Intrinsic Sensitivity Limits for Multiparameter Quantum Metrology

The quantum Cram'er-Rao bound is a cornerstone of modern quantum metrology, as it provides the ultimate precision in parameter estimation. In the multiparameter scenario, this bound becomes a matrix inequality, which can be cast to a scalar form with a properly chosen weight matrix. Multiparameter estimation thus elicits tradeoffs in the precision with which each parameter can be estimated. We show that, if the information is encoded in a unitary transformation, we can naturally choose the weight matrix as the metric tensor linked to the geometry of the underlying algebra $\mathfrak{s}\mathfrak{u}(n)$, with applications in numerous fields. This ensures an intrinsic bound that is independent of the choice of parametrization.