Portable atomic clocks could reduce reliance on satellite navigation

Photonics team use laser technology to produce significant improvement in accuracy of counting element in compact atomic clocks

Timekeeping has been vital to navigation for centuries. The Royal Observatory at Greenwich proudly displays the chronometers designed and built by John Harrison in the 18th century that allowed longitude to be determined by ships at sea, with the result that the zero meridian runs through London. Today, the hyper-accurate clocks that allow us navigate are mainly housed on satellites, such as the American-owned global positioning system (GPS) and the European Union’s Galileo consolation, the subject of some controversy in the UK owing to legal aspects of Brexit.

atomic clocks
Artist’s impression of a soliton wave propagating inside the Sussex team’s chip

A team at the emerging photonics laboratory (EPic Lab) at Sussex University is working on technology to make super accurate clocks portable: something that would free users from the dependants on GPS or Galileo for navigation. This would reduce their exposure to the whims of international politics, while also allowing them to navigate where there is no satellite signal.

Their breakthrough is in improving the accuracy of the lancet – the device in the clock responsible for counting, analogous to the pendulum in a traditional mechanical clock – by 80 per cent, outperforming other teams around the world working on similar projects.

The device is a component of an optical atomic clock, the most accurate kind of timekeeper ever devised. Currently, such clocks are huge, weighing hundreds of kilograms. Their lancets derive from the quantum property of a single confined atom, using a device known as an optical frequency comb – a laser emitting many colours, equally and precisely separated in frequency – to observe the electromagnetic field of a light beam oscillating hundreds of millions of times a second.

The EPic Lab team is working with micro-combs – devices that reduce the size of frequency comb equipment using a microprocessor component called an optical micro-resonator. Previously, these have proved too delicate, difficult to operate and not as accurate as optical frequency combs.

In a paper in Nature Photonics, the team explains how it achieved its improved results by using a laser cavity soliton. “Solitons are special waves that are particularly robust to perturbation. Tsunamis, for instance, are water solitons. They can travel unperturbed for incredible distances; after the Japan earthquake in 2011 some of them even reached as far as the coast of California,” explained Dr Alessia Pasquazi, a mathematician and a member of the research team.

“Instead of using water, in our experiments performed by Dr Hualong Bao, we use pulses of light, confined in a tiny cavity on a chip. Our distinctive approach is to insert the chip in a laser based on optical fibres, the same used to deliver internet in our homes.” The soliton wave produced in the chip can generate many colours, while also offering the robustness and versatility of control of much larger pulsed lasers, she added.

The next stage in the project is to integrate the soliton device with an ultra-compact atomic reference developed by Prof Matthias Keller at Sussex University.

“Working together, we plan to develop a portable atomic clock that could revolutionise the way we count time in the future,” said Prof Marco Peccianti of the EPic Lab. This could lead to partnerships with the aerospace industry within five years and eventually portable atomic clocks that could be housed in mobile phones or vehicles, he added. Advantages of this might include ambulances being able to access mapping inside tunnels or out in the countryside where there is no reliable mobile phone signal, said Pasquazi.