On June 3 1955, physicist Louis Essen demonstrated the world’s first practical caesium atomic clock, which started a new era of time-keeping. Today, 60 years on, the technology is being challenged by new developments.
The first practical atomic clock - now on display in London’s Science Museum - was built and operated at the UK’s National Physical Laboratory, which still enjoys the status as one of the world’s major hubs of precise time-keeping.
“Essen’s demonstration enabled a fundamental change in the way we keep time,” said 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.”
Essen’s clock, 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, was accurate to one millisecond a day - equal to one second in about 300 years.
“Atomic clocks really changed the world,” said 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 invention arrived at the right time, as the world’s astronomers, responsible for global time-keeping, already knew that the definition of one second as a fraction of the Earth’s rotation would soon fail to meet the needs of the era of precise science.
“Atoms have very precise resonances, each associated with one particular frequency, which is a fundamental constant of nature,” explained 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.”
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’.
Soon after Essen published his results in the journal Science, physical laboratories around the world started building their own caesium atomic clocks and an idea soon emerged the labs could exchange their respective measurements via radio frequencies to make sure time all over the world was in sync. This marked the birth of the Coordinated Universal Time (UTC).
Essen’s clock kept time at NPL for about ten years. Although its accuracy had been gradually improved over the years, new caesium clocks were eventually developed offering better results.
“The first caesium clock worked by forming a beam of caesium atoms,” Whibberley explained. “You have an oven at one end of the system and you heat 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, you have a fundamental limit to how precisely you can measure the frequency.”
Today, time at NPL is kept by a caesium fountain clock, which doesn’t lose or gain a second in 158 million years.
“In a caesium fountain, you form a cloud of slowly moving caesium atoms that is thrown upwards through the microwave cavity,” Whibberley described. “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.”
Despite the almost ‘supernatural’ precision of current caesium atomic clocks, scientists are already exploring new technologies that could perform even better.
“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,” said 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.”
The best-performing device currently available - a strontium clock developed by the US NIST-JILA laboratory - will neither gain nor lose a second in 15 billion years. Within the next decade, the second may thus be redefined again.
Read E&T's exclusive feature on the past and future of atomic time keeping
E&T interview with NPL’s Professor Patrick Gill