  Quantum entanglement is the basic resource for most quantum information
schemes. A fundamental problem of using photonic states as carriers of quantum
information is that they interact weakly with matter and that the interaction
volume is typically limited by the wavelength of light. The use of metallic
structures in quantum plasmonics has the potential to alleviate these problems.
Here, we present the first results showing that a single subwavelength
plasmonic nanoaperture can controllably modify the quantum state of light. In
particular, we experimentally demonstrate that two-photon entanglement can be
either completely preserved or completely lost after the interaction with the
nanoaperture solely depending on the relative phase between the quantum states.
We achieve this effect by using a specially engineered two photon state to
match the properties of the nanoaperture. The effect is fundamentally mediated
by quantum interference which occurs at scales smaller than the wavelength of
light. This connection between nano-photonics and quantum optics not only
demonstrates an unprecedented control over light-matter interaction in the
quantum limit, but also probes the fundamental limits of the phenomenon of
quantum interference.
