  The optical resonances supported by nanostructures offer the possibility to
enhance the interaction between matter and the quantum states of light. In this
work, we provide a framework to study the scattering of quantum states of light
with information encoded in their helicity by a nanostructure. We analyze the
purity of the scattered output quantum state, and we find that the purity of
the incident state can be lost, when it interacts with the optical resonances
of the nanostructure. To explain the loss of quantum purity, we develop a
physical picture based on time delays and frequency shifts between the output
two-photon modes. The framework and analysis proposed in this work establishes
a tool to address the interaction between quantum light and nanoenvironments.
