Spontaneous Parametric Down-Conversion (SPDC) is a quantum optical process where a single photon from a laser beam interacts with a nonlinear crystal. This interaction causes the photon to split into two lower-energy photons, known as signal and idler photons. These two new photons are entangled, meaning their properties are linked, even when separated by large distances. SPDC is widely used in quantum optics and quantum information science. It serves as a source of entangled photon pairs, which are essential for experiments in quantum computing, quantum cryptography, and quantum teleportation. This process helps researchers explore the fundamental principles of quantum mechanics.