Asymptotic Metrological Scaling and Concentration in Chaotic Floquet Dynamics

Abstract

We study quantum sensing with Floquet chaotic dynamics generated by Haar random unitary gates. The metrological resources consist of three ingredients: A given initial state, a set number of Haar random unitary gates and the sensing gates. There are two natural ways of organizing the resources: the first one is the "control" protocol, where the random unitary gates act as random controls and intertwine with the deterministic sensing gates and the second one is the "state-preparation" protocol, where random unitary gates play the role of preparing the metrological useful states. In each protocol, we consider both global Haar random unitary gates and a set of local two-site Haar random unitary gates that forms a Floquet random quantum circuit (RQC) respectively. We find linear, shot-noise scaling of the metrological precision, quantified by the quantum Fisher information (QFI), in the asymptotic limit when the Hilbert space dimension becomes large, and quantum advantages beyond linear scaling in the non-asymptotic regimes. We also bound the fluctuation of the QFI using concentration inequalities. Our analytical findings are corroborated by numerical simulations. Finally, along the way of analyzing the precision limit, we prove an empirical conjecture of RQC: In the asymptotic limit of large local Hilbert space dimension, the Floquet operator of a Floquet RQC essentially behaves like a global unitary operator.