Photon subtraction is a technique in quantum optics where a single photon is removed from a quantum state of light. This process can be achieved using various methods such as a beam splitter or a photon absorber. Photon subtraction is often used in quantum information processing tasks, such as quantum teleportation and quantum cryptography, as well as in quantum communication and quantum computing applications. By selectively removing single photons from a quantum state, researchers can manipulate and control the properties of the light and improve the performance of quantum systems.