Novel phases of quantum matter in open quantum systems: spontaneous symmetry breaking and beyond

Abstract: Significant progress has recently been made in our understanding of quantum phases of matter in open systems. This emerging field, partly motivated by the development of noisy intermediate-scale quantum (NISQ) devices, lies at the crossroads of condensed matter physics and quantum information science. In this colloquium, I will provide an overview of the concept of "phases of matter" in many-body quantum systems, addressing both the traditional closed systems and the more recent developments in open systems. Symmetry, as in nearly all areas of physics, plays a crucial role in this discussion. After briefly reviewing how the concept of 'spontaneous symmetry breaking' differentiates phases of matter in conventional settings, I will introduce a novel form of symmetry breaking, known as "strong-to-weak spontaneous symmetry breaking”, which is an example of quantum phases unique to open quantum systems.