The Floquet central spin model: A platform to realize eternal time crystals, entanglement steering, and multiparameter metrology
We propose and characterize protocols to engineer exact quantum revivals, discrete time crystals (DTCs), and entanglement oscillations in the periodically driven central spin model. While period-doubling DTCs have been observed in this system before, we uncover a unifying interaction-induced echo mechanism underlying several distinct dynamical regimes. We first demonstrate that this echo can enable exact period-doubling revivals when the Ising interaction strength, $λ$, between the central spin and the $N_{\rm sat}$ satellite spins is tuned to $2 π$. Notably, these revivals persist for arbitrary $N_{\rm sat}$ and transverse field $g$. This not only stabilizes the DTC response over a wide parameter regime, but also leads to a dynamical freezing regime. Furthermore, when $λ=(2m+1)π$ and $g= (2n+1)π/2$ ($\forall \, m,n \in \mathbb{Z}$), this echo induces a Clifford group structure. Consequently, higher-period revivals emerge that naturally steer the system through an entangled manifold of Bell-cat and spin-cat states. We establish that due to parity-dependent phases accumulated during the echo, the recurrence period is $12 T\,\, (24 T)$ for even (odd) $N_{\rm sat}$. Finally, we demonstrate that the multipartite entanglement generated by the Clifford dynamics can be harnessed for multiparameter metrology, with odd $N_{\rm sat}$ enabling Heisenberg-limited sensitivity.