Quantum interference through plasmonic nanostructures

Quantum technologies like quantum computing, quantum communication or quantum metrology promise astonishing advantages over their classical counterparts. However, they all require excellent control and protection of the involved quantum states. In this respect, photons are ideal carriers of quantum information due to their robustness against decoherence and the ease with which they can be transferred over long distances. At the same time they suffer from weak interactions with matter and the large structures necessary, as given by the wavelength of light. Combining quantum optics with plasmonic structures could open an avenue to address these drawbacks while still benefiting from the advantages of photons. We present for the first time the transmission of an entangled two-photon state through a plasmonic aperture that is smaller than the wavelength of the light. Entanglement is the key resource for many quantum information schemes and its protection of great interest. Strong interactions with the nanoaperture usually destroy the entanglement of an arbitrary state. We tailor a special state for the interaction – taking into account the specific properties of the aperture – that leads to quantum interference and eventually protects the entanglement from degradation. We experimentally demonstrate creation of this state, transmission through the nanoaperture and successful protection of the entanglement. On our way to this achievement, we improve our control over the spontaneous parametric down-conversion source of photon pairs. We report a surprising dependence of the time delay distribution between the photons of the pair on the position of the non-linear crystal. We experimentally confirm the effect via quantum interference experiments and challenging direct measurements of the arrival time. Furthermore, a novel reconstruction scheme for the complex spectral biphoton wave function allows us to study the temporal correlations in more detail and to shape the wave function. We experimentally demonstrate the reconstruction in different situations and find an unexpected temporal distribution with a detection mode carrying orbital angular momentum.