An integrated photonics platform that can store light and electrically control its frequency in an integrated circuit is the newest development from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS). According to the researchers, their work represents the first time that microwaves have been used to shift the frequency of light in a programmable manner on a chip. A new integrated photonics platform that can store light and electrically control its frequency (or color) in an integrated circuit. Courtesy of Loncar Lab/Harvard SEAS. The researchers developed a technique to fabricate high-performance optical microstructures using lithium niobate, a material with powerful electro-optic properties. The team led by professor Marko Loncar had previously demonstrated the ability to propagate light through lithium niobate nanowaveguides with very little loss and to control light intensity with on-chip lithium niobate modulators. In the latest research, the team combined and further developed these technologies to build a molecule-like photonic system. The researchers demonstrated a “photonic molecule” with two distinct energy levels using coupled lithium niobate microring resonators, and controlled the molecule by external microwave excitation. They showed that the frequency and phase of light could be precisely controlled by programmed microwave signals. Through such coherent control, the team was able to show on-demand optical storage and retrieval by reconfiguring the photonic molecule into a bright-dark mode pair. “The unique properties of lithium niobate, with its low optical loss and strong electro-optic nonlinearity, give us dynamic control of light in a programmable electro-optic system,” said professor Cheng Wang at City University of Hong Kong. “This could lead to the development of programmable filters for optical and microwave signal processing and will find applications in radio astronomy, radar technology, and more.” Many quantum photonic and classical optics applications require shifting of optical frequencies, a historically difficult task. “We show that not only can we change the frequency in a controllable manner, but using this new ability we can also store and retrieve light on demand, which has not been possible before,” said Mian Zhang, CEO of Hyperlight Corporation. Next, the researchers aim to develop even lower-loss optical waveguides and microwave circuits using the same architecture to enable even higher efficiencies and, ultimately, achieve a quantum link between microwave and optical photons. “The energies of microwave and optical photons differ by five orders of magnitude, but our system could possibly bridge this gap with almost 100 percent efficiency, one photon at a time,” Loncar said. “This would enable the realization of a quantum cloud — a distributed network of quantum computers connected via secure optical communication channels.” The research was published in Nature Photonics (https://doi.org/10.1038/s41566-018-0317-y).