# Tag Info

Accepted

### Schmidt decomposition for tripartite system $ABC$ with vanishing mutual information between $A$ and $C$

TL;DR: The key observation is that Schmidt basis on a subsystem consists of eigenvectors of the reduced state of that subsystem. Consequently, if the reduced state is a product state then its Schmidt ...
Accepted

### What exactly is the relation between the Holevo quantity and the mutual information?

Right, they are quite similar. The Holevo bound is a bound on the amount of accessible information between your quantum system and your classical system. The I(X;B) object written in the HSW theorem ...
The mutual information can be written in terms of the relative entropy, please see Nielsen and Chuang (the entropy Venn diagram figure 11.2). I am writing the equation in the question's notation: $$I(... 4 votes Accepted ### Does quantum mutual information encompass information only about quantum correlations, or does it encompass both classical and quantum correlations? Definition and Intuition Quantum mutual information is defined as:$$ S(A:B) = S(A) + S(B) - S(A,B) $$Here,  S(A)  is the Von Neumann entropy of the quantum state described by density matrix  A ,... 3 votes Accepted ### How to prove the positivity of the conditional quantum mutual information, I(A;B|C)\ge0? Here's a relatively simple proof just based on the data processing inequality (DPI) for the relative entropy D(\rho\|\sigma) = \mathrm{tr}[\rho (\log \rho - \log \sigma)] -- if you're willing to ... 3 votes Accepted ### Question regarding the monotonicity of Mutual Information of a tripartite state under multiple non-local commuting unitaries Define the relative entropy between two states \rho and \sigma as$$ D(\rho\|\sigma) := \mathrm{tr}[\rho(\log \rho - \log \sigma)]\,. $$Then we can write a the mutual information between X and ... 2 votes Accepted ### Can separable states have quantum mutual information larger than one? If \rho_{AB} is separable then$$ I(A:B) \leq \min\{H(A), H(B)\}. $$To see this first note that$$ I(A:B) = H(A) + H(B) - H(AB) = H(A) - H(A|B). $$Now consider the conditional entropy term H(A|B) ... 2 votes Accepted ### Comparison of Quantum Mutual Information and Coherent Information with Classical Mutual Information You are asking about the quantum analogue of a classical quantity, but quantum information contains features that have no classical analogue and so you won't find an objective answer. Here are some ... 1 vote Accepted ### Is quantum mutual information an entanglement measure? Coherent information is the measure of quantum correlations. Positivity of coherent information indicates that quantum correlations are present. But this is not an iff relation. Coherent information ... 1 vote ### How to evaluate the mutual information of a classical-quantum state? Let us express the state \rho^{XB} in terms of eigen decomposition of \rho_i: \rho^{XB}{=}\sum_{i,\lambda^{(i)}}p_i\lambda^{(i)}|i\rangle\langle i|{\otimes}|\lambda^{(i)}\rangle\langle\lambda^{(i)... 1 vote ### How to prove the positivity of the conditional quantum mutual information, I(A;B|C)\ge0? To expand on @Purva Tharke's comment, the strong subadditivity inequality states:$$H(ABC)+H(C) \le H(AC) + H(BC)=H(ABC)+H(C) +H(C) -H(C) \le H(AC) + H(BC)=H(AB|C) \le H(A|C) + H(B|C)=0\...
Here's a guess: they might be related to entanglement-breaking channels (also known as measure-and-prepare channels, quantum-classical channels, etc.). Any channel of the form,  \Phi(\rho) = \sum\...