyl quantization

*Moyal bracket

*Modified Wigner distribution function

*Negative probability **References***E.P. Wigner, "On the quantum correction for thermodynamic equilibrium", "Phys. Rev."

**40**(June 1932) 749-759.

*H. Weyl, "Z. Phys."**46**, 1 (1927).

*H. Weyl, "Gruppentheorie und Quantenmechanik" (Leipzig: Hirzel)(1928).

*H. Weyl, "The Theory of Groups and Quantum Mechanics" (Dover, New York, 1931).

*H.J. Groenewold, "On the Principles of elementary quantum mechanics","Physica",**12**(1946) 405-460.

*J. Ville, "Théorie et Applications de la Notion de Signal Analytique", "Cables et Transmission",**2A**, 61-74 (1948).

* J.E. Moyal, "Quantum mechanics as a statistical theory", "Proceedings of the Cambridge Philosophical Society",**45**, 99-124 (1949).

*W. Heisenberg, "Über die inkohärente Streuung von Röntgenstrahlen", "Physik. Zeitschr."**32**, 737-740 (1931).

*P.A.M. Dirac, "Note on exchange phenomena in the Thomas atom", "Proc. Camb. Phil. Soc."**26**, 376-395 (1930).

* C. Zachos, D. Fairlie, and T. Curtright, "Quantum Mechanics in Phase Space" ( World Scientific, Singapore, 2005).

* http://qis.ucalgary.ca/quantech/wiggalery.html

* http://gerdbreitenbach.de/gallery/

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