Semi-classical geometric tensor in multiparameter quantum information

Abstract: The quantum geometric tensor (QGT) captures the variations of quantum states with parameters, serving as a central concept in modern quantum physics. Its real part, the quantum Fisher information matrix (QFIM), has a measurement-dependent counterpart that links statistics to distinguishability. However, an analogous extension for the QGT is hindered by the fundamental inaccessibility of its imaginary part through measurement probabilities. Here we introduce a counterpart to the QGT that includes measurement operators, termed the \textit{semi-classical} geometric tensor (SCGT). We show that the SCGT provides a lower bound to the QGT that is tight for pure states. Moreover, we use the SCGT to derive sharp multiparameter information bounds and discuss extensions of the Berry phase.