 Choi state

ID: choi-state

A channel's Choi state is its normalized Choi matrix,

C_{Λ} = (Λ \otimes id) (∣ Φ_{d} ⟩ ⟨ Φ_{d} ∣), ∣ Φ_{d} ⟩ = \frac{1}{d} \sum_{i} ∣ ii ⟩ .

It is a density operator and satisfies

Tr_{out} C_{Λ} = I_{in} / d

. Some authors use an unnormalized Choi matrix of trace

d

, so reconstruction factors depend on the convention.

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