  Many technologies emerging from quantum information science heavily rely upon
the generation and manipulation of entangled quantum states. Here, we propose
and demonstrate a new class of quantum interference phenomena that arise when
states are created in and coherently converted between the propagating modes of
an optical microcavity. The modal coupling introduces several new creation
pathways to a nonlinear optical process within the device, which quantum
mechanically interfere to drive the system between states in the time domain.
The coherent conversion entangles the generated biphoton states between
propagation pathways, leading to cyclically evolving path-entanglement and the
manifestation of coherent oscillations in second-order temporal correlations.
Furthermore, the rich device physics is harnessed to tune properties of the
quantum states. In particular, we show that the strength of interference
between pathways can be coherently controlled, allowing for manipulation of the
degree of entanglement, which can even be entirely quenched. The states can
likewise be made to flip-flop between exhibiting initially correlated or
uncorrelated behavior. Based upon these observations, a proposal for extending
beyond a single device to create exotic multi-photon states is also discussed.
