In quantum detectors, what specific energy transition occurs when an incoming infrared photon interacts with the material?

An electron jumps from a bound state (valence band) to a conducting state (conduction band).

Quantum detectors fundamentally rely on the principles of quantum mechanics applied to semiconductor materials, which allows for extremely fast and sensitive detection compared to thermal methods. When an infrared photon possesses sufficient energy, it is absorbed by the detector material. This absorbed energy excites an electron that was previously held in a fixed, bound energy level, known as the valence band. The energy propels this electron to a higher energy level where it becomes free to move, entering the conduction band. This creation of a mobile charge carrier generates the measurable electrical signal, whether it is a current change (in photoconductive types) or a voltage change (in photovoltaic types).