On the cover of the Pink Floyd album Dark Side of the Moon, a prism splits a ray of light into all the colors of the rainbow. This multicolored medley, which owes its emergence to the fact that light travels as a wave, is almost always hiding in plain sight; a prism simply reveals that it was there. For instance, sunlight is a mixture of many different colors of light, each bobbing up and down with their own characteristic frequency. But taken together the colors merge into a uniform yellowish glow. A prism, or something like it, can also undo this splitting, mixing a rainbow back into a single beam. Back in the late 1970s, scientists figured out how to generate many colors of light, evenly spaced in frequency, and mix them together—a creation that became known as a frequency comb because of the spiky way the frequencies lined up like the teeth on a comb. They also overlapped the crests of the different frequencies in one spot, making the colors come together to form short pulses of light rather than one continuous beam. As frequency comb technology developed, scientists realized that they could enable new laboratory developments, such as ultra-precise optical atomic clocks, and by 2005 frequency combs had earned two scientists a share of the Nobel Prize in physics. These days, frequency combs are finding uses in modern technology, by helping self-driving cars to “see” and allowing optical fibers to transmit many channels worth of information at once, among others. Now, a collaboration of researchers at the University of Maryland (UMD) has proposed a way to make chip-sized frequency combs ten times more efficient by harnessing the power of topology—a field of abstract math that underlies some of the most peculiar behaviors of modern materials. The team, led by JQI Fellows Mohammad Hafezi and Kartik Srinivasan, as well as Yanne Chembo, an associate professor of electrical and computer engineering at UMD and a member of the Institute for Research in Electronics and Applied Physics, published their result recently in the journal Nature Physics.
