Entropy–energy inequalities for qudit states

We establish a procedure to find the extremal density matrices for any finite Hamiltonian of a qudit system. These extremal density matrices provide an approximate description of the energy spectrum of the Hamiltonian. In the case of restricting the extremal density matrices by pure states, we show that the energy spectrum of the Hamiltonian is recovered for d = 2 and 3. We conjecture that by means of this approach the energy spectrum can be recovered for the Hamiltonian of an arbitrary finite qudit system. For a given qudit system Hamiltonian, we find new inequalities connecting the mean value of the Hamiltonian and the entropy of an arbitrary state. We demonstrate that these inequalities take place for both the considered extremal density matrices and generic ones. (paper)