Homodyne Measurement of a Non-Hermitian Qubit Undergoing Fluorescence

Abstract: Implementation of a two-level non-Hermitian qubit via postselection of a three-level system has been demonstrated. The postselection procedure, which discards quantum jump to the ground-state manifold while retaining excitations in the first and second excited-state manifolds, effectively generates a non-Hermitian qubit exhibiting PT symmetry. In this work, we perform continuous homodyne mea- surement of this non-Hermitian qubit and analyze the interplay between decay introduced by posts- election and measurement backaction. We compare the ensemble-averaged dynamics obtained from measurement trajectories with the the Liouvillian average. We formulate the no-jump stochastic differential equation describing the postselected non-Hermitian qubit and show that its ensemble- averaged dynamics agree with those of the jump-updated postselected evolution at drive strengths far from the Liouvillian exceptional point (EP). The degree of deviation near the EP depends sensitively on the nature of the drive. This discrepancy is attributed to the interplay between measurement backaction and the non-Hermitian decay introduced by postselection. Furthermore, we determine the optimal path of the non-Hermitian qubit by extremizing the action within the path-integral formulation of the quantum trajectory framework Our results provide insights into how measurement backaction and non-Hermitian dynamics together shape the transient behavior of open quantum systems and enable controlled manipulation of qubits near exceptional points.