Greatest distance to sychronize two optical clocks

The greatest distance to wirelessly synchronize two optical clocks is 113 km (70.2 mi), achieved by a team from the University of Science and Technology of China. The team used a refined, high powered laser to send a super-precise time signal between two clocks in Xinjiang, China. The results of the experiment were published in Nature on 5 Oct 2022.

Optical clocks (sometimes called optical atomic clocks) are a development of the atomic clocks currently used by research laboratories around the world. Existing atomic clocks work by using microwaves to push a stream of atoms (most often cesium, but other elements can be used) into a specific energy state, which occurs at a universally consistent frequency. The stream of atoms enter a resonator in a low energy state, and the microwave energy is used to push them over into a higher energy state. This transition happens when the microwave energy is tuned to a precise frequency (defined as 9,192,631,770 oscillations per second). The clocks are calibrated by monitoring the energy state of the output atoms, once they are all transitioning as expected, the clock registers a second for every 9,192,631,770 oscillations.

Optical clocks replace the microwave emitters with lasers. Optical light oscilates at a higher frequency for a given energy level, meaning that the transition frequency can be defined with a far greater degree of precision. Microwave-based atomic clocks are so accurate that they drift off by only 1 second for every 100 million years. Optical clocks, on the other hand, are capable of a drift of one second every 80 billion years.

The problem with achieving such an extraordinary increase in precision is that difficulty involved in keeping the world's master clocks synchronized also increases. There isn't much point in having super-accurate clocks if the time at the National Institutes of Standards and Technology in the USA isn't the same as the time at the National Time Service Center in China, for example, and for many practical purposes it's important that this time signal can be beamed to satellites in space.

Atomic clocks are currently synchronized using microwave signals, but these are too low-frequency to keep optical clocks in sync. The only thing that can be used is light itself, but optical light is absorbed by impurities in the air, and can be deflected off target by atmospheric conditions.

The method used in this experiment relied on devices called optical frequency combs, which refine laser beams into extremely stable and precise pulses. Once refined, the pulse was sent through a series of amplifiers to boost its power. The time on one clock was encoded into the frequency of the laser and then that laser was beamed to the other clock. These time signals were then checked against a similar light pulse sent through a high-quality fibre-optic cable.

Once the synchronization issue has been resolved, optical clocks promise to enable important scientific advances, as well as improvements to the lives of ordinary people. If the atomic clocks that currently coordinate the Global Positioning System, for example, were replaced with optical clocks then the uncertainty of a consumer-device GPS fix could go from 5 metres to a few centimetres.