Bipartite quantum system
= Bipartite quantum system
{title2=$\mathcal H_{AB}=\mathcal H_A\otimes\mathcal H_B$}
A <quantum system> with two specified subsystems has a <tensor product> <Hilbert space>. A joint <density operator> determines subsystem $A$'s measurement statistics through its <partial trace> $\rho_A=\operatorname{Tr}_B\rho_{AB}$. A joint <pure state> need not give a pure reduced state: the <Bell state> $(|00\rangle+|11\rangle)/\sqrt2$ gives $\rho_A=I/2$.