The subentropyJozsaRobbWootters1994 of a quantum state is defined similarly to the von Neumann entropy, as

$$Q(\rho) = -\sum_{k=1}^n \left( \prod_{l \neq k} \frac{\lambda_k}{\lambda_k - \lambda_l} \right) \lambda_k \log \lambda_k,$$
where {λ_k} is the set of eigenvalues of ρ. In the case that some of the eigenvalues are degenerate, the limit from different eigenvalues is taken.

The subentropy can be used to get a lower bound on the accessible information from an ensemble of quantum states.

Category:Entropy