Lüders bounds of Leggett-Garg inequalities, $\mathcal{PT}$- symmetric evolution and arrow-of-time

Abstract: Leggett Garg inequalities (LGIs) test the incompatibility between the notion of macrorealism and quantum mechanics. For unitary dynamics, the optimal quantum violation of an LGI is constrained by the L\"uders bound. However, the LGIs does not provide the necessary and sufficient for macrorealism. A suitably formulated set of no-signaling in time (NSIT) conditions along with the arrow-of-time (AOT) condition provides the same. In this paper, we study two formulations in the three-time LG scenario, viz., the standard LGIs and the recently formulated variant of LGIs when the system evolves under $\mathcal{PT}$-symmetric Hamiltonian. We first demonstrate that the quantum violations of both forms of LGIs exceed their respective L\"uders bounds and can even reach their algebraic maximum. We further show that for the case of standard LGI, the violation of L\"uders bound can be obtained when both NSIT and AOT conditions are violated. Interestingly, for the case of a variant of LGI, for suitable choices of relevant parameters, the quantum violation can even be obtained when only the AOT is violated but all NSIT conditions are satisfied. This feature has not hitherto been demonstrated. We discuss the further implication of our study.