Quantum Computing Asked on March 12, 2021
The max-relative entropy between two states is defined as
$$D_{max }(rho | sigma):=log min {lambda: rho leq lambda sigma},$$
where $rholeq sigma$ should be read as $sigma – rho$ is positive semidefinite. Consider the following quantity for a bipartite state $rho_{AB}$ with reduced states $rho_{A}$ and $rho_{B}$.
$$I_{max}(rho_{AB}) = D_{max}(rho_{AB}||rho_{A}otimesrho_{B})$$
I would like to know if this satisfies a continuity bound. That is, given $rho_{AB}approx_{epsilon}sigma_{AB}$ in some distance measure, can we bound $|I_{max}(rho_{AB}) – I_{max}(sigma_{AB})|$?
Motivation for question: Recall the quantum relative entropy $D(rho||sigma) = text{Tr}(rhologrho – rhologsigma)$ with the convention that $0log 0 = 0$. Let us define the mutual information as follows
$$I(rho_{AB}) = D(rho_{AB}||rho_{A}otimesrho_{B}) = -S(rho_{AB}) + S(rho_A) + S(rho_B),$$
where $S(rho) = -text{Tr}(rhologrho)$ is the von Neumann entropy. In this case, we may use Fannes inequality to find a bound on $|I(rho_{AB}) – I(sigma_{AB})|$ in terms of $|rho_{AB} – sigma_{AB}|_1$. I’m wondering if the move from $D(.||.)$ to $D_{max}(.||.)$ can be made while still having some Fannes type bound.
Unfortunately $D_{max}$ is not a continuous function and so functions built from it tend not to be continuous. For example consider consider the two states
$$ rho_{AB} = |00 rangle langle 00|, $$ and $$ tau_{AB}(epsilon) = (1-epsilon) |00 rangle langle 00 | + epsilon | 11rangle langle 11 |. $$ A quick calculation gives $I_{max}(rho_{AB}) = 1$ and $I_{max}(tau_{AB}(epsilon)) = - log(epsilon)$. So $I_{max}(rho_{AB}) neq lim_{epsilon rightarrow 0} I_{max}(tau_{AB}(epsilon)) = infty$.
However, some authors have studied a smoothed version of $I_{max}$ (see https://arxiv.org/pdf/1308.5884.pdf).
Correct answer by Rammus on March 12, 2021
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