update?

content/Math401/Math401_P1_1.md

@@ -145,4 +145,8 @@ QED
 
 ## Drawing the connection between the space $S^{2n+1}$, $CP^n$, and $\mathbb{R}$
 
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+A pure quantum state of size $N$ can be identified with a Hopf circle on the sphere $S^{2N-1}$.
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+A random pure state $|\psi\rangle$ of a bipartite $N\times K$ system such that $K\geq N\geq 3$.
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+The partial trace of such system produces a mixed state $\rho(\psi)=\operatorname{Tr}_K(|\psi\rangle\langle \psi|)$, with induced measure $\mu_K$. When $K=N$, the induced measure $\mu_K$ is the Hilbert-Schmidt measure.