Quantum Computing Asked on April 14, 2021
Part three (going by N&C page 544) states that
$$tr(S(n)rho^{otimes n})=tr(S(n)rho^{otimes n}P(n,epsilon))+tr(S(n)rho^{otimes n}(I-P(n,epsilon))).$$
Now I understand how the term on the left of + goes to 0 as n $to infty$. However, I am confused how the term on the right does. N&I states that you can set
$$0 le tr(S(n)rho^{otimes n}(I-P(n,epsilon))) le tr(rho^{otimes n}(I-P(n,epsilon))) rightarrow 0,,text{ as } nto infty.$$
I don’t quite understand why this is the case. My only assumption is that the eigenvalues of $rho^{otimes n}(I-P(n,epsilon))$ are bounded in such a way that as $n to infty$ it will go to zero. However, I am unsure how to go about calculating this bound, though I assume it is of a similar form to the eigenvalues of $rho^{otimes n}P(n,epsilon)), 2^{-n(S(rho)-epsilon)}$
By part 1, we have that for any $delta > 0$, then for sufficiently large $n$, $tr( rho ^ {otimes n} P(n, epsilon)) geq 1 - delta$.
This means that $tr( rho ^ {otimes n} P(n, epsilon)) rightarrow 1$ as $n rightarrow infty$, since it is at most 1.
Correct answer by Simon Crane on April 14, 2021
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