  Photonic structures have an inherent advantage to realize PT-phase transition
through modulating the refractive index or gain-loss. However, quantum PT
properties of these photonic systems have not been comprehensively studied yet.
Here, in a bi-photonic structure with loss and gain simultaneously existing, we
analytically obtained the quantum PT-phase diagram under the steady state
condition. To characterize the PT-symmetry or -broken phase, we define an
Hermitian exchange operator expressing the exchange between quadrature
variables of two modes. If inputting several-photon Fock states into a
PT-broken bi-waveguide splitting system, most photons will concentrate in the
dominant waveguide with some state distributions. Quantum PT-phase diagram
paves the way to the quantum state engineering, quantum interferences, and
logic operations in non-Hermitian photonic systems.
