A Quantum Well Infrared Photodetector (QWIP) is an infrared photodetector, which uses electronic intersubband transitions in quantum wells to absorb photons. In order to be used for infrared detection, the parameters of the quantum wells in the quantum well infrared photodetector are adjusted so that the energy difference between its first and second quantized states match the incoming infrared photon energy. QWIPs are typically made of gallium arsenide, a material commonly found in smartphones and high-speed communications equipment. Depending on the material and the design of the quantum wells, the energy levels of the QWIP can be tailored to absorb radiation in the infrared region from 3 to 20 µm. QWIPs are one of the simplest quantum mechanical device structures that can detect mid-wavelength and long-wavelength infrared radiation. They are known for their stability, high pixel-to-pixel uniformity, and high-pixel operability. History In 1985, Stephen Eglash and Lawrence West observed strong intersubband transition in multiple quantum wells (MQW) that prompted more serious consideration into using quantum wells for infrared detectors. Previously, attempts to use quantum wells for infrared detection were based on free absorption in quantum wells that bring the electrons over the top of the barriers. However, resulting detectors displayed low sensitivity. By 1987, the basic operating principles for a quantum well infrared photodetector that demonstrated sensitive infrared detection were formulated. In 1990, the low-temperature sensitivity of the technology was further improved by increasing the barrier thickness, which suppressed the tunneling current. At this point, these devices were formally known as quantum well infrared photodetectors. In 1991, the first infrared image was obtained using this approach. In 2002, researchers at the U.S. Army Research Laboratory (ARL) developed a voltage-tunable, two-color QWIP with effective wavelength switching for remote temperature sensing. The instrument exhibited a peak detection wavelength of 7.5 micrometers for positive bias at 10 K when the electrons resided in one of the quantum wells and switched to 8.8 micrometers at a large negative bias when the electrons were transferred to the other well. Yet despite its use in civilian applications, QWIP technology was considered insufficient by the U.S. military for military use. At the time, the photodetectors could only sense the one-dimensional quantization when the light traveled in parallel to the material layers, which typically occurred when light was shined at the edge of the detector. As a result, the QWIP technology had a quantum efficiency of only 5