
A family of entropy measures generalizing von Neumann entropy, parameterized by a positive order α.Rényi entropy is a family of information-theoretic measures that generalize the von Neumann entropy, parameterized by a positive real order α. For α approaching 1, it reduces to the standard Shannon or von Neumann entropy, while other α values emphasize different aspects of a probability distribution, such as rare or common events. In physics, it quantifies entanglement and complexity in quantum systems, from spin chains to black holes.
Introduced by Alfréd Rényi in 1961 as a generalization of Shannon entropy. Its application to quantum systems grew in the 2000s with advances in entanglement theory.
For a quantum state with density matrix ρ, the Rényi entropy is defined as S_α(ρ) = (1/(1-α)) log Tr(ρ^α). By varying α, one can probe the spectrum of eigenvalues of ρ: lower α highlights larger probabilities, higher α highlights smaller ones. In many-body systems, the scaling of Rényi entropy with subsystem size reveals universal properties like central charge in conformal field theories.
It is used to measure the complexity of turbulent flows and to detect hidden order in phase transitions, as in the article 'Танцующий хаос: как измерить сложность турбулентности'.
In quantum field theory, the Rényi entropy for α=2 is often easier to compute than the von Neumann entropy, and it can be measured in experiments via 'entanglement spectroscopy'.



