  Electromagnetically induced transparency, as a quantum interference effect to
eliminate optical absorption in an opaque medium, has found extensive
applications in slow light generation, optical storage, frequency conversion,
optical quantum memory as well as enhanced nonlinear interactions at the
few-photon level in all kinds of systems. Recently, there have been great
interests in exceptional points, a spectral singularity that could be reached
by tuning various parameters in open systems, to render unusual features to the
physical systems, such as optical states with chirality. Here we theoretically
and experimentally study transparency and absorption modulated by chiral
optical states at exceptional points in an indirectly-coupled resonator system.
By tuning one resonator to an exceptional point, transparency or absorption
occurs depending on the chirality of the eigenstate. Our results demonstrate a
new strategy to manipulate the light flow and the spectra of a photonic
resonator system by exploiting a discrete optical state associated with
specific chirality at an exceptional point as a unique control bit, which opens
up a new horizon of controlling slow light using optical states. Compatible
with the idea of state control in quantum gate operation, this strategy hence
bridges optical computing and storage.
