A photodetector is an electronic device that can be used to convert light radiation into corresponding electrical signals.



In photodetectors, quantum limit refers to the minimum noise limit that can be achieved through the detection of a single photon under ideal conditions. In photodetectors, when photons are absorbed, electron and hole pairs are generated, and charges are generated under the action of an electric field.

Quantum limit refers to the noise carried by the extracted electrical signal when the number of photons is extremely small. According to the theory of optoelectronics, noise at the quantum limit is mainly caused by the random nature of photon absorption, namely the statistical fluctuation of photon counting. The quantum limit can be described by Quantum Efficiency and Equivalent Noise Photon Number (ENPN). Quantum efficiency is the efficiency of a photodetector in absorbing photons and generating electrical signals.



In an ideal photodetector, the efficiency of photoelectric conversion can approach 100%. The equivalent number of noise photons is an indicator that describes the noise performance of a photodetector, measuring the impact of noise photons on the detection signal. In an ideal scenario, the equivalent number of noise photons is equal to 1, which means that each detected photon signal can be accurately distinguished and counted.

However, in practical photodetectors, due to factors such as device noise and electronic lifetime, the actual equivalent noise photon number will be greater than 1, exceeding the quantum limit. Therefore, improving the performance of photodetectors and approaching quantum limits is one of the important goals of photodetector research.

In recent years, scientists have proposed some technologies and methods, such as cooling detector temperatures, reducing the impact of noise sources, optimizing photodetector structures, and using quantum effects, to approach and achieve quantum limit performance. The application of these methods has made photodetectors widely used in many fields, such as optical communication, spectral analysis, and quantum information.