Given two distinct and noninteracting quantum mechanical
systems mathfrakS1 and mathfrakS2 with state spaces
mathcalH1 and mathcalH2, respectively, the state space
of the combined system mathcalS1+mathcalS2 is the
tensor product Hilbert space
mathcalH=mathcalH1otimesmathcalH2. Density operators
WinmathcalD(mathcalH), and effects
FinmathcalE(mathcalH). Similarly, there are corresponding
symbols WiinmathcalD(mathcalHi),FiinmathcalE(mathcalHi) for subsystems
mathfrakSi(i=1,2), respectively.
Given any quantum operation, Phi:mathcalD(mathcalH)rightarrowmathcalD(mathcalH), of
the composite system mathcalS1+mathcalS2.
Problem: (1) Do there exist whether or not two quantum
operation phi1 and phi2, of the subsystems mathfrakS1
and mathfrakS2, respectively, such that the following
diagram is commutative:
begindiagramnodemathcalD(mathcalH1)arrow[4]e,tphi1node[4]mathcalD(mathcalH1) node nodemathcalD(mathcalH1otimesmathcalH2)arrow[2]n,lTr2arrow[4]e,tPhiarrow[2]s,lTr1node[4]mathcalD(mathcalH1otimesmathcalH2)arrow[2]s,rTr1arrow[2]n,rTr2 node nodemathcalD(mathcalH2)arrow[4]e,bphi2node[4]mathcalD(mathcalH2)enddiagram
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 phii:mathcalD(mathcalHi)rightarrowmathcalD(mathcalHi)(i=1,2) and Tri:mathcalD(mathcalH)rightarrowmathcalD(mathcalHi) is a partial trace with respect to the subsystem mathfrakSi(i=1,2).
(2) If quantum operation phi1 and phi2 exist, give the
relationship among the quantum operations Phi,phi1 and
phi2.
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