Hidden Single‐Qubit Topological Phase Transition without Gap Closing in Anisotropic Light‐Matter Interactions

Conventionally the occurrence of topological phase transitions (TPTs) requires gap closing, whereas there are also unconventional cases without need closing. Although traditionally TPTs lie in many-body systems condensed matter, both may find analogs few-body systems. Indeed, ground-state node number provides a classification for single-qubit While no-node theorem spinless is shown to restrict fundamental quantum Rabi model light-matter interactions, it demonstrated that limitation can be broken not only small counter-rotating term (CRT) but large-CRT regime, which striates rich diagram with different TPTs. these mostly accompanied closing and parity reversal, hidden node-phase transition revealed has neither nor change, turns out an analog TPT matter. A hysteresis sign unveiled via from amplitude squeezing gapped phase. The imprints Wigner function addressed. clarified mechanisms provide some special insights subtle role CRT.