Generalized phase estimation in noisy quantum gates

We examine metrological scenarios where the parameter of interest is encoded onto a quantum state through the action of a noisy quantum gate and investigate the ultimate bound to precision by analyzing the behavior of the quantum Fisher information (QFI). We focus on qubit gates and consider the possibility of employing successive applications of the gate. We go beyond the trivial case of unitary gates and characterize the robustness of the metrological procedure introducing noise in the performed quantum operation, looking at how this affects the QFI at different steps (gate applications). We model the dephasing and tilting noise affecting qubit rotations as classical fluctuations governed by a von Mises--Fisher distribution. Compared to the noiseless case, in which the QFI grows quadratically with the number of steps, we observe a nonmonotonic behavior, and the appearance of a maximum in the QFI, which defines the ideal number of steps that should be performed in order to precisely characterize the action of the gate. Analyzing also the combination of different types of gate imperfections we defined a form of generic noise, finding also nontrivial regimes in which the combination of the two different noises leads to a more resilient QFI or, more remarkably, to an enhancement of the QFI with respect to the case in which only one type of noise is considered.