Marco Schiro (IPhT 17/05/2021) Measurement-Induced Phase Transitions in a Quantum Ising Spin Chain

The effect of a measurement apparatus on a quantum system can be dramatic: a well known example is the Quantum Zeno Effect where by repeatedly measuring a quantum system it is possible to completely freeze its dynamics into a well defined state. In the many-body context the competition between unitary dynamics and stochastic measurements can give rise to new dynamical phases characterised by qualitatively different entanglement properties and to sharp phase transitions between them. In this talk I will discuss this measurement-induced criticality in the context of a Quantum Ising chain with continuous monitoring of the transverse magnetization. I will compare different limits of the measurement problem and show how they provide a remarkably similar phenomenology as the measurement strength is increased, namely a sharp transition from a critical phase with logarithmic scaling of the entanglement to an area-law phase. I will argue how the essential features of this problem can be understood by looking at the associated non-Hermitian Quantum Ising model and its subradiance spectral transition.