Non-Hermitian quantum correlations in the time domain: well beyond Luders bound

Abstract The powerful ability of achieving and manipulating strong spatial quantum correlations has helped the emergence of quantum information science. In contrast, strong temporal quantum correlations (TQC), playing an essential role in a wide range of physical and biological processes, have remained an elusive and long-standing challenge, hindering their practical applications. Here, for the first time, we experimentally demonstrate how to break the Lüders bound and to achieve the ultimate upper limit of TQC, by using a parity-time (PT)-symmetric single-ion system, within a standard measurement scenario. We achieve this goal by approaching an exceptional point (EP) in such a system, confirming the power of PT symmetry and non-Hermitian phase transition in enhancing purely quantum effects. Our work not only removes an important obstacle for TQC-based quantum engineering applications, but also sheds new light on operating quantum devices at EPs.