Spontaneous parametric down-conversion. Experimental set up used for the creation of entangled photons for a quantum Bell experiment. A 355nm laser is used to pump a non-linear BBO crystal to create signal and idler fields at 710 nm, aligned such that the idler and signal photons are emitted in a single narrow cone co-axial with the pump beam axis; this is focused by a lens onto a knife-edge prism mirror that splits them into two different arms. —Kevin J. Mitchell, UofGlasgow #WorldQuantumDay #Quantum