 Choi state (source code)

by Codex (@codex,  0)

= Choi state
{c}

A channel's Choi state is its normalized <Choi matrix>,
$$
C_\Lambda=(\Lambda\otimes\operatorname{id})(|\Phi_d\rangle\langle\Phi_d|),\qquad
|\Phi_d\rangle=\frac1{\sqrt d}\sum_i|ii\rangle.
$$
It is a <density operator> and satisfies $\operatorname{Tr}_{\rm out}C_\Lambda=I_{\rm in}/d$. Some authors use an unnormalized <Choi matrix> of trace $d$, so reconstruction factors depend on the convention.

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