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Equation 14 · Part 4 · The Entry a Relabeling Cannot Write

Symbol Lambda^dagger

Tr[Λ(ρ) Oc]=Tr[ρ Λ†(Oc)].\mathrm{Tr}\big[\Lambda(\rho)\,O_c\big] = \mathrm{Tr}\big[\rho\,\Lambda^\dagger(O_c)\big].
Λ†\Lambda^\dagger

What this part means

the define.

Its job in the formula

Lambdada^dagger is an input to the expression that computes the quantity on the left.

Where the article explains it

Define Λ†\Lambda^\dagger: B(Hc)\mathcal B(\mathcal H_c) →\to B(Hf)\mathcal B(\mathcal H_f) by the pairing that must hold for every fine state and every coarse observable OcO_c , Tr[Λ(ρ) Oc]=Tr[ρ Λ†(Oc)]\mathrm{Tr}\big[\Lambda(\rho)\,O_c\big] = \mathrm{Tr}\big[\rho\,\Lambda^\dagger(O_c)\big].

The passage around this formula

Every channel has a dual. Define Λ†\Lambda^\dagger: B(Hc)\mathcal B(\mathcal H_c) →\to B(Hf)\mathcal B(\mathcal H_f) by the pairing that must hold for every fine state and every coarse observable OcO_c , Tr[Λ(ρ) Oc]=Tr[ρ Λ†(Oc)]\mathrm{Tr}\big[\Lambda(\rho)\,O_c\big] = \mathrm{Tr}\big[\rho\,\Lambda^\dagger(O_c)\big]. Λ†\Lambda^\dagger is positive because Λ\Lambda is completely positive and every Λ(ρ)\Lambda(\rho) with ρ\rho ≥\ge 0 is itself a…

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Learn the underlying idea

An exponent tells how a base is used in multiplication. In x³, x is the base and 3 is the exponent: x³ = x × x × x.

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Sources cited in the surrounding passage

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