Louis Essen and his colleague Jack Parry with the world's first practical atomic clock

Atomic time: 60 years of the ultra-precise atomic clock

On June 3 2015, the world celebrates an anniversary of one invention that truly changed the world: the birth of atomic time. Where has the technology moved since then and where will it go in the future?

When physicist Louis Essen demonstrated his new toy to his colleagues on June 3 1955, he already knew he was about to enter the history books as the man who killed astronomical time.

This toy, which Essen had spent the previous two years building in the basement of the National Physical Laboratory’s (NPL) historical building in Teddington, UK, was no less than the world’s first properly functioning atomic clock.

“Essen’s demonstration enabled a fundamental change in the way we keep time,” says Peter Whibberley, senior research scientist at NPL’s time and frequency group.

“Scientists worked on the atomic clock technology probably since the 1930s, but this clock built by the NPL was the first man-made clock that was much better at keeping time than the Earth itself.”

Over the centuries, mankind developed various means for keeping track of time. In Essen’s time, the quartz clock was the number one technology, known for its exceptional stability over short periods of time. However, over the long term the quartz clock performed less impressively. Moreover, each device worked slightly differently, depending on the smallest variations in the shape and size. Researchers were thus on a quest to find a more precise technology and atoms drew their attention for their unique properties.

“Atoms have very precise resonances, each associated with one particular frequency which is a fundamental constant of nature,” explains Whibberley. “An atom will always have the same transition frequency anywhere in the universe and so by starting from an atom, we have a fundamentally constant reference for time keeping.”

Essen’s clock was a two-metre-long horizontal apparatus with a source of caesium atoms on one end, a microwave cavity in the middle probing the atoms’ frequency and a sensitive detector at the other end. When it was first put in to operation, it was accurate to one millisecond a day - equal to one second in about 300 years.

“Atomic clocks really changed the world,” says Professor Patrick Gill, Head of the Time and Frequency group at NPL. “Without atomic clocks, GPS wouldn’t be possible and so neither would be all the vast range of location, tracking and timing based applications it enables, including network synchronisation or trading on the stock exchange.”

The redefinition of the second

The breakthrough came at the right time. Renowned astronomers, including those from Britain’s Greenwich Observatory, who were the world’s main time-setters at the time, were well aware that the existing definition of a second as a fraction of the Earth’s rotation would soon fail to meet the needs of the era of precise science. Due to various geological and atmospheric processes, the Earth slows down and speeds up unpredictably. Moreover, it is gradually slowing down in the long term due to the friction caused by the tides. As a result, the duration of the second was not stable. It was thus clear that the second would soon have to be redefined.

“During the late 1950s, the caesium frequency determined by Essen’s clock was measured in terms of the astronomical second based on the Earth’s rotation” explains Whibberley. “This was then turned around and the value was adopted as the definition of the second.”

In 1967, the definition of the second was changed from one based on the Earth’s rotation to ‘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’.

Coordinated Universal Time

Soon after Essen published the results of his experiment in the journal Science, laboratories all over the world started building their own caesium atomic clocks. An idea soon emerged that labs could compare the frequencies of their respective atomic clocks via radio signals in order to make sure time all over the world was in sync.

In 1967, the International Bureau of Weights and Measures in Paris assumed responsibility for creating the Coordinated Universal Time (UTC). Currently, it compiles data from about 70 timing laboratories around the world. The UK’s NPL is still among its most accurate contributors.

“In our time scales lab, we take the second defined by our caesium fountain primary standard as the basis for our timescale” explains Elizabeth Laier English, scientist in the NPL time and frequency group. “We generate UTC(NPL), which is the UK version of the international time scale, and compare this with other labs around the world to make sure all the time scales are within a few nanoseconds of each other.”

Leap second

Some unexpected challenges arose from the 'supernatural' precision of caesium atomic clocks: the time of the clock and the time of the Earth started to diverge. While the deviation was imperceptible to most people, scientists were aware that if left untreated, it would keep accumulating.

“It’s a very slow divergence, that might reach around a minute in a century,” says Whibberley. “It would probably take a thousand years to reach an hour but it is a steadily increasing divergence. No one quite knows the exact rate because the Earth’s rotation is so unpredictable.”

To keep the Universal Coordinated Time and the Earth’s time in sync, scientists at first adjusted the atomic time by applying increments of fractions of a second.

In the early 1970s the science community agreed that such time jumps should take place less frequently and equal exactly one second. One such leap second will be inserted at the end of June, which will see its last minute have 61 seconds instead of the usual 60.

“We insert this leap second through our software,” explains Whibberley. “The caesium clock continues to tick at its normal frequency. The software in the clock can be programmed to have a minute with an extra second in it, so the clocks can handle that extra second just in the software and the physical processes behind carry on just the same.”

It is fair to say that the insertion of the leap second, which takes place once or twice a year, is a source of a headache for the time-keeping community. In the current interconnected world, any mistakes with the leap second could wreak havoc with computer systems, stock markets and even air-traffic control computers.

“Sometimes devices like Internet time servers, running the NTP network time protocol that synchronises time over the Internet, may not handle the leap second correctly,” describes Whibberley. “That results in different systems operating with slightly different times and that could have big implications in today’s interconnected world.”

Caesium fountain

Essen’s atomic clock, currently on display at London’s Science Museum, kept time at NPL for around a decade. Even though its precision had been improved over the years, new caesium clocks were eventually developed offering far better accuracy.

“The first caesium clock worked by forming a beam of caesium atoms,” Whibberley explains. “It has an oven at one end of the system and heats up the caesium to form a beam of atoms that passes through a microwave cavity. Then you tune the microwaves until you hit the transition frequency. But because the atoms are passing very quickly through the microwaves, in about a millisecond, there is a fundamental limit to how precisely you can measure the frequency.”

The late 1980s saw another major breakthrough in atomic time-keeping: the invention of the caesium fountain.

Caesium fountain was a novel, vertical, type of atomic clock taking advantage of what were, at the time, state-of-the-art laser cooling systems which allowed for the slowing down of atoms, thus increasing the time available to conduct the measurement.

“In a caesium fountain, you form a cloud of slowly moving caesium atoms that is thrown upwards through the microwave cavity,” Whibberley describes. “The cold cloud of atoms rises and then falls back under gravity through the microwave cavity again. The total interaction time with the microwaves is about half of a second, rather than a millisecond. So you have an improvement of almost a thousand and that gives a thousand times sharper measurement.”

The current caesium fountain at the NPL gains or loses only one second in 158 million years and is one of the most accurate in the world.

The future is in optics

The amazing precision of current atomic clocks doesn’t deter the physicist from looking for other, even more precise ways to measure time. Such is the nature of scientific minds that they are never happy with the ‘state-of-the-art’ and caesium clocks have already ceased to satisfy their appetite for improvement.

“There have been thoughts for a long time that if you moved your clock frequency into the optical domain with frequencies much larger than microwaves, you would get an improved resolution in terms of subdividing the second,” says Professor Gill. “That’s exactly what has been happening in the national measurement labs in the past decades to the point that now we have several different optical atomic clocks with uncertainties that are much smaller than what we can achieve with the caesium fountain.”

Optical clocks, using lasers to measure the atomic transitions, could be, according to estimates, up to 100 times more accurate than current caesium fountains. The best-performing device currently available – a strontium clock developed by the US National Institute of Standards and Technology – would neither gain nor lose a second in 15 billion years.

The developments are so promising that scientists have already started considering the possibility of changing the definition of a second again – this time based on optical clocks.

“If we want to change the definition, we have to agree on one particular system that will become the new standard,” explains Professor Gill. “But there are currently about ten or so promising systems and we still see rapid improvement in their performance. We should wait for this rapid development to flatten out a bit. We don’t want to make this decision too early,” he says adding that it might take up to a decade for the second to be redefined again.

Atomic clocks in space

In their search for the perfect accuracy, the world’s time-keepers are considering some rather revolutionary concepts. As the precision of atomic clocks depends on the strength of gravity, an idea has emerged that to actually improve the (already staggering) accuracy of atomic clocks beyond what is currently possible; the future clocks should actually be placed in space.

“There is a concept of a series of master clocks in space,” explains Professor Gill. “They would suffer less from the gravitational potential than the clocks on the Earth’s surface and could provide a reference point for clocks in aircraft or on the ground which could be calibrated by accessing information from the master clock.”

The idea presents many challenges. The current time-setting atomic clocks fill the whole room with equipment. The caesium fountain at NPL is about a metre wide and three metres high and requires a lot of associated paraphernalia. However, launching anything to space costs money and the more equipment that needs to be launched, the higher the cost.

“We have to figure out how to collapse all this equipment into something of a useful size to put it on a satellite,” says Professor Gill.

“We are currently working with the European Space Agency to understand how to make these clocks small and what the trade-off might be in terms of capability and accuracy.”

The upcoming laser-based optical atomic clock might in fact be better suited for such a task. Due to the smaller wavelengths compared to the current standard microwaves, the whole kit could fit into a much smaller box.

Commercial atomic clocks

And that’s still not all that awaits the atomic clocks in the upcoming years and decades. Now, 60 years after the technology was first demonstrated, there are “chip-scale” atomic clocks the size of a matchbox already available on the market, although much less accurate than the standards. This doesn’t necessarily mean that in the future we will all be wearing atomic wrist watches. However, the scope of applications benefitting from precise time keeping using such small clocks will greatly increase.

“We are involved in developing small clocks in both the microwave and optical domains” says Professor Gill.

“We are also interested in improving the accuracy both of those very small devices, and the accuracy of slightly larger optical clock packages, the size of a shoe-box or an aircraft suitcase.”

The industry’s appetite for such innovation is huge, especially in defence, security and communications sectors, which is perhaps overly reliant on time synchronisation through the GPS, and looking for their own independent source of time, to deal with situations where satellite navigation signals are lost.

“I expect to see those packages reaching maturity with respect to the market where we would be able to provide them with handheld systems or systems for incorporation into small travelling units within five years or so,” concludes Professor Gill.

Watch our interview with Professor Patrick Gill about the future of atomic time-keeping:

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