NPL Atomic Timekeepingresource.npl.co.uk/.../metromnia/issue19_atomic_timekeeping_poste… · the quartz and atomic clocks invented during the 1930s and 1950s are even better timekeepers

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The spinning Earth gives our most basic measurement of time - the day -and for thousands of years it was our most stable timekeeper. However,the quartz and atomic clocks invented during the 1930s and 1950s are

even better timekeepers which show that the Earth does not rotate steadily but wobbles! NPL built the world’s first accurate caesium atomicclock in 1955 and paved the way for a new and better definition of the second based on the caesium 133 atom.

How a caesium clock worksCaesium atoms can be made to flip between two possible energies by illuminating themwith microwaves tuned to a specific frequency. In a caesium atomic clock, a beam ofatoms is created by boiling them off from an oven. Atoms with one energy are selectedusing a special magnetic gate and sent through a chamber containing microwaves. A second magnetic gate selects those atoms which have flipped energy levels after passing through the microwave chamber and then sends them to a detector to becounted. The microwaves are tuned to the exact frequency that defines thesecond when the detectorcounts the maximum number of atoms!

There are 24 hours in a day, 60 minutes in an hour and60 seconds in a minute - but how long is a second?

The second is 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.

How do you keep time around the world?Everyone around the world needs to keep to an agreed timescale. On the1st January 1972, Coordinated Universal Time (UTC) was adopted as the

official time for the world. The International Bureau of Weights andMeasures (BIPM) acts as the official time keeper of atomic time for

the world. Some 65 laboratories with 230 atomic clocks are usedto calculate this composite timescale. The time measured

by the rotation of the Earth about its axis slowly drifts awayfrom UTC and leap seconds are inserted, as required, to keepUTC to within 0.9 seconds of the Earth’s time.

Access to the national time scale,UTC(NPL)

NPL has been disseminating the UK national time scale from a transmitterat Rugby since 1950. This transmitter, which has the call sign MSF, operates

24 hours a day at a frequency of 60 kHz; transmissions can be receivedas far away as Iceland and Gibraltar. Everyone can have access to

the national time scale either via a radio controlled clock or by using NPL’s TRUETIME Service to update a PC clock.

Why do we need the accuracy of atomic clocks?Time measurement has become a basic part of everyday life and accuracies of the nearest minute or a few seconds are usually good enough for most human activities, but highly accurate timing plays a vital role in many other aspects of the modern world.

Global Positioning System satellites broadcast timing signals from their atomic clocks which enable ships, aeroplanes andeven hikers to establish their position to the nearest tens of metres. The challenge for the next generation of aircraft

landing systems is to guarantee reliable and accurate timing and positional data from satellites to avoidbumpy landings.

Whether surfing the Internet or making a telephone call, telecommunicationsrely upon accurate timing to ensure that digital messages are safely delivered to their destination. The recording of the timing and the order of a transaction is critical when transferring millions of pounds.

Pulsars as clocks?First discovered in 1967, pulsars are thoughtto be compact, highly magnetised, rapidly rotating neutron stars. As the pulsar rotates,

radiation is beamed along the direction of the two magnetic polesof the star. Astronomers detectthese pulses of radiation as if from acosmic lighthouse. For a special classof pulsars, called millisecond pulsars,the time intervals between these pulses of radiation are found to be extremely stable. It is this stability that could be used to provide the basis of an accurate clock at least comparable to some atomic clocks on Earth.

How do you generate a more accurate second?In current caesium clocks the atoms are heated to 100 °C. This causes theatoms to move quickly through the microwave chamber of the clock whichlimits its accuracy. At NPL, a new form of atomic clock, known as the caesium fountain, is under development. The fountain uses laser beams tocool the atoms to overcome this problem. In this clock, a cloud of atomsis projected up into the microwave chamber and allowed to fall downunder gravity. The slow movement of the

atoms allows a more accurate measurement of the resonant frequency of the atoms, i.e., the clock signal. In the future, putting these types of clocks in space, where

gravity is reduced, will allow further increases in accuracy.

The futureClocks for the 21st century are being developed in the form of ion traps. Ions arecharged atoms which can be trapped by electromagnetic fields almost indefinitely.Once trapped a laser beam can then be used to cool the ion down close to absolutezero, keeping it stationary. At NPL the element ytterbium has been chosen to develop ion trap clocks as its ions can have very stable states. These clocks may have accuracies of around 1000 times higher than the best current atomic clocks. That is equivalent to losing no more than one second in the lifetime of the universe.

Time Line 3500 BC

1762± 1 second in 3 days

17

1930s± 1 second in 3 yrs

± 1 second in 30 yrs

1955± 1 second in 300 yrs

1980s± 1 second in 300,000 yrs

1995± 1 second in 15 million yrs

± 10 seconds per day

Sundials

Atomic Timekeeping

Pendulum clocks

Harrison’schronometer

Earth’s rotation

Quartz

Essen’s caesium clock

Caesiumfountain

NPL - the nation’stimekeepersNPL’s atomic clocks help the UK run ontime through dissemination of thenational time scale and by contributingto Coordinated Universal Time. NPL also carries out an extensive programme of research into evenmore accurate ways to measure time.

The Nat ional Phys ica l Laboratory is operated on behalf of the DTI by NPL Management L im ited , a whol ly owned subsid iary of Serco Group p lc

If you have a measurement related scientific question contact us on: Telephone: 020 8943 6880 E-mail: time@npl.co.ukor visit our web site which has lots of measurement related information at www.npl.co.uk

Caesiumfountain

Ion Trap

PULSAR

Caesium clock

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