A Single-Photon Avalanche Diode (SPAD) is a highly sensitive semiconductor device designed to detect single photons, which are the smallest units of light. It operates by using a process called avalanche multiplication, where a single photon can trigger a cascade of charge carriers, resulting in a measurable electrical signal. SPADs are commonly used in applications like quantum optics, medical imaging, and LIDAR systems. These devices are typically made from materials such as silicon or InGaAs, allowing them to operate in various wavelengths of light. SPADs are known for their fast response times and high detection efficiency, making them essential tools in fields like telecommunications and particle physics.