Given two distinct and noninteracting quantum mechanical
systems $mathfrak{S}_1$ and $mathfrak{S}_2$ with state spaces
$mathcal H_1$ and $mathcal H_2$, respectively, the state space
of the combined system $mathcal S_1+mathcal S_2$ is the
tensor product Hilbert space
$mathcal H=mathcal H_1otimesmathcal H_2$. Density operators
$Winmathcal D(mathcal H)$, and effects
$Finmathcal E(mathcal H)$. Similarly, there are corresponding
symbols $W_iinmathcal D(mathcal H_i),
F_iinmathcal E(mathcal H_i)$ for subsystems
$mathfrak{S}_i(i=1,2)$, respectively.
Given any quantum operation, $Phi:
mathcal D(mathcal H)rightarrow mathcal D(mathcal H)$, of
the composite system $mathcal S_1+mathcal S_2$.
Problem: (1) Do there exist whether or not two quantum
operation $phi_1$ and $phi_2$, of the subsystems $mathfrak{S}_1$
and $mathfrak{S}_2$, respectively, such that the following
diagram is commutative:
$$
begin{diagram}
node{mathcal D(mathcal H_1)} arrow[4]{e,t}{phi_1}node[4]{mathcal D(mathcal H_1)}\
node{}\
node{mathcal D(mathcal H_1otimesmathcal H_2)}
arrow[2]{n,l}{Tr_2} arrow[4]{e,t}{Phi} arrow[2]{s,l}{Tr_1}
node[4]{mathcal D(mathcal H_1otimesmathcal H_2)} arrow[2]{s,r}{Tr_1} arrow[2]{n,r}{Tr_2}
\
node{}\
node{mathcal D(mathcal H_2)} arrow[4]{e,b}{phi_2}
node[4]{mathcal D(mathcal H_2)}
end{diagram}
$$
i.e.
$$begin{eqnarray}
Tr_2(Phi(W))&=&frac{tr(Phi(W))}{tr(phi_1(Tr_2(W)))}phi_1(Tr_2(W)),\
Tr_1(Phi(W))&=&frac{tr(Phi(W))}{tr(phi_2(Tr_1(W)))}phi_2(Tr_1(W)),
end{eqnarray}
$$
where $phi_i: mathcal D(mathcal H_i)rightarrow
mathcal D(mathcal H_i)(i=1,2)$ and $Tr_i:
mathcal D(mathcal H)rightarrow
mathcal D(mathcal H_i)$ is a partial trace with respect to the subsystem $mathfrak{S}_i(i=1,2)$.
(2) If quantum operation $phi_1$ and $phi_2$ exist, give the
relationship among the quantum operations $Phi, phi_{1}$ and
$phi_2$.
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