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| The '''optical equivalence theorem''' in [[quantum optics]] asserts an equivalence between the [[expectation value]] of an operator in [[Hilbert space]] and the expectation value of its associated function in the [[phase space formulation]] with respect to a [[quasiprobability distribution]]. The theorem was first reported by [[George Sudarshan]] in 1963 for [[normal order|normally ordered]] operators<ref>E. C. G. Sudarshan "Equivalence of Semiclassical and Quantum Mechanical Descriptions of Statistical Light Beams", ''Phys. Rev. Lett.'','''10''' (1963) pp. 277–279. {{doi|10.1103/PhysRevLett.10.277}}</ref> and generalized later that decade to any ordering.<ref>K. E. Cahill and R. J. Glauber "Ordered Expansions in Boson Amplitude Operators", ''Phys. Rev.'','''177''' (1969) pp. 1857–1881. {{doi|10.1103/PhysRev.177.1857}}</ref><ref>K. E. Cahill and R. J. Glauber "Density Operators and Quasiprobability Distributions", ''Phys. Rev.'','''177''' (1969) pp. 1882–1902. {{doi|10.1103/PhysRev.177.1882}}</ref><ref>G. S. Agarwal and E. Wolf "Calculus for Functions of Noncommuting Operators and General Phase-Space Methods in Quantum Mechanics. I. Mapping Theorems and Ordering of Functions of Noncommuting Operators", ''Phys. Rev. D'','''2''' (1970) pp. 2161–2186. {{doi|10.1103/PhysRevD.2.2161}}</ref><ref>G. S. Agarwal and E. Wolf "Calculus for Functions of Noncommuting Operators and General Phase-Space Methods in Quantum Mechanics. II. Quantum Mechanics in Phase Space", ''Phys. Rev. D'','''2''' (1970) pp. 2187–2205. {{doi|10.1103/PhysRevD.2.2187}}</ref> Let Ω be an ordering of the non-commutative [[creation and annihilation operators]], and let <math>g_{\Omega}(\hat{a},\hat{a}^{\dagger})</math> be an operator that is expressible as a power series in the creation and annihilation operators that satisfies the ordering Ω. Then the optical equivalence theorem is succinctly expressed as
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| <math>\langle g_{\Omega}(\hat{a},\hat{a}^{\dagger}) \rangle = \langle g_{\Omega}(\alpha,\alpha^*) \rangle.</math>
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| Here, α is understood to be the [[eigenvalue]] of the annihilation operator on a [[coherent states|coherent state]] and is replaced formally in the power series expansion of ''g''. The left side of the above equation is an expectation value in the Hilbert space whereas the right hand side is an expectation value with respect to the quasiprobability distribution. We may write each of these explicitly for better clarity. Let <math>\hat{\rho}</math> be the [[density operator]] and <math>{\bar{\Omega}}</math> be the ordering reciprocal to Ω. The quasiprobability distribution associated with Ω is given, at least formally, by
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| :<math>f_{\bar{\Omega}}(\alpha,\alpha^*) = \frac{1}{\pi} \int \rho_{\bar{\Omega}}(\alpha,\alpha^*) |\alpha\rangle\langle\alpha| \, d^2\alpha.</math> | |
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| The above framed equation becomes
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| :<math>\mathrm{tr}( \hat{\rho} \cdot g_{\Omega}(\hat{a},\hat{a}^{\dagger})) = \int f_{\bar{\Omega}}(\alpha,\alpha^*) g_{\Omega}(\alpha,\alpha^*) \, d^2\alpha.</math>
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| For example, let Ω be the [[normal order]]. This means that ''g'' can be written in a power series of the following form:
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| :<math>g_{N}(\hat{a}^\dagger, \hat{a}) = \sum_{n,m} c_{nm} \hat{a}^{\dagger n} \hat{a}^m. </math>
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| The quasiprobability distribution associated with the normal order is the [[Glauber-Sudarshan P representation]]. In these terms, we arrive at
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| :<math>\mathrm{tr}( \hat{\rho} \cdot g_N(\hat{a},\hat{a}^{\dagger})) = \int P(\alpha) g(\alpha,\alpha^*) \, d^2\alpha.</math>
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| This theorem implies the formal equivalence between expectation values of normally ordered operators in quantum optics and the corresponding complex numbers in classical optics.
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| ==References==
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| <references/>
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| {{DEFAULTSORT:Optical Equivalence Theorem}}
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| [[Category:Quantum optics]]
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| [[Category:Physics theorems]]
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