Quantum Fisher Information and the Curvature of Entanglement

Abstract: We explore the relationship between quantum Fisher information (QFI) and the second derivative of concurrence with respect to the coupling between two qubits, referred to as the curvature of entanglement (CoE). For a two-qubit quantum probe used to estimate the coupling constant appearing in a simple interaction Hamiltonian, we show that at certain times CoE = -QFI; these times can be associated with the concurrence, viewed as a function of the coupling parameter, being a maximum. We examine the time evolution of the concurrence of the eigenstates of the symmetric logarithmic derivative and show that, for both initially separable and initially entangled states, simple product measurements suffice to saturate the quantum Cram\'er-Rao bound when CoE = -QFI, while otherwise, in general, entangled measurements are required giving an operational significance to the points in time when CoE = -QFI.