  The radiation of an emitter does not depend only on its intrinsic properties
but also on the surrounding photonic environment, the notion of which is
essential in the developments of lasers, quantum optics and other light-matter
interaction related fields. However, in conventional wisdom, an emitter
radiates into photonic eigenstates in the weak coupling regime and does not
alter the property of the latter. Here, we report a counterintuitive phenomenon
where the radiation field of a dipole in a parity-time symmetric ring resonator
displays the opposite handedness to the eigenstates of the system. This
chiral-reversing radiation takes place at an exception point of the underlying
non-Hermitian system, where the singularity at the exceptional point forces a
phase locking of the coalesced eigenstates when interacting with the dipole
emitter. Such an intriguing phenomenon has been employed to construct vortex
radiation with controllable topological charge from a single quantum dot
embedded plasminic nanocavity with Purcell enhancement factor up to 1000. Our
scheme enriches the interesting physics of an exception point in the quantum
region and may open a new paradigm for chiral quantum optics and vortex lasers
at nanoscale.
