Fig. 4: A typical quantum toolbox.
The core quantum toolbox (top panel) typically exploits the generation of entangled photons, shown in red, by spontaneous parametric downconversion (SPDC) of a high-energy photon (shown in blue), often using non-linear crystals. Expressing and controlling the entanglement in the spatial basis comprising patterns of light (bottom panel) allows for high-dimensional entanglement to be exploited in quantum protocols. The more patterns in the superposition, the higher the dimensionality of the quantum state
