Quantum entanglement is an intriguing and the most confusing character in quantum theory. The "spooky action at a distance" is controversial since has been proposed, leading to the generation of the "EPR paradox". In 1964, theoretical physicist J.S. Bell who worked at CERN deduced an inequality on the basis of "local realism" and "hidden variables theory" which indicates that the local realism keep with its derivative result while it can be broken by quantum mechanics, thus a method was proposed to check whether the description of quantum mechanics is complete or not.

The miniaturized BBO crystal entangled source is a quantum optical system that combines the generation, collection, detection and inspection of polarization entangled photon pairs in one. Twin-photon polarization entanglement state can be prepared based on spontaneous parametric down conversion(SPDC) process of BBO crystal. When we measure the entangled photons we can find that whether the Bell's inequality has been broken or not. As a set of high brightness, high quality source of entangled photons, the miniaturized entangled source can be used for other frontier scientific experiments, such as single-photon interference, two-photon interference, decoherence experiment, etc. Furthermore, with the aid of the experimental system, researchers can combine the theory with application, understand quantum mechanics deeply and develop basic optic experimental ability.