Controlling sloppiness in two-phase estimation with a tunable weak measurement

The description of complex systems requires a progressively larger number of parameters. However, in practice, it often happens that a small subset of parameters suffice to describe the dynamics of the system itself: these combinations are usually referred to as stiff combinations. In turn, the remaining combinations, called sloppy, only play a minor role in the dynamics of the system, hence provide little information on it. While this effect can reduce model complexity, it can also limit the estimation precision when the stiff and sloppy combinations are unknown to the experimenter, and one is forced to estimate the potentially sloppy model parameters. We explored how such a sloppy behavior can be controlled and counteracted via quantum weak measurements in the estimation of two sequential phases. We showed that the introduction of a weak measurement of variable strength in between the two phases allows to switch from a fully sloppy setup to a fully determined one where both phases can be estimated with quantum-limited precision. Our work provides an important insight of sloppiness detection in quantum systems, with promising applications in quantum metrology and imaging, as well as in quantum security and quantum monitoring.