Single Photon Nonlinearities and Macroscopic Entanglement

I will discuss a couple of recent experiments that are at opposite extremes of quantum photonics. We have demonstrated an integrated nonlinear optics scheme – based on sum-frequency generation - capable of photon-photon interactions. This approach has the potential for significant impact in quantum communication, e.g. for heralding entanglement over large distances and providing a significant step towards device-independent quantum key distribution. It also has the added advantage that it is incredibly flexible in terms of the wavelengths and bandwidths that can be incorporated, thus providing a potential solution for a wide range of experiments and applications beyond quantum communication. In a more fundamental experiment, we have tested a way of characterising macroscopic entanglement – the approach firstly amplifies a "microscopic" - single photon - entangled state, to the level of ~500 photons, before reducing this state back to the microscopic regime where a well known entanglement witness can be used to characterise the macroscopic state.