Researchers from the universities of Strathclyde, Loughborough, and Sussex have demonstrated how optical clocks can be reliably switched on and made to keep running. The collaborators’ work resolves what had emerged as a persistent problem in the development of ultraprecise optical clocks and, specifically, the microcombs on which they rely to move from an “off” state to an “on” state. The collaborators’ work used a fiber laser to realize a reliable, self-starting oscillation of microcavity solitons. The solitons are naturally robust to perturbations, the researchers said, and they recover spontaneously even after complete disruption. Microcombs are the components that allow one to count the oscillation of the “atomic pendulum” of an atomic clock. The combs enable the conversion of the atomic oscillation at hundreds of trillions of times per second to a billion times a second — a gigahertz frequency that can be easily measured by modern electronic systems. The difficulty is that microcombs, similar to the engine of a car, prefers the “off” state. To start a car, a starter motor is required to coax the larger engine into its “on” state. “At the moment, microcombs do not have a good ‘starter motor.’ It is like having your car with the battery constantly broken, and you need someone to push it downhill every time you need to use it, hoping that it will start,” said Alessia Pasquazi, who began the project at the University of Sussex before moving to Loughborough University. “A well-behaved microcomb uses a special type of wave, called a cavity-soliton,” she said. “If you imagine that usually a cavity-soliton disappears in a microcomb laser when someone simply talks in the room, you see that we have a problem here.” An international team has demonstrated how optical clocks, which are designed to replace satellite navigation systems such as GPS and Galileo, can be reliably switched on and kept running. The researchers' advancement involves the 'self-emergence' of solitons in a microcavity. Courtesy of the University of Strathclyde. According to Marco Peccianti, now the director of the Emergent Photonic Research Centre at Loughborough University, the researchers demonstrated a different type of wave that could be used with microcombs, which they called laser cavity solitons because they directly embedded the microchip into a standard laser. With those waves, the team saw a significant boost in efficiency. “We have shown now that our soliton can be naturally turned into the only state of the system, and we call this process ‘self-emergence,’” Peccianti said. “It works like a simple thermodynamical system, which is ruled by ‘global variables,’ like temperature and pressure,” said Juan Sebastian Totero Gongora, a research fellow in quantum technologies at Loughborough. Work to commercialize the microcomb continues; according to Maxwell Rowley, who worked on the research at the University of Sussex and now works with CPI TMD Technologies, after setting the electrical current driving the laser to the appropriate value, it is guaranteed that the microcomb will operate in the desired soliton state. “We have basically an ‘eternal engine’ — like ‘Snowpiercer’ if you watch it — which always comes back to the same state if something happens to disrupt it,” Pasquazi said. The self-emergent microcombs will be directly used in optical fiber-based calcium ion references, which are being pursued under Innovate UK support and the leadership of Matthias Keller at the University of Sussex with CPI TMD Technologies, as well as in a collaboration on quantum technologies that includes co-author professor Roberto Morandotti at the Canadian Institut national de la recherche scientifique (INRS). The research was published in Nature (www.doi.org/10.1038/s41586-022-04957-x).