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Quantum information is the study of the informational content of quantum states. The most common object of study is the "qubit", the information in a two-state quantum system such as spin-1/2 or photon polarization.

2 votes
1 answer
80 views

Is there a formula for the eigenvalues of $\rho_A,\rho_B$ in terms of those of $\rho_{AB}$?

Let $\rho_{AB}$ be a bipartite density matrix with marginal states $\rho_A=\text{tr}_B(\rho_{AB})$ and $\rho_B=\text{tr}_A(\rho_{AB})$. In such a case, is there a formula for the eigenvalues of $\rho_ …
5 votes
1 answer
325 views

An extension of von Neumann entropy to observables

Suppose we define the "entropy" of a self-adjoint matrix $\rho$ as the real number $S(\rho)$ given by: $$S(\rho)=-\text{tr}(\rho\log|\rho|)$$ (notice the absolute value on $\rho$, as $\rho$ may have n …
9 votes
3 answers
1k views

Are two states with the same measurement probabilities necessarily equal up to unitary equiv...

Let $\rho$ and $\rho'$ be $n\times n$ density matrices, and suppose that for every observable $A$ and every $\lambda$ in the spectrum of $A$ we have $$ \text{tr}(\rho P_{\lambda})=\text{tr}(\rho' P_{\ …