DescriptionRecently, a new class of nonequilibrium quantum phase transitions was identified in random quantum circuits undergoing unitary evolution interspersed with local projective measurements.The transition is characterized by a change in the scaling of the entanglement entropy from volume-law to area-law scaling as the measurement rate $p$ is increased above a critical value $p_c$.
In this thesis, we propose the tripartite mutual information as an unbiased diagnostic of the transition that has weak finite-size effects and simple scaling properties.
Using this quantity we locate the critical point of the transition for a collection of models and uncover that the critical exponents characterizing the transition are numerically similar between the generic Haar and stabilizer circuit models as well as 2-D bond percolation. To distinguish between the different universality classes of measurement-induced criticality we use a numerical transfer matrix method to calculate the effective central charge of the logarithmic conformal field theory at the critical point as well as the first few low-lying scaling dimensions of operators in the theory.Our results provide convincing evidence that the generic (Haar) and Clifford transition in qubit models lie in different universality classes and are distinct from the percolation transition for qudits in the limit of large onsite Hilbert space dimension.
Importantly, in the generic qubit model, we find evidence of multifractal scaling of correlation functions indicating a continuous spectrum of scaling dimensions at the critical point, which are absent in the percolation limit. We then proceed to modify the structure of the dynamics by introducing symmetry, quasiperiodicity, and disorder. For dynamics conserving a $U(1)$ global charge we identify a new charge-sharpening transition within the volume-law entangled phase.For dynamics with quasiperiodicity in the measurement rate we find the critical properties remain unchanged consistent with the expectation from the Luck criterion.
On the other hand, for dynamics with disorder in the measurement rate, we find evidence of a new universality class as predicted by the Harris criterion, and signatures of an infinite randomness fixed point.
Our results represent a significant step forward in the characterization of models of measurement-induced criticality.