National Physical Laboratory

Atomic Timeline 

Essen Timeline - 1955

1955
Birth of Atomic Time

Louis Essen and Jack Parry design and build the world's first caesium atomic clock at NPL. Essen invites Director Edward Bullard 'to come and witness the death of the astronomical second and the birth of atomic time' 

 

Essen Timeline - 1957

1957

Schematic of the first caesium atomic clock 

 

1958
9 billion beats per second

NPL and the US Naval Observatory complete a three-year study comparing the astronomical second with the frequency of Essen's clock. One second is found to be equal to 9,192,631,770 cycles of the transition frequency of a caesium atom 

 

Essen Timeline - 1962

1962
Transferring time

Using the Telstar 1 satellite, NPL and the US Naval Observatory carry out the first two-way transatlantic clock comparisons. Two-way satellite time transfer goes on to become one of the most precise and accurate ways of comparing frequency standards 

 

Essen Timeline - 1967

1967
Time redefined

The SI second, previously based on the Earth's motion, is redefined in terms of the caesium transition frequency measured using Essen's clock in 1958. The caesium atom now underpins the definition of time

Photo: Members of the Consultative Committee for the Definition of the Second - Essen is third from the left. Courtesy of BIPM. 

 

Essen Timeline - 1967a

1967

Essen (centre) and Sir Alan Cook (right) hand over a rubidium atomic clock for control of the MSF time signal to Rugby Radio Station 

 

Essen Timeline - 1972

1972
Introducing the leap second

Leap seconds are introduced into the global time scale, Coordinated Universal Time (UTC). This ensures the world's time scale stays in sync with the Earth's rotation rate, which fluctuates unpredictably and, in the long term, is slowing down

Find out more about leap seconds

Photo: One of NPL's atomic clocks shows a leap second making a 61-second minute in 1994 

 

Essen Timeline - 1976

1976

NPL designs and installs fail-safe brakes for the chiming mechanism of Big Ben after a mechanical failure 

 

1983
Length redefined

The SI metre is redefined in terms of the SI second as 'the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second'. This enables scientists to accurately measure length using time

Find out more about NPL's speed of light research 

 

Essen Timeline - 1985

1985
NPL going solo

The Royal Greenwich Observatory (RGO) announces that it will no longer maintain an atomic time scale and is allowing its atomic clocks to run down. This decision leaves only NPL operating an atomic time scale in the UK 

 

Essen Timeline - 1991

1991
Hydrogen clocks

NPL adds a hydrogen maser to its atomic clock ensemble, greatly improving the performance of the UK's time scale, UTC(NPL). Hydrogen masers, which use the hydrogen atom as their frequency standard, offer increased stability over caesium clocks 

 

Essen Timeline - 1996

1996

Jon 'Doctor Who' Pertwee visits the NPL atomic clocks as part of a BBC Radio 2 science programme 

 

Essen Timeline - 2001

2004

Part of a femtosecond comb (a laser producing pulses of light of extremely short duration) used to measure the frequencies of NPL's optical clocks 

 

Essen Timeline - 2004

2004
The next generation

NPL announces the world's most accurate optical frequency measurement, made in a trapped laser-cooled strontium ion. Optical atomic clocks could form the basis of a future redefinition of the SI second

Photo: The optical cavity used to stabilise the laser for the NPL strontium ion optical clock 

 

2007
Time from NPL

The MSF Radio Time Signal, also known as 'The Time from NPL', moves from Rugby Radio Station to Anthorn Radio Station in Cumbria. The broadcast provides an accurate and reliable source of time, based on UTC(NPL), to radio-controlled clocks across the UK 

 

Essen Timeline - 2011

2011
World's most accurate clock

NPL's caesium fountain atomic clock - the UK's primary frequency standard - is revealed to be the most accurate timekeeper in the world, achieving an accuracy equivalent to losing or gaining only one second in 138 million years 

 

2014
Beginning of NPLTime®

NPL launches a new service, NPLTime®, providing trusted time for a new age of financial services. NPLTime® offers a precise time signal, directly traceable to Coordinated Universal Time (UTC) and independent of GPS, delivered through optical fibres

Find out more about NPLTime® 

 

Essen Timeline - 2014

2014

A trap for a single ytterbium ion used by NPL to set new limits on the time variation of fundamental physical constants and form the basis of the next generation of atomic clocks 

 

Essen Timeline - 2015

2015
Future of atomic time

The new caesium fountain atomic clock NPL-CsF3 becomes operational, allowing continuous operation of a primary clock at NPL. Work continues on the next generation of optical atomic clocks at NPL, which should achieve accuracies equivalent to losing or gaining one second in the age of the universe

 

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