Numerical Shadow

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numerical-shadow:properties

Properties of numerical shadow

  1. By construction the distribution $P_A(z)$ is supported on the numerical range of $W(A)$ and it is normalized, $$\int_{W(A)} P_A(z) d^2 z =1.$$
  2. The (numerical) shadow is unitarily invariant, $P_A(z)=P_{UAU^*}(z)$. This is a consequence of the fact that the integration measure ${\rm d} \mu(\psi)$ is unitarily invariant.
  3. For any normal matrix $A$, such that $AA^*=A^*A$, its shadow covers the numerical range $W(A)$ with the probability corresponding to a projection of a regular $N$–simplex of classical states ${\cal C}_N$ (embedded in ${\mathbb R}^{N-1}$) onto a plane.
  4. For a non–normal matrix $A$, its shadow covers the numerical range $W(A)$ with the probability corresponding to an orthogonal projection of the complex projective manifold $\Omega_N={\mathbb C}P^{N-1}$ onto a plane.
  5. For any two matrices $A$ and $B$, the shadow of their tensor product does not depend on the order, $$ P_{A \otimes B}(z)=P_{B \otimes A} (z)\; .$$ To show this property define a unitary swap operator $S$ which acts on a composite Hilbert space and interchanges the order in the tensor product, $S(\ket{z} \otimes \ket{y})=\ket{y} \otimes \ket{z}$. Thus $ \bra{x} A \otimes B \ket{x} = \bra{x} S^{*} B \otimes A S \ket{x}$, and since $S$ is unitary it does not influence the numerical shadow induced by the unitarily invariant Fubini-Study measure on complex projective space.
numerical-shadow/properties.txt · Last modified: 2014/02/04 02:07 by lpawela