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Coherent numerical range


Coherent numerical range $W_{\mathrm{CS}}(A)$ of a matrix $A$ is a subset of the complex plane defined as $$ W_\mathrm{CS}(A)=\{z:z=\bra{\vartheta, \varphi}A\ket{\vartheta, \varphi},\braket{\vartheta, \varphi}{\vartheta, \varphi}=1\}, $$ where $\ket{\vartheta, \varphi}$ is an SU(2) coherent state.

Definition of $SU(2)$ coherent states

$SU(2)$ coherent states are related to the $SU(2)$ algebra of the components of the angular momentum operator $J = \{J_x , J_y , J_z \}$. Let us choose a reference state $\ket{\kappa}$, usually taken as the maximal eigenstate $\ket{j,j}$ of the component $J_z$ acting on $\mathcal{H}_d$ , $d = 2j + 1$, $j = 1/2, 1/3, \ldots$. This state, pointing toward the ‘north pole’ of the sphere, enjoys the minimal uncertainty equal to $j$ . Then, the vector coherent state is defined by the Wigner rotation matrix $R_{\vartheta, \varphi}$ $$ \ket{\vartheta,\varphi} = R_{\vartheta,\varphi} \ket{\kappa} = (1 + |\gamma|^2)^{-j} e^{\gamma J_-} \ket{j, j} $$ where $R_{\vartheta,\varphi} = \exp[( \ii \vartheta( \cos\varphi J_x - \sin \varphi J_y)]$ , $J_- = J_x - \ii J_y$ and $\gamma = \tan(\vartheta/2)e^{\ii\varphi}$, for $(\vartheta,\varphi) \in S^2$ (we use the spherical coordinates).

numerical-range/generalizations/restricted-numerical-range/coherent-numerical-range.txt · Last modified: 2018/10/08 08:09 by plewandowska