  Biphoton process is an essential benchmark for quantum information science
and technologies, while great efforts have been made to improve the coherence
of the system for better quantum correlations. Nevertheless, we find that the
non-Hermitian features induced by the atomic quantum interference could be well
employed for the direct control of entanglement. We report the demonstration of
exceptional point (EP) in biphotons by measuring the light-atom interaction as
a natural non-Hermitian system, in which the electromagnetically induced
transparency regime provides a powerful mechanism to precisely tune the
non-Hermitian coupling strength. Such biphoton correlation is tuned within an
unprecedented large range from Rabi oscillation to
antibunching-exponential-decay, also indicating high-dimensional entanglement
within the strong and weak light-matter coupling regimes. The EP at the
transition point between the two regimes is clearly observed with the biphoton
quantum correlation measurements, exhibiting a single exponential decay and
manifesting the coalesced single eigenstate. Our results provide a unique
method to realize the controllability of natural non-Hermitian processes
without the assistance of artificial photonic structures, and paves the way for
quantum control by manipulating the non-Hermitian features of the light-matter
interaction.
