In an attempt to move timekeeping away from traditional definitions expressed in terms of the period of the Earth’s rotation, physicist Louis Essen spent the early 1950s developing his pioneering atomic clock at the National Physical Laboratory (NPL). On 3 June 1955, Essen started to control the UK radio time signals using atomic units of time derived from the atomic clock, effectively marking what Essen called “the death of the astronomical second and the birth of atomic time.”
Essen showed that atoms, which have a set of discrete energy levels, could provide a much more stable reference time interval. By using microwaves to excite electrons from one energy level to another within atoms of caesium, Essen was able to stabilise the microwaves at a precise and reproducible frequency. Much like a grandfather clock depends on the swinging of a pendulum, Essen’s prototype atomic clock relied on this underlying frequency to mark the passing of time.
Although different elements have since been used in other types of atomic clock, the caesium clock has remained the fundamental standard of time and frequency. Since 1967, the SI second has been 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.
Such high levels of accuracy may seem unnecessary in everyday life, but in reality the clocks in train stations, video recorders and mobile phones all use atomic time to ensure they are perfectly synchronised. When it comes to more specialised functions, atomic time is also necessary for banking, controlling air traffic, maintaining the Internet, synchronising the distribution of electricity and the Global Positioning System (GPS) for navigation. Without the invention of atomic timekeeping, none of these things would be possible.
Since Essen’s pioneering work, the accuracy of atomic clocks has steadily improved by a factor of 10 or so every decade. The major improvement in the accuracy of caesium clocks has been achieved through the use of laser-cooled caesium atoms in complex laboratory clocks known as caesium fountains. NPL’s current primary clock, the caesium fountain NPL-CsF2, contributes to the generation of the international time scale UTC and is the reference for the UK’s national time scale UTC(NPL). It is over 300,000 times more precise than Essen’s original clock, which is now housed in the Science Museum in London.
“It is hard to overstate the significance of Louis Essen’s contribution,” said Professor Patrick Gill at NPL. “Although other researchers were experimenting with atomic timekeeping at roughly the same time, it was Essen who first developed the working prototype that has come to define the digital age.”
As the UK’s home of precise timing, NPL is continuing to develop ways to measure time ever more accurately and improve the performance of its atomic clocks. The next generation of these devices – optical clocks based on laser-cooled trapped ions or neutral atoms – should achieve accuracies equivalent to losing or gaining no more than one second in the lifetime of the universe. Efforts to develop accurate atomic clocks which can be used outside of the lab to widen their applications are also under way.
Dr Leon Lobo, Strategic Business Development Manager for Time & Frequency at NPL, said: “As well as continuing to improve accuracy, we are also looking to miniaturise atomic clocks. By making accurate atomic clocks portable, we could unlock the benefits of precise timing for countless applications. In the near future, we could use miniature atomic clocks to send unhackable communications, help timestamp high frequency trades, improve deep space navigation, and eventually integrate them into smartphones, increasing data transfer rates in communications networks.”
Contact: Leon Lobo
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