Multistability in quantum systems and related cavity QED experiments

First order phase transitions are ubiquitous in nature, however, this notion is ambiguous and highly debated in the case of open quantum systems. We present a paradigmatic example which allows for elucidating the key concepts. We show that atoms in an optical cavity can manifest a first-order dissipative phase transition where the stable co-existing phases are quantum states with high quantum purity. These states include atomic hyperfine ground states
and coherent states of electromagnetic field modes. We present the experimental demonstration of characteristic features of a quantum bistability, including the enhanced quantum fluctuations during the phase transition and
hysteresis in the order parameter.