Charge and Spin Sharpening Transitions on Dynamical Quantum Trees

The dynamics of monitored systems can exhibit a measurement-induced phase transition (MIPT) between mixed and pure phases, tuned by the measurement rate. When the dynamics obeys a continuous symmetry, the mixed phase further splits into a fuzzy phase and a sharp phase based on the scaling of fluctuations of the symmetry charge. While the sharpening transition for Abelian symmetries is well understood analytically, no such understanding exists for the non-Abelian case. In this work, building on a recent analytical solution of the MIPT on tree-like circuit architectures (where qubits are repatedly added or removed from the system in a recursive pattern), we study purification and sharpening transitions in monitored dynamical quantum trees obeying U(1) and SU(2) symmetries. The recursive structure of tree tensor networks enables powerful analytical and numerical methods to determine the phase diagrams in both cases. In the U(1) case, we analytically derive a Fisher-KPP-like differential equation that allows us to locate the critical point and identify its properties. We find that the purification and sharpening transitions generically occur at distinct measurement rates. In the SU(2) case, we find that the fuzzy phase is generic, and a sharp phase is possible only in the limit of maximal measurement rate. In this limit, we analytically solve the boundaries separating the fuzzy and sharp phases, and find them to be in agreement with exact numerical simulations.