  Parity-Time (PT) symmetric quantum mechanics is a complex extension of
conventional Hermitian quantum mechanics in which physical observables possess
a real eigenvalue spectrum. However, an experimental demonstration of the true
quantum nature of PT symmetry has been elusive thus far, as only
single-particle physics has been exploited to date. In our work, we demonstrate
two-particle quantum interference in a PT-symmetric system. We employ
integrated photonic waveguides to reveal that PT-symmetric bunching of
indistinguishable photons shows strongly counterintuitive features. We
substantiate our experimental results by modelling the system by a quantum
master equation, which we analytically solve using Lie algebra methods. Our
work paves the way for nonlocal PT-symmetric quantum mechanics as a novel
building block for future quantum devices.
