Measurement induced criticality in monitored quantum systems

Abstract: A novel aspect of recent experiments with quantum devices is that measurements can play an active role in preparing the state of the system, not just in diagnosing it. Unlike unitary evolution, the quantum collapse induced by local measurements can have a highly non-local impact on entangled quantum states, instantaneously destroying or creating new long distance correlations. I will review the surprising collective effects that can arise, such as measurement induced phase transitions and new entanglement structures. There is, however, a fundamental challenge to observing post-measurement correlations, conditioned on the outcome of many-measurements with exponentially small Born probability of recurring. I will discuss how to resolve this post-selection problem by cross-correlating experimental data with results of an approximate classical model. This allows us to reframe the measurement induced transition as a transition in the ability of a classical intelligent agent to learn the quantum state.