A group of scientists, including those from the University of Strathclyde, have developed smaller optical clocks, which could usher in the next generation of satellite navigation.

The research helps resolve a major stumbling block in the development of portable ultra-precise optical clocks

Working with researchers from the universities of Loughborough and Sussex, the scientists have determined how optical clocks, which are designed to replace satellite navigation systems such as GPS and Galileo, can be reliably switched ‘on’ – and kept running.

Optical atomic clocks are among the most precise time-measuring devices, losing less than one second every ten billion years. However, at present, they are massive devices, weighing hundreds of kilograms, and need to be housed within precise conditions.

Portable optical clocks have been heralded as the future of precision clocks – a reliable alternative to satellite geo-mapping, with scientists around the world working to develop clocks that will work in real-world settings.

Until now, a crucial component of the time keeping element of optical clocks, microcombs – ultra-fast laser beams that act as optical rulers by simultaneously emitting many precise colours, evenly spaced in frequency – has proved problematic. Their physics makes them incapable of starting and – crucially – staying in a running state.

The international collaboration resolved this issue in a new paper published in the journal Nature, developing a way to make these ultra-fast laser beams self-start and also ensuring their robustness, paving the way for real-world portable optical clocks that will have applications in ultra-sensitive medical instruments, ultra-fast online communications networks and navigation systems.

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Professor Alessia Pasquazi from the University of Loughborough, the leader of the project, said: “A well-behaved microcomb uses a special type of wave, called a cavity-soliton, which is not simple to get. Like the engine of a petrol car, a microcomb prefers to stay in an ‘off-state’. When you start your car, you need a starter motor that makes the engine rotate properly.

“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. 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.

“Now we have found a way to allow the system to self-start and to remain in the desired cavity-soliton state forever – self-recovering – independently of external perturbations.”

Professor Gian-Luca Oppo, the Strathclyde member of the international collaboration, added: “Cavity solitons, the crucial elements of these microcomb devices, have been theoretically discovered at Strathclyde in the mid 1990s.

“It is extraordinary to see these mathematical wave forms realised experimentally in these self-starting devices, in one-to-one correspondence with the results of our simulations and for the benefit of society at large.”

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