Dynamics of Open Quantum Systems: Measurement, Entanglement, and Criticality

Open quantum systems refer to quantum systems that couple with their surrounding environment. They are ubiquitous, especially for quantum devices. Due to coupling with the external environment, the dynamics of open quantum systems becomes non-unitary, which leads to additional complexity and novel possibilities compared to the unitary dynamics of closed systems. The study of open quantum systems is therefore of both theoretical and practical interest.

In this dissertation, using paradigmatic models of (Markovian) open quantum systems, I study three aspects of open quantum systems: (i) measurement of emitted particles from an open quantum system, to probe its dynamics; (ii) quantum entanglement in open quantum systems, which demonstrates the significance of information gained from measurement; and (iii) quantum critical phenomena in an open quantum many-body system. The first part is of importance for probing dynamics of open quantum systems and for engineering quantum states of emitted particles using engineered open quantum systems. The second part is from the quantum information point of view, which clearly demonstrates the subtle relation between quantum entanglement of mixed states and measurement in open quantum systems. An entanglement generation protocol is provided, which can be useful for quantum information processing. The last part is concerned with open quantum many-body physics, which demonstrates the basic mechanism behind phase transitions in open quantum systems. The differences and similarities between Lindbladian and Hamiltonian phase transitions are shown from various perspectives.