# Glauber–Sudarshan P representation

# Glauber–Sudarshan P representation

The **Glauber–Sudarshan P representation** is a suggested way of writing down the phase space distribution of a quantum system in the phase space formulation of quantum mechanics. The P representation is the quasiprobability distribution in which observables are expressed in normal order. In quantum optics, this representation, formally equivalent to several other representations,^{[1]}^{[2]} is sometimes championed over alternative representations to describe light in optical phase space, because typical optical observables, such as the particle number operator, are naturally expressed in normal order. It is named after George Sudarshan^{[3]} and Roy J. Glauber,^{[4]} who were working on the topic in 1963. It was the subject of a controversy when Glauber was awarded a share of the 2005 Nobel Prize in Physics for his work in this field and George Sudarshan's contribution was not recognized.^{[5]}
Despite many useful applications in laser theory and coherence theory, the **Glauber–Sudarshan P representation** has the drawback that it is not always positive, and therefore is not a true probability function.

Definition

`We wish to construct a functionwith the property that thedensity matrixisdiagonalin the basis ofcoherent states, i.e.,`

We also wish to construct the function such that the expectation value of a normally ordered operator satisfies the optical equivalence theorem. This implies that the density matrix should be in *anti*-normal order so that we can express the density matrix as a power series

Inserting the identity operator

we see that

and thus we formally assign

More useful integral formulas for *P* are necessary for any practical calculation. One method^{[6]} is to define the characteristic function

and then take the Fourier transform

Another useful integral formula for *P* is^{[7]}

`Note that both of these integral formulas do`

*not*converge in any usual sense for "typical" systems . We may also use the matrix elements ofin theFock basis. The following formula shows that it is*always*possible^{[3]}to write the density matrix in this diagonal form without appealing to operator orderings using the inversion (given here for a single mode),where r and θ are the amplitude and phase of α. Though this is a full formal solution of this possibility, it requires infinitely many derivatives of Dirac delta functions, far beyond the reach of any ordinary tempered distribution theory.

Discussion

`If the quantum system has a classical analog, e.g. a coherent state orthermal radiation, thenPis non-negative everywhere like an ordinary probability distribution. If, however, the quantum system has no classical analog, e.g. an incoherentFock stateorentangled system, thenPis negative somewhere or more singular than a Dirac delta function. (By atheorem of Schwartz, distributions that are more singular than the Dirac delta function are always negative somewhere.) Such "negative probability" or high degree of singularity is a feature inherent to the representation and does not diminish the meaningfulness of expectation values taken with respect toP. Even ifPdoes behave like an ordinary probability distribution, however, the matter is not quite so simple. According to Mandel and Wolf: "The different coherent states are not [mutually] orthogonal, so that even ifbehaved like a true probability density [function], it would not describe probabilities of mutually exclusive states."`

^{[8]}Examples

Thermal radiation

From statistical mechanics arguments in the Fock basis, the mean photon number of a mode with wavevector * k* and polarization state s for a black body at temperature T is known to be

The P representation of the black body is

In other words, every mode of the black body is normally distributed in the basis of coherent states. Since P is positive and bounded, this system is essentially classical. This is actually quite a remarkable result because for thermal equilibrium the density matrix is also diagonal in the Fock basis, but Fock states are non-classical.

Highly singular example

Even very simple-looking states may exhibit highly non-classical behavior. Consider a superposition of two coherent states

where *c0* , *c*1 are constants subject to the normalizing constraint

`Note that this is quite different from aqubitbecauseandare not orthogonal. As it is straightforward to calculate, we can use the Mehta formula above to compute`

*P*,Despite having infinitely many derivatives of delta functions, P still obeys the optical equivalence theorem. If the expectation value of the number operator, for example, is taken with respect to the state vector or as a phase space average with respect to P, the two expectation values match:

See also

Quasiprobability distribution#Characteristic functions

Nonclassical light

Wigner quasiprobability distribution

Husimi Q representation

Nobel Prize controversies

## References

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