- Wigner quasi-probability distribution
Wigner distribution, a disambiguation page."The Wigner quasi-probability distribution (also called the Wigner function or the Wigner-Ville distribution) is a special type of quasi-probability distribution. It was introduced by Eugene Wignerin 1932 to study quantumcorrections to classical statistical mechanics. The goal was to supplant the wavefunctionthat appears in Schrödinger's equationwith a probability distribution in phase space.
It is a
generating functionfor all spatial autocorrelationfunctions of a given quantum-mechanical wavefunction ψ(x). Thus, it maps on the quantum density matrixin the map between real phase-space functions and Hermitian operators introduced by Hermann Weylin 1927, in a context related to representation theory in mathematics (cf. Weyl quantizationin 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 in 1948as a quadratic (in signal) representation of the local time-frequency energy of a signal.
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 (cf Weyl quantization). It has applications in statistical mechanics, quantum chemistry, quantum optics, classical opticsand signal analysis in diverse fields such as electrical 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 the uncertainty 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 (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 ,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 ,and thus renders such "negative probabilities" less paradoxical.
The Wigner distribution "P"("x", "p") is defined as:
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:
where Φ is the
Fourier transformof ψ.
In the general case, which includes mixed states, it is the Wigner transform of the
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 in phase space. In 1949, José Enrique Moyalelucidated how the Wigner function provides the integration measure in phase space to yield expectation values from phase-space c-numberfunctions "g(x,p)" (uniquely associatedto suitably ordered operators through Weyl's transform (cf. Weyl quantizationand property 7 below), in a manner evocative of classical probability theory.
Specifically, an operator's expectation value is a "phase-space average" of the Wigner transform of that operator,
1. "P"("x", "p") is real
2. The "x" and "p" probability distributions are given by the marginals:
*. If the system can be described by a
pure state, one gets
*. If the system can be described by a
pure state, one gets
*Typically the trace of ρ is equal to 1.
*1. and 2. imply the P(x,p) is negative somewhere, with the exception of the
coherent state(and mixtures of coherent states) and the squeezed vacuum state.
3. "P"("x", "p") has the following reflection symmetries:
4. "P"("x", "p") is Galilei-invariant:
*It is not
5. The equation of motion for each point in the phase space is classical in the absence of forces:
*In fact, it is classical even in the presence of harmonic forces.
6. State overlap is calculated as:
7. Operator expectation values (averages) are calculated as phase-space averages of the respective Wigner transforms:
8. In order that "P"("x", "p") represent physical (positive) density matrices:
*,where |θ> is a pure state.
9. By virtue of the Schwarz inequality, for a pure state, it is constrained to be bounded,
*. This bound disappears in the classical limit, . 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 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 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 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
*FROG Frequency-resolved optical gating
The Wigner-Weyl transformation
The Weyl-Wigner Transformation (also called Wigner transformation) is a general transformation of an operator on a Hilbert-space to a function on
phase spaceand is given by
The inverse of this transformation is called the Weyl transformation, not to be confused with another definition of the
Weyl transformation. The Wigner function is the Weyl-Wigner Transform of the density matrix.
Other related quasi-probability distributions
Quasi-probability distributionfor 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
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
Heisenbergand Dirac, 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.
Modified Wigner distribution function
*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).
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