Quantum control of photonic entanglement with a single sub-wavelength structure

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.