Zhang and his colleagues created a gate from a single layer of molybdenum disulfide—consisting of a sheet of molybdenum atoms sandwiched between two layers of sulfur atoms—placed on top of silica. When the researchers shone two light beams at the gate, the handedness (also known as chirality) of the output beam was determined by the chirality of the input beams. When both input beams had the same chirality, the output was right-handed, but when both input beams had different chirality, the output beam was left-handed.