Atomic Clocks

1
Atomic Clocks

• Chris Wills

2
Before Atomic Clocks

• In 1844, the Royal Greenwich Observatory was
  determined to be on the prime meridian, 0
  longitude.
• All time was to be based off of GMT.
• However, because of different definitions of
  years, months, days and seconds, there is no one
  exact standard demarcation of time.
• There are rate changes in the length that it
  takes for the earth to make a rotation around the
  sun, which also change the length of a second, a
  minute and so forth.
• Also, in 1986, Britain went off the clocks.
  Meaning that the clocks had to be replaced and
  they chose not to do so.
• A new measurement of standard time had to be
  created.

3
History of the Atomic Clock (Through 1959)

• 1945 - Isidor Rabi, a physics professor at
  Columbia University, suggests a clock could be
  made from a technique he developed called atomic
  beam magnetic resonance.
• 1949 -- Using Rabis technique, the National
  Bureau of Standard announces the worlds first
  atomic clock using the ammonia molecule as the
  source of vibrations.
• 1952 -- NIST completes the first accurate
  measurement of the frequency of the cesium clock
  resonance. The clock is named
• NBS-1.
• 1955 --The National Physical Laboratory in
  England builds the first cesium-beam clock used
  as a calibration source.
• 1958 -- Commercial cesium clocks become
  available, costing 20,000 each.
• 1959 -- NBS-1 goes into regular service as NIST's
  primary frequency standard.

4
History of the Atomic Clock (Through Present)

• 1963- Scientists continue to make changes and
  alterations of the clock resulting in
• NBS-3.
• 1967 -The 13th General Conference on Weights and
  Measures defines the second on the basis of
  vibrations of the cesium atom the worlds
  timekeeping system no longer has an astronomical
  basis.
• 1972- Atomic timing adopted as the standard
  reference for all scientific timing
• 1975 - NBS-6 begins operation an outgrowth of
  NBS-5, it is one of the worlds most accurate
  atomic clocks, neither gaining nor losing one
  second in 300,000 years.
• 1999 - NIST-F1 begins operation with an
  uncertainty of 1.7 x 10-15, or accuracy to about
  one second in 20 million years, making it one of
  the most accurate clocks ever made.

NIST-F1
5
Progression of the Clock
6
Types of Atomic Clocks

• There are three different types of atomic clocks
  cesium atomic clocks, hydrogen atomic clocks and
  rubidium atomic clocks.
• Cesium Atomic Clocks- sends out a beam of cesium
  atoms. The clock separates the cesium atoms of
  different energy levels by a magnetic field.
• Hydrogen atomic clocks- sustain hydrogen atoms at
  their ideal condition in special containers so
  the atoms dont lose their high energy state
  rapidly.
• Rubidium atomic clocks- the simplest form of
  atomic clocks. They use a glass cell of rubidium
  gas that changes its absorption of light at the
  optical frequency when the surrounding microwave
  frequency is just right.

7
How does it work?

• NIST-F1 is a fountain clock, meaning that the way
  lasers and atoms looks like a fountain.
• Six infrared laser beams are pointed at right
  angles to each other at the center of the
  chamber. The lasers gently push the cesium atoms
  together into a ball, causing the lasers to slow
  down the movement of the atoms and cool them to
  temperatures near absolute zero.
• Two vertical lasers are used to gently toss the
  ball upward (the "fountain" action), and then all
  of the lasers are turned off. The ball then
  falls back down through the microwave cavity.
  This lasts about a second.
• After they fall back down, another laser is
  pointed at the atoms. Those atoms whose atomic
  state were altered by the microwave signal emit
  light (a state known as fluorescence), measured
  by a detector.
• The process is repeated until a state of maximum
  florescence is reached that affects all the
  atoms.

8
How is atomic time measured?

• The correct frequency for the atomic time is now
  defined as 9,192,631,770 Hz.
• This way there is exactly 1 Hz of output, or 1
  cycle per second.
• The long-term accuracy achievable by modern
  cesium atomic clocks is better than one second
  off per one million years.

• The United States main atomic clock is
  located at the National
• Institute of Standards and Technology in
  Boulder, Colorado.
• The United States Naval Observatory also
  tracks time using
• portable atomic clocks for their missions as
  well as supplying the
• time for Department of Defense and various
  naval and military
• bases.

9
So how can we figure out what time it is?

• Radio stations are used to determine the time for
  the rest of the country (WWV, WWVB, WWVH).
• The radio stations frequently check to make sure
  that they the same as the national frequency
  standard.
• The signal then reaches a time code which
  determines which time zone the signal will be
  broadcast.

• The signal then is amplified over one of five
  different frequencies.
• Next, the signal is sent to the antenna using
  a 50 Ohm
• transmission line, connecting it to the
  station house with the
• antenna towers.
• The antenna radiates the time signal from
  single antenna towers.
• The radio signal travels through the
  atmosphere at the speed of
• People then receive the transmitted signal
  through wall clocks,
• wrist watches and laboratory receivers.

10
The Future of Atomic Clocks

• In 2008 NASA is planning the launch of Primary
  Atomic Reference Clock in Space (PARCS).
• The mission involves a laser-cooled cesium atomic
  clock, and a time-transfer system using Global
  Positioning System (GPS) satellites.
• The purpose of this mission will be to test the
  gravitational theory, study laser-cooled atoms in
  microgravity, and to improve the accuracy of
  timekeeping on earth.
• The proposed location for the experiment is on
  the External Facility of the Japanese
  Experimental Module (JEM).

11
So what time is it now?

• The Real Time!

12
Bibliography

• Fraser, J.T. Time the Familiar Stranger. Redmond
  Tempus Books, 1987.
• A Brief History of Atomic Clocks at NIST.
  Physics Laboratory, Time and Frequency Division.
  July 2005, National Institute of Standards and
  Technology. October 11, 2005 lthttp//tf.nist.govgt.
  
• Dick, Steven. Timekeeping at the U.S. Naval
  Observatory. Time Service Department. United
  States Naval Observatory. October 11, 2005
  lthttp//tycho.usno.navy.milgt.
• Dwyer, Douglas. How Atomic Clocks Work. How
  Stuff Works. 2005. October 11, 2005
  lthttp//science.howstuffworks.comgt.
• Atomic Clock. Wikipedia. October 11, 2005
  lthttp//en.wikipedia.orggt.
• The Official U.S. Time. Physics Laboratory,
  Time and Frequency Division. October 11, 2005.
  National Institute of Standards and Technology.
  October 11, 2005 lthttp//nist.time.govgt.