Physics Colloquium 26th May 2017 delivered by Professor Miles Padgett, University of Glasgow Ghost imaging and ghost diffraction were first demonstrated by Shih and co-workers using photon pairs created by parametric down-conversion. They were able to obtain an image or a diffraction pattern using photons that had never interacted with the object, relying instead on the correlations with photons that have. In a typical ghost-imaging configuration, the down-converted photons are directed into two separate optical arms. The object is placed in one arm and a single-pixel “heralding” detector detects the photons transmitted through this object. The signal from this detector triggers a camera positioned in the other arm, which then detects the spatial position of the correlated photon. The image is recovered from the coincidence detection of the two photons. But what sets the resolution of the resulting images? The resolution of the heralding arm, the resolution of the camera optics, or something else? This talk will present an examination of the resolution limits of the ghost imaging and ghost diffraction. Beyond consideration of these limits, our ghost diffraction is an implementation of Popper’s thought experiment, and while our results agree with his experimental predictions, we show how these results do not contradict the Copenhagen Interpretation.