The first all-optical photonic switch made of cadmium sulfide nanowires has not only been produced but combined with others into a logic gate for on-chip information processing, which could help bring quantum computing a step closer. An all-optical photonic nanowire switch could help bring quantum computing a step closer. Here, laser light is emitted from the end of a cadmium sulfide nanowire. (Image: University of Pennsylvania) A team at the University of Pennsylvania built upon its earlier research showing that cadmium sulfide nanowires exhibited extremely strong light-matter coupling, making them especially efficient at manipulating light. This quality is crucial for the development of nanoscale photonic circuits, as existing mechanisms for controlling the flow of light are bulkier and require more energy than their electronic analogs. “The biggest challenge for photonic structures on the nanoscale is getting the light in, manipulating it once it's there and then getting it out,” said associate professor Ritesh Agarwal of Penn’s School of Engineering and Applied Science. “Our major innovation was how we solved the first problem, in that it allowed us to use the nanowires themselves for an on-chip light source.” First, they precisely cut a gap into a nanowire; then, they pumped enough energy into the first nanowire segment that it began to emit laser light from its end and through the gap. Because the researchers started with a single nanowire, the two segment ends were perfectly matched, allowing the second segment to efficiently absorb and transmit the light down its length. “Once we have the light in the second segment, we shine another light through the structure and turn off what is being transported through that wire,” Agarwal said. “That's what makes it a switch.” The researchers were able to measure the intensity of the light coming out of the end of the second nanowire and to show that the switch could effectively represent the binary states used in logic devices. “Putting switches together lets you make logic gates, and assembling logic gates allows you to do computation,” said graduate student Brian Piccione. “We used these optical switches to construct a NAND gate, which is a fundamental building block of modern computer processing.” A NAND gate, which stands for “not and,” returns a “0” output when all its inputs are “1.” The researchers constructed it by combining two nanowire switches into a Y-shaped configuration. NAND gates are important for computation because they are “functionally complete,” which means that, when put in the right sequence, they can do any kind of logical operation and thus form the basis for general-purpose computer processors. “We see a future where ‘consumer electronics’ become ‘consumer photonics,’ ” Agarwal said. “and this study shows that is possible.” Postdoctoral fellows Chang-Hee Cho and Lambert van Vugt contributed to the study, which was published in the journal Nature Nanotechnology. The research was supported by the US Army Research Office and the National Institutes of Health’s New Innovator Award Program. For more information, visit: www.upenn.edu