摘要： Parity-Time (PT)-symmetric systems have been intensely studied since their first description by Bender & Boettcher in 1998 [1]. Especially in photonics, numerous PT-symmetric effects and systems were investigated [2]. However, all of these experiments used classical light or single photons. Here, we present the first observation of two-photon interference in a lossy directional coupler and its associated Hong-Ou-Mandel (HOM) dip. In optics, PT symmetric potentials can be realized by enforcing a symmetric real part of the refractive index and an antisymmetric imaginary part (gain/loss) [3]. Notably, the transition to the quantum-optical regime precludes one from deploying gain [4], thus, we make use of entirely passive systems: By choosing an appropriate loss distribution, the behaviour of the system can be separated into a global loss factor and the desired non-Hermitian dynamics [5]. In this vein, the object of our following considerations is a passive PT-symmetric coupler in which two waveguides interact over a distance z, see Fig. 1 A. Rapid microscopic undulations of the waveguide trajectory serve to introduce effectively constant Markovian losses, resulting in the desired complex index distribution (B). Fig. 1. (A) In a pair of coupled waveguides, rapid undulations introduce bending losses and the (B) effective complex index distribution. (C) Dependence of the two-photon coincidence on the propagation length in the coupler. In contrast to the Hermitian case, PT-symmetry systematically displaces the dip minimum towards shorter propagation lengths, as indicated by the vertical dashed red line. In the experiment (D), corresponding coincidence data were obtained for Hermitian and PT-symmetric couplers. We achieved a dip with a visibility of 87±2 % and the minimum at 3 cm in the Hermitian case, and in the PT case 90±4 % at a position of 2.75 cm. We theoretically describe light propagation in the lossy directional coupler by a quantum master equation in Lindblad form. Our rigorous approach provides information on the full quantum state of the system. It is based on a Lie algebra treatment that provides us with an eigen-decomposition of the density matrix. The analytical solution holds for the unbroken PT-symmetry case, i.e. for losses that do not exceed twice the coupling. The resulting coincidence function for a |1,1> input state is plotted in Fig. 1 C: In the Hermitian case, the HOM dip occurs at exactly half the coupling length. In contrast, the interference minimum occurs after a shorter propagation length in the PT-symmetric system, as the vertical dashed line indicates. For the experiment, a set of lossy directional couplers of identical couplings with different propagation lengths was implemented to sample light at different z positions. A corresponding set of conventional Hermitian couplers served as baseline reference. Our measurements were executed with pairs of indistinguishable photons obtained by type-I spontaneous parametric down conversion, the coincidences at the sample output were recorded by avalanche photo detectors. In conclusion, our theoretical and experimental findings indicate that the asymmetric loss distribution in the PT-symmetric case can systematically accelerate the quantum interference dynamics in comparison to the Hermitian case.