University of Strathclyde: Physicists ‘switch on’ the future for ultra-precise optical clocks

A major stumbling block in the development of portable ultra-precise optical clocks has been solved by an international collaboration of scientists, including the Universities of Strathclyde, Loughborough and Sussex.

The researchers have worked out 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 the pinnacle of time measuring devices, losing less than one second every ten billion years, but they are currently massive devices, weighing hundreds of kilograms, which need to be housed within precise conditions.

Portable optical clocks have been heralded as the future of ultra-precise clocks – an ultra-reliable alternative to satellite geo-mapping, with scientists around the world racing 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.
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.”

“Cavity solitons, the crucial elements of these microcomb devices, have been theoretically discovered at Strathclyde in the mid 1990s” said Professor Gian-Luca Oppo, the Strathclyde member of the international collaboration, of the University’s Department of Physics.

“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.”

The paper, Self-emergence of robust solitons in a micro-cavity, is published in Nature as a testimony to the impact and wide application of the scientific results.

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