On-chip topological light exhibits less energy loss and is more consistent than light at the interior of an array, a recent experimental study shows. These qualities of robustness and consistency are vital for the integration of such arrays for processing quantum information. Topological transport of light is the photonic equivalent of topological electron flow in certain semiconductors: Light flows in a metamaterial consisting of an array of tiny glass loops fabricated on a silicon substrate. If the loops are engineered just right, light sent into the array easily circulates around the edge with very little energy loss, while light taking an interior route suffers loss. Light enters a 2-D array from the lower left and exits at the lower right. Light that follows the edge of the array (blue) does not suffer energy loss and exits after a consistent amount of delay. Light that travels into the interior of the array (green) suffers energy loss. Images courtesy of University of Maryland’s Joint Quantum Institute. A 2011 paper published in Nature Physics by a research team at the University of Maryland’s Joint Quantum Institute pointed out the potential application of robustness in delay lines and described a scheme to implement quantum Hall models in arrays of photonic loops. With the photonic quantum Hall effect, light that circulates around one unit cell of a loop array will undergo a slight phase change: The light signal, in coming around the unit cell, re-arrives where it started, but is advanced or delayed just a bit from its original condition. Just this amount of change imparts the topological robustness to the global transmission of the light in the array. In 2013, the same research team reported in Nature Photonics on results from an actual experiment: Because the tiny loops weren’t perfect, they allowed a bit of light to escape vertically out of the plane of the array. This faint light allowed the team to image the light’s course, confirming that light persists when it goes around the edge of the array but suffers energy loss when traveling through the bulk. The team’s newest paper, to be published in Physical Review Letters, delivers detailed measurements of the transmission and delay for edge-state light and for bulk-route light. This 2-D array consists of resonator rings, where light spends more time, as well as link rings, where light spends little time. Undergoing a circuit around a complete unit cell of rings, light will return to the starting point with a slight change in phase. In the new study, the researchers found that for light taking the bulk route in the array, the delay and transmission of light can vary widely, whereas for light taking the edge route, the amount of energy loss is regularly less and the time delay for signals more consistent. "Apart from the potential photonic-chip applications of this scheme," said team member Dr. Mohammad Hafezi, “this photonic platform could allow us to investigate fundamental quantum transport properties." For more information, visit www.jqi.umd.edu.