Single-photon detectors are specialized devices designed to detect individual photons, which are the smallest units of light. These detectors are crucial in various fields, including quantum optics, telecommunications, and medical imaging. They operate by converting the energy from a single photon into an electrical signal, allowing researchers to measure very low levels of light with high precision. There are several types of single-photon detectors, including avalanche photodiodes and superconducting nanowire single-photon detectors (SNSPDs). Each type has its advantages and limitations, such as sensitivity, speed, and operational temperature. These detectors enable advancements in technologies like quantum computing and quantum cryptography.