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A numerical study of measurement-induced phase transitions in the Sachdev-Ye-Kitaev model

Continuous monitoring of an otherwise closed quantum system has been found to lead to a measurement-induced phase transition (MIPT) characterized by abrupt changes in the entanglement or purity of the many-body quantum state. For an entanglement MIPT, entangling dynamics compete with measurement dynamics, pushing the system either to a phase with extensive entanglement or to a phase with low-level entanglement. For purification MIPTs, projective measurements effectively cool and localize the system, inducing a transition from a mixed state to an uncorrelated pure state. In this work, we numerically simulate monitored dynamics in the all-to-all Sachdev-Ye-Kitaev (SYK) model for finite N. We witness both entanglement and purification MIPTs in the steady-state. It is often said that there is an equivalence between entanglement and purification MIPTs, however we provide numerical evidence to the contrary, implying that entanglement and purification MIPTs are indeed two distinct phenomena. The reason for such a distinction is quite simple: entanglement can revive after a completely projective measurement -- if measurements do not occur too often in time -- but impurity cannot.