Observation of $\mathcal{PT}$-symmetric quantum coherence in a single ion system

TL;DR

This paper reports the experimental observation of $ ext{PT}$-symmetry breaking and perfect quantum coherence at the exceptional point in a single cold calcium ion, revealing new quantum effects with potential quantum engineering applications.

Contribution

First experimental demonstration of $ ext{PT}$-symmetry breaking and quantum coherence at the exceptional point in a single ion system.

Findings

Observation of spontaneous $ ext{PT}$ symmetry breaking in a single ion.

Detection of perfect quantum coherence at the exceptional point.

Experimental results match theoretical predictions.

Abstract

Parity-time($\mathcal{PT}$)-symmetric systems, featuring real eigenvalues despite its non-Hermitian nature, have been widely utilized to achieve exotic functionalities in the classical realm, such as loss-induced transparency or lasing revival. By approaching the exceptional point (EP) or the coalescences of both eigenvalues and eigenstates, unconventional effects are also expected to emerge in pure quantum $\mathcal{PT}$ devices. Here, we report experimental evidences of spontaneous $\mathcal{PT}$ symmetry breaking in a single cold $^{40}\mathrm{Ca}^{+}$ 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 quantum memory or processor, our experiment provides a new platform to explore and utilize pure quantum EP effects, with diverse applications in quantum engineering of trapped ions.