  In conformal field theories, in contrast to \emph{adding} some auxiliary
states into the bipartite mixed state $\rho_{AB}$ as the usual purifications
do, we show a pure entangled state $\psi_{AB}$ can be constructed by
\emph{subtracting} the undetectable regions. In this pure state $\psi_{AB}$,
the von Neumann entropy $S_{\text{vN}}(A)$ naturally captures quantum
entanglement between $A$ and $B$. We verify that $S_{\text{vN}}(A)$ is equal to
the entanglement wedge cross-section $E_{W}$ in AdS spacetime, which is
conjectured to be the holographic dual of the entanglement of purification. We
show such constructed entanglement entropy has a phase transition. The ordinary
entanglement entropies of critical and non-critical QFTs are simply limits of
the two phases.
