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

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.