Essen's demonstration in 1955

The atomic (r)evolution in time-keeping

June 2015 marks the 60th anniversary of the invention of atomic time. The technology changed the world, but how has it advanced since then and where will it go in the future?

When physicist Louis Essen demonstrated his new toy, the world's first properly functioning atomic clock, to colleagues at the National Physical Laboratory in Teddington on 3 June 1955, he already knew he was about to enter history books as 'the man who killed astronomical time'.

"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 atomic clock technology 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."

The invention enabled a plethora of now omnipresent applications such as GPS and a vast range of location, tracking and timing based applications, including network synchronisation or trading on the stock exchange.

1955 Caesium I

Essen's clock, known as Caesium I, was a two-metre-long horizontal apparatus with a source of caesium atoms at one end, a microwave cavity in the middle probing the atoms' frequency and a sensitive detector at the other end. It was accurate to one millisecond a day – equal to one second in about 300 years.

"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."

1967 The redefinition of the second

Before Essen, one second was defined as a fraction of the rotation of the Earth. However, in the 1950s, the world's astronomers, then the main time-setters, were already aware that this definition 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 because of the friction caused by the tides.

"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 second was defined as "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".

1967 Coordinated Universal Time (UTC)

Soon after Essen published the results of his experiment, laboratories all over the world started building their own caesium atomic clocks. The 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 the 1960s, the International Bureau of Weights and Measures in Paris assumed responsibility for creating Coordinated Universal Time (UTC). Currently, it compiles data from about 70 timing laboratories around the world.

1972 Leap second

As a result of the preternatural stability and precision of caesium atomic clocks the time of the clock and the time of the Earth started to diverge.

"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 Coordinated Universal 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 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 instead of the usual 60 seconds.

1980s Caesium fountain

A major improvement in atomic time-keeping came in the late 1980s with the invention of the caesium fountain.

Taking advantage of what were then state-of-the-art laser cooling systems, physicists were able to slow down the caesium atoms in order to increase the time available for the measurement of their frequencies inside the microwave cavity.

While in Essen's clock the caesium atoms travelled fast in the form of a beam providing only a millisecond for the measurement, in a caesium fountain this time was increased to half a second.

"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 current caesium fountain at the NPL gains or loses only one second in 158 million years.

2010s The future is in optics

Despite the precision of a caesium fountain clock, technology is already being developed with even better performance.

"There have been thoughts for a long time that if you moved your clock frequency into the optical domain, you would get an improved resolution in terms of subdividing the second," exaplains Professor Patrick Gill, head of the Time and Frequency group at NPL. "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 currently best performing device, a strontium clock developed by the US NIST-JILA laboratory, would neither gain nor lose a second in 15 billion years.

2020s Atomic clocks in space

To improve precision of time measurements even further, scientists have been considering putting atomic clocks 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."

But there is a catch. The caesium fountain at NPL is about a metre wide and 3 metres long and requires a lab full of equipment. To make one fit for space, it would have to be squeezed into a much smaller package.

2020s Commercial atomic clock

60 years after the atomic clock 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 wristwatches. However, the scope of applications benefitting from precise time keeping using such small clocks will greatly increase.

Image credits: NPL, ESA

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