 The other main avenue of approach, which paved the way to a conclusive test of Bell inequalities, involved innovations in tests using photons. First, efficient sources of photon entangled states were realized by exploiting Parametric Down Conversion, a non-linear optical phenomenon in which a photon of higher energy converts into two lower frequency photons inside a non-linear medium in such a way that energy and momentum are conserved. This allows a high collection efficiency due to wave vector correlation of the emitted photons. Next, high efficiency single photon Transition Edge Sensors were produced. These advances led to detection loophole-free experiments with photons (Giustina et al. 2013, Christensen et al. 2013) and finally to the conclusive tests discussed in the next section.