The scientists building a clock so accurate it could redefine the second

The second - the modern world's most fundamental unit of time - is up for a refresh. This opens up new technological possibilities, and raises some philosophical questions along the way

When you’re just trying to ensure you’re not late for your 9am meeting, a watch that might lose a second or two here or there is perfectly adequate. But sometimes you need a little more accuracy.

In a building outside London is a room-sized clock so accurate that it would have only lost or gained a second if it had been ticking since the beginning of the universe, 13.7 billion years ago. The ‘pendulum’ is a strontium atom, and the clock in the National Physics Laboratory in Teddington (and other clocks like it) could eventually redefine the official definition of a second.

In labs around the world, scientists are working to create new versions of the optical atomic clock, and connect existing ones into a global network for even higher accuracy. The clocks could enable new and world-changing technologies, such as gravity sensors and GPS accurate down to a centimetre. Such clocks could even help humanity weather the devastating effects of a solar flare, by helping networks to synchronise without the help of satellites.

The National Physics Laboratory in Teddington
The National Physics Laboratory in Teddington
Andrew Brookes

In the future, optical atomic clocks will change the official way we define a second, says Dr. Rachel Godun, Senior Research Scientist in the Time and Frequency Group at the National Physics Laboratory, where an optical atomic clock has operated since 2004. Optical atomic clocks ‘tick’ a great deal faster than the microwave atomic clocks which are used as the basis of the official international definition of the second today.

The first microwave atomic clock was unveiled at NPL in 1955, Dr Godun says, but today’s optical atomic clocks oscillate up to 100 times faster. Once a network of these clocks is created, the new optical clocks could be used as the basis for the SI (The International System of Units/Systeme Internationale) second. The current official definition of a second is ‘the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.’

Dr Godun says the shift to a new definition doesn’t mean current seconds are inaccurate - it’s that optical atomic clocks are less prone to disturbance. ‘There is no inaccuracy in the current definition of the second. The frequency of the caesium transition is defined as an exact number of cycles per second. In practice, however, it is difficult to measure the frequency of an atom without disturbing it slightly.

Strontium End Cap Ion Trap
The ’endcap’ chamber in which a single strontium ion is trapped, from which the clock takes its measurements.

‘The best caesium clocks are currently capable of measuring one second to 16 decimal places. Optical clocks, however, are more immune to disturbances and are already making measurements at 18 decimal places.

‘By basing the definition of the second on an atom in an optical clock, the top-level experimental realisation of the second will be more accurate. This will ripple down to all time and frequency measurements that are traced back to the top-level standard.’ In greatly simplified terms, the optical clock works by measuring vibrations given off by a supercooled strontium ion, which is trapped in a purpose-built chamber between two poles 0.56mm apart. More (much, much more) information is available here, and here.

Optical clocks will offer a boost in sectors which already rely on atomic clocks - such as satellite navigation systems, telecoms networks and energy networks, Dr Godun says. The clocks could also enable entirely new technologies, because they are so accurate they can be used to ‘sense’ gravity, Dr Godun says. ‘Next-generation optical clocks will enable entirely new applications.

‘For example, optical clocks could be useful tools for surveyors. This is because relativity dictates that time runs at different rates in different gravity potentials, or heights above the Earth’s surface. The best optical clocks are currently sensitive enough to resolve changes equivalent to a height difference of just 1 cm. They could therefore be used by surveyors in specific locations, such as the opposite ends of a long-distance pipeline or a bridge under construction. ‘

In Europe, scientists are working on a simplified version of optical clocks, which could be available outside laboratories: ‘superradiant optical clocks’, which could potentially be more compact than existing clocks, and more robust. Dr Florian Schreck of the IQClock consortium says that his team has funding for three years to build a continuously operating superradiant optical clock.

Unwerk AMC Clock
In 2018 Urwerk released a watch which came with an atomic clock regulating counterpart, complete with satellite linkup.

Schreck says that the clocks could not only enable new technologies, but could actually test whether fundamental constants of science are actually constant… or not. Dr Godun agrees, saying that increasing accuracies offer, ‘the ability to make measurements more precisely than ever before, which allows the laws of physics to be tested at unprecedented levels.

‘Can we find discrepancies between experimental measurements and the predictions of quantum or gravitational theories? Can we detect Dark Matter? These are some of the exciting questions that optical clocks are hoping to answer.’

For your mechanical watch, of course, the question is entirely academic - but for those of us who do value the principle of accurate timekeeping, and for anyone who in their more philosophical moments has paused to wonder what the concept of "accuracy" really means, it is fascinating to consider that the standard from which all notions of horological accuracy are ultimately derived is potentially about to change, no matter how infinitesimally.

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