Observation of PT -symmetric quantum coherence in a single-ion system

Parity-time (PT) -symmetric systems, featuring real eigenvalues despite their non-Hermitian nature, undergo spontaneous PT symmetry breaking at the exceptional point (EP). The phase transition from PT symmetry to PT symmetry breaking has shown exotic functionalities in the classical realm, such as loss-induced transparency or lasing revival. This transition is also expected to produce a more unconventional effect in pure quantum PT devices. Here, we report experimental evidences of spontaneous PT symmetry breaking in a single cold ^40Ca^+ ion and, more importantly, a counterintuitive effect of perfect quantum coherence occurring at the EP. Excellent agreement between experimental results and theoretical predictions is identified. In view of the versatile role of cold ions in building a quantum memory or processor, our experiment provides an alternative platform to explore PT-symmetric physics and utilize pure quantum EP effects.