Ancilla quantum measurements on interacting chains: Sensitivity of entanglement dynamics to the type and concentration of detectors

We consider a quantum many-body lattice system that is coupled to ancillary degrees of freedom (``detectors’'), which are periodically measured by means of strong projective measurements. The concentration ${\rho }_a$ of ancillae and their coupling $M$ to the main system are considered as parameters. We explore the dynamics of density and of entanglement entropy in the chain, for various values of ${\rho }_a$ and $M$ for two models of the detector-chain interaction that couple the local density in the chain to a detector degree of freedom. It is found that, for the density-density ($S_z{s}_z$-type in spin language) coupling, the critical values $M_c$ for the measurement-induced entanglement transition depends sensitively on ${\rho }_a$. Moreover, our results indicate that for a sufficiently small ${\rho }_a$ the transition in this model disappears, i.e., a finite density of detectors is needed to reach a disentangling phase. The behavior is qualitatively different for the second model, with density-hopping ($S_z{s}_x$-type) coupling. Specifically, the dynamics is much less sensitive to the concentration ${\rho }_a$ of detectors than in the first model. Furthermore, the dependence of entanglement on the coupling strength $M$ is strongly non-monotonic, indicating re-entrance of the entangling phase at large $M$.