C 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 atomic clock 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 works Caesium atoms can be made to flip between two possible energies by illuminating them with microwaves tuned to a specific frequency. In a caesium atomic clock, a beam of atoms is created by boiling them off from an oven. Atoms with one energy are selected using 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 be counted. The microwaves are tuned to the exact frequency that defines the second when the detector counts the maximum number of atoms! There are 24 hours in a day, 60 minutes in an hour and 60 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 the 1st January 1972, Coordinated Universal Time (UTC) was adopted as the official time for the world. The International Bureau of Weights and Measures (BIPM) acts as the official time keeper of atomic time for the world. Some 65 laboratories with 230 atomic clocks are used to calculate this composite timescale. The time measured by the rotation of the Earth about its axis slowly drifts away from UTC and leap seconds are inserted, as required, to keep UTC 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 transmitter at 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 received as 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 and even 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 avoid bumpy landings. Whether surfing the Internet or making a telephone call, telecommunications rely 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 thought to be compact, highly magnetised, rapidly rotating neutron stars. As the pulsar rotates, radiation is beamed along the direction of the two magnetic poles of the star. Astronomers detect these pulses of radiation as if from a cosmic lighthouse. For a special class of 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 the atoms to move quickly through the microwave chamber of the clock which limits its accuracy. At NPL, a new form of atomic clock, known as the caesium fountain, is under development. The fountain uses laser beams to cool the atoms to overcome this problem. In this clock, a cloud of atoms is projected up into the microwave chamber and allowed to fall down under 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 future Clocks for the 21st century are being developed in the form of ion traps. Ions are charged 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 absolute zero, 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’s chronometer Earth’s rotation Quartz Essen’s caesium clock Caesium fountain NPL - the nation’s timekeepers NPL’s atomic clocks help the UK run on time through dissemination of the national time scale and by contributing to Coordinated Universal Time. NPL also carries out an extensive programme of research into even more accurate ways to measure time. The National Physical Laboratory is operated on behalf of the DTI by NPL Management Limited, a wholly owned subsidiary of Serco Group plc If you have a measurement related scientific question contact us on: Telephone: 020 8943 6880 E-mail: [email protected] or visit our web site which has lots of measurement related information at www.npl.co.uk Caesium fountain Ion Trap PULSAR Caesium clock 4949/AAR15653/1.5K/0803
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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: [email protected] visit our web site which has lots of measurement related information at www.npl.co.uk
Caesiumfountain
Ion Trap
PULSAR
Caesium clock
4949
/AA
R15
653/
1.5K
/080
3
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