Abstract

In a measurement process, a quantum system is subject to an unavoidable back-action due to the quantum fluctuations of the detector. As a consequence, the system evolves along stochastic quantum trajectories controlled by the measurement strength. This features can be used as a tool to engineer and control quantum states, e.g. via feedback mechanisms, exploited in experiments in optics and superconducting devices.

In this talk, I will present two novel transitions triggered by the measurement strength in single-qubit and many-body systems. I will show that a time-dependent sequence of measurement of a qubit can induce a geometric phase equivalent to the Berry phase of driven quantum systems. I will discuss the probability distribution of such geometric phases and show that, when tuning the measurement strength, the mapping between the measurement sequence and the geometric phase undergoes a topological transition [1]. I will then discuss the effect of measurement in an interacting many-body 1-dimensional spin chain. I will show evidence from a numerical analysis that weak measurement can induce a quantum phase transition from an ergodic thermal phase with a large entropy to a nonergodic localized phase with a small entropy. I will discuss the phase transition in the measurement probability – measurement strength space and its identification in terms of entropy fluctuations and mutual information [2].

[1] Gebhart et al., arXiv:1905.01147
[2] M. Szyniszewski, A. Romito, H. Schomerus, Phys. Rev. B 100, 064204 (2019)