Ehud ALTMAN
An external observer can have a profound impact on the quantum state that it is observing. Recent studies of random quantum circuits have shown that if the object of observation is a large many-body system, then mere observation can induce a phase transition in the large-scale quantum correlations of the system. In this talk I will argue that observing a quantum ground state can similarly lead to a phase transition in the structure of long range correlations in the state. As examples I will discuss the impact of weak measurement on a one dimensional quantum liquid and on a critical ground state of the transverse field Ising model. Finally I will present new results on decoherence induced criticality in a quantum paramagnet.