Researchers have demonstrated the first single-photon transistor using a semiconductor chip. The device has numerous holes in it, making it appear much like a honeycomb. Light entering the chip bounces around and gets trapped by the hole pattern. A small crystal sits inside the area where the light intensity is strongest, and, analogous to conventional computer memory, this crystal stores information about photons as they enter the device. It can then effectively tap into that memory to mediate interactions with other photons that later arrive at the chip.
The team observed that a single photon could, by interacting with the crystal, control the transmission of a second light pulse through the device. The first light pulse acts like a key, opening the door for the second photon to enter the chip. If the first pulse didn't contain any photons, the crystal blocked subsequent photons from getting through. This behavior is similar to a conventional transistor where a small voltage controls the passage of current through its terminals. Here, the researchers successfully replaced the voltage with a single photon and demonstrated that their quantum transistor could switch a light pulse containing around 30 photons before the device's memory ran out.