A nano-scale crystalline structure made from cadmium selenide that absorbs white light and then reemits it a couple of nanoseconds later in a specific color. The quantum dot has been around since the 1980s when scientists were looking into the technology as a way to build nano-scale computing applications where light is used to process information. More recently, however, the technology is being used in medicine. The crystals are one ten-millionth of an inch in size and can be dissolved in water. When illuminated, they act as molecule-sized LEDs and can be used as probes to track antibodies, viruses, proteins, or DNA within the human body.