Theory of free fermions dynamics under partial post-selected monitoring

TL;DR

This paper develops a formalism for analyzing the dynamics of monitored quantum systems under partial post-selection, revealing universal features of measurement-induced phase transitions in free fermion chains.

Contribution

It introduces a partial post-selected stochastic Schrödinger equation connecting monitored and post-selected dynamics, and applies it to free fermions to analyze phase transition universality.

Findings

Universality of non-Hermitian MiPT is stable against weak stochasticity.

Passage to monitored universality occurs abruptly at finite partial post-selection.

Formalism enables study of MiPTs for arbitrary quantum trajectories.

Abstract

Monitored quantum systems undergo Measurement-induced Phase Transitions (MiPTs) stemming from the interplay between measurements and unitary dynamics. When the detector readout is post-selected to match a given value, the dynamics is generated by a Non-Hermitian Hamiltonian with MiPTs characterized by different universal features. Here, we derive a partial post-selected stochastic Schr\"odinger equation based on a microscopic description of continuous weak measurement. This formalism connects the monitored and post-selected dynamics to a broader family of stochastic evolution. We apply the formalism to a chain of free fermions subject to partial post-selected monitoring of local fermion parities. Within a 2-replica approach, we obtained an effective bosonized Hamiltonian in the strong post-selected limit. Using a renormalization group analysis, we find that the universality of the non-Hermitian MiPT is stable against a finite (weak) amount of stochasticity. We further show that the passage to the monitored universality occurs abruptly at finite partial post-selection, which we confirm from the numerical finite size scaling of the MiPT. Our approach establishes a way to study MiPTs for arbitrary subsets of quantum trajectories and provides a potential route to tackle the experimental post-selected problem.