In the last few years, materials with strong second-order optical nonlinearity such as gallium arsenide, barium titanate and transition metal dichalcogenides have attracted significant attention, because they for the first time allowed efficient nonlinear optical interactions on the sub-micron scales. One of such nonlinear optical interactions - spontaneous parametric down-conversion (SPDC) - allows the generation of pairs of correlated photons and can enable photon entanglement . This is the foundation of many quantum optical applications ranging from secure communication to ultrafast quantum computing . The key challenges in this field are efficiency and the generation of on-demand quantum states.
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